Changelog.md

Release 3.15, 2024-12-13

C language support:

• Minimal syntactic support for _Float16 type (half-precision FP numbers). Function declarations using _Float16 are correctly parsed, but any actual use of _Float16 is rejected later during compilation. (#525)
• Support C99 array declarator syntax involving static and *. These annotations are correctly parsed, but ignored during typing and compilation. (#539)

Code generation and optimization:

• Better support for _Bool type in the back-end and in the memory model. This avoids redundant normalizations to _Bool.
• Take types of function parameters into account during value analysis. This avoids redundant normalizations on parameters of small integer types.
• More conservative static analysis of pointer comparisons. (#516)
• Refined heuristic for if-conversion. Turn if-conversion off in some cases where it would prevent later optimization of conditional branches in the continuation of the if.
• CSE: remember pointer alias information in Load equations, making it possible to remove more redundant memory loads. (#518)
• More precise value analysis of integer multiplication, division, modulus.
• Constant propagation: optimize "known integer or undefined" results. For example, &x == &x, which is either 1 or undefined, is now replaced by 1.
• Optimize (x ^ cst) != 0 into x != cst.
• Avoid quadratic compile-time behavior on deeply-nested if statements. (#519)

Bug fixes:

• More robust determination of symbols that may be defined in a shared object. (#538)
• Escape $NNN identifiers in generated x86 and ARM assembly code (#541).
• Wrong parameter scoping in function definitions such as int (*f(int y))(int x) { ... } (a function returning a pointer to a prototyped function).

Usability:

• Mark stack as non-executable in binaries produced by ccomp.
• Check that preprocessed sources (.i files) do not contain backslash-newline sequences.
• ./configure arm-linux now selects the hard-FP ABI, since Linux distributions no longer use the soft-FP ABI.

Supporting libraries:

• ARM library for 64-bit integer arithmetic: faster division, cleaned-up code.

Coq development:

• Support Coq 8.20.
• Build: support TIMING and PROFILING like coq_makefile. (#512)
• Make dependency on Extraction explicit. (#515)
• Install .glob and .v files along .vo files. (#527)

Release 3.14, 2024-05-02

ISO C conformance:

• free has well-defined semantics on blocks of size 0 (#509).

Code generation and optimization:

• More simplifications of comparison operations and selection operations during the CSE pass.
• Replace selection operations with moves if both branches are equal.
• ARM 32 bits: several minor improvements to the generated code (#503 and more).

Bug fixes:

• x86 under Windows: the wrong sub instruction was generated for Pallocframe.
• ARM: fix PC displacement overflow involving floating-point constants.
• ARM: fix error on printing of "s17" register.
• RISC-V: do not use 64-bit FP registers for memcpy if option -fno-fpu is given.

Usability:

• Added generation of CFI debugging directives for AArch64 and RISC-V.
• Removed the command-line option -fstruct-return, deprecated since release 2.6.

Formal semantics:

• The big-step semantics for Clight now supports the two models for function arguments (either as stack-allocated variables or as register-like temporaries).

Coq development:

• Support Coq 8.17, 8.18, and 8.19.
• Revised most uses of the intuition tactic (#496 and more).
• Address most other deprecation warnings from Coq 8.18 and 8.19.
• Updated local copy of MenhirLib to version 20231231.
• Updated local copy of Flocq to version 4.1.4.

Distribution:

• The small test suite was moved to a separate Git repository. Use git submodule init && git submodule update to download it.

Release 3.13, 2023-07-04

Code generation and optimization:

• Slightly more precise value analysis, with a better distinction between "any integer value" and "any integer or pointer value". (#490)
• Recognize a few more nested conditional expressions that can be if-converted into a conditional move.
• Register allocation: better support for "preferred" registers, e.g. registers R0-R3 on ARM Thumb, which enable more compact instruction encodings.
• ARM Thumb: use more instructions that can be encoded in 16 bits, producing more compact code.
• x86: use a shorter instruction encoding for loading unsigned 32-bit constants. (#487)

Bug fixes:

• _Noreturn on function definitions (but not declarations) was ignored sometimes.
• The argument of __builtin_va_end was discarded, is now evaluated for its side effects.
• Tail call optimization must be disabled in variadic functions, otherwise a va_list structure can get out of scope.
• Nested compound initializers were elaborated in the wrong order, causing compile-time failures later in the compilation pipeline.
• The signedness of a bit field was determined differently when its type was an enum type and when it was a typedef to an enum type.

ABI conformance:

• Define wchar_t exactly like the ABI says. On most platforms, CompCert was defining wchar_t as signed int, but it must be unsigned int for AArch64-ELF and unsigned long for PowerPC-EABI.

Supported platforms:

• Removed support for Cygwin 32 bits, which has reached end of life. Cygwin 64 bits remains supported.

Coq development:

• Support Coq 8.16.0 and 8.16.1, addressing most of the new warnings in 8.16.
• When installing the Coq development, also install coq-native generated files.
• Tweak some proof scripts for compatibility with future Coq versions. (#465, #470, #491, #492)
• Updated the vendored copy of Flocq to version 4.1.1.

Release 3.12, 2022-11-25

New features:

• Support unstructured switch statements such as Duff's device. This is achieved via an optional, unverified code rewrite, activated by option -funstructured-switch. (#459)
• Support C11 Unicode string literals and character constants, such as u8"été" or u32'❎'.

Usability:

• Support the -std=c99, -std=c11 and -std=c18 option. These options are passed to the preprocessor, helping it select the correct version of the standard header files. It also controls CompCert's warning for uses of C11 features. (#456)
• The source character set of CompCert is now officially Unicode with UTF-8 encoding, A new warning invalid-utf8 is triggered if byte sequences that are not valid UTF-8 are found outside of comments. Other source character sets and stricter validation can be supported via the -finput-charset option, see next.
• If the GNU preprocessor is used, the source character set can be selected with the -finput-charset= option. In particular, -finput-charset=utf8 checks that the source file is correctly UTF-8 encoded, and -finput-charset=ascii that it contains no Unicode characters at all.
• Support mergeable sections for string literals and for numerical constants.
• AArch64, ARM, RISC-V and x86 ELF targets: use .data.rel.ro section for const data whose initializers contain relocatable addresses. This is required by the LLVM linker and simplifies the work of the GNU linker.
• configure script: add option -sharedir to specify where to put the compcert.ini configuration file (#450, #460)
• ARM 32 bits: emit appropriate Tag_ABI_VFP attribute (#461)

Optimizations:

• Recognize more if-else statements that can be if-converted into a conditional move. In particular, debug statements generated in -g mode no longer prevent this conversion.

Bug fixes:

• Revised simplification of nested conditional, ||, && expressions to make sure the generated Clight code is well typed and is not rejected later by ccomp (#458).
• In -g mode, when running under Windows, the ccomp executable could fail on an uncaught exception while inserting lines of the source C file as comments in the generated assembly file.
• Reintroduced DWARF debug information for bit fields in structs (it was missing since 3.10).

Coq development:

• RTLgen: use the state and error monad for reserving goto labels (#371) (by Pierre Nigron)
• Add Declare Scope statements where appropriate, and re-enable the undeclared-scope warning.

Release 3.11, 2022-06-27

New features:

• Support _Generic expressions from ISO C11.

ISO C conformance:

• Enumeration types are compatible only with int but not with other integer types.
• Fixed confusion between unprototyped function pointer types T (*)() and prototyped, zero-argument function pointer types T (*)(void) in type casts (#431).

Usability:

• Improved control-flow analysis of calls to "noreturn" functions, resulting in more accurate warnings.
• More detailed warning for unprototyped function definitions, now shows the prototyped type that is given to the function.
• Extended the warning above to definitions of the form T f() { ... }, i.e. unprototyped but with no parameters. (Before, the warning would trigger only if parameters were declared.)
• Check (and warn if requested) for arguments of struct/union types passed to a variable-argument function.

Bug fixes:

• RISC-V: fixed an error in the modeling of float32 <-> float64 conversions when the argument is a NaN (#428).
• x86: changed the compilation of __builtin_fmin and __builtin_fmax so that their NaN behavior is the one documented in the manual.
• Improved reproducibility of register allocation. (Before, compiling CompCert with two different OCaml versions could have resulted in correct but different allocations.)
• Hardened the configure script against Cygwin installations that produce \r\n for end-of-lines (#434).
• RISC-V: tail calls to far-away functions were causing link-time errors (#436, #437).

Coq development:

• Updated the Flocq library to version 4.1.
• Support for Coq 8.14.1, 8.15.0, 8.15.1, 8.15.2.
• Minimal Coq version supported is now 8.12.0.

Release 3.10, 2021-11-19

Major improvement:

• Bit fields in structs and unions are now handled in the formally-verified part of CompCert. (Before, they were being implemented through an unverified source-to-source translation.) The CompCert C and Clight languages provide abstract syntax for bit-field declarations and formal semantics for bit-field accesses. The translation of bit-field accesses to bit manipulations is performed in the Cshmgen pass and proved correct.

Usability:

• The layout of bit-fields in memory now follows the ELF ABI algorithm, improving ABI compatibility for the CompCert target platforms that use ELF.
• Handle the # 0 line directive generated by some C preprocessors (#398).
• Un-define the __SIZEOF_INT128__ macro that is predefined by some C preprocessors (#419).
• macOS assembler: use ## instead of # to delimit comments (#399).

ISO C conformance:

• Stricter checking of multi-character constants 'abc'. Multi-wide-character constants L'ab' are now rejected, like GCC and Clang do.
• Ignore (but still warn about) unnamed plain members of structs and unions (#411).
• Ignore unnamed bit fields when initializing unions.

Bug fixing:

• x86 64 bits: overflow in offset of leaq instructions (#407).
• AArch64, ARM, PowerPC, RISC-V: wrong expansion of __builtin_memcpy_aligned in cases involving arguments that are stack addresses (#410, #412)
• PowerPC 64 bits: wrong selection of 64-bit rotate-and-mask instructions (rldic, rldicl, rldicr), resulting in assertion failures later in the compiler.
• RISC-V: update the Asm semantics to reflect the fact that register X1 is destroyed by some builtins.

Compiler internals:

• The "PTree" data structure (binary tries) was reimplemented, using a canonical representation that guarantees extensionality and improves performance.
• Add the ability to give formal semantics to numerical builtins with small integer return types.
• PowerPC: share code for memory accesses between Asmgen and Asmexpand
• Declare __compcert_i64* helper runtime functions during the C2C pass, so that they are not visible during source elaboration.

The clightgen tool:

• Add support for producing Csyntax abstract syntax instead of Clight abstract syntax (option -csyntax to clightgen) (contributed by Bart Jacobs; #404, #413).

Coq development:

• Added support for Coq 8.14 (#415).
• Added support for OCaml 4.13.
• Updated the Flocq library to version 3.4.2.
• Replaced Global Set Asymmetric Patterns by more local settings (#408).

Release 3.9, 2021-05-10

New features:

• New port: AArch64 (ARM 64 bits, "Apple silicon") under macOS.
• Support bitfields of types other than int, provided they are no larger than 32 bits (#387)
• Support __builtin_unreachable and __builtin_expect (#394) (but these builtins are not used for optimization yet)

Optimizations:

• Improved branch tunneling: optimized conditional branches can introduce further opportunities for tunneling, which are now taken into account.

Usability:

• Pragmas within functions are now ignored (with a warning) instead of being lifted just before the function like in earlier versions.
• configure script: add -mandir option (#382)

Compiler internals:

• Finer control of variable initialization in sections. Now we can put variables initialized with symbol addresses that need relocation in specific sections (e.g. const_data on macOS).
• Support re-normalization of function parameters at function entry, as required by the AArch64/ELF ABI.
• PowerPC 64 bits: remove Pfcfi, Pfcfiu, Pfctiu pseudo-instructions, expanding the corresponding int<->FP conversions during the selection pass instead.

Bug fixing:

• PowerPC 64 bits: incorrect ld and std instructions were generated and rejected by the assembler.
• PowerPC: some variadic functions had the wrong position for their first variadic parameter.
• RISC-V: fix calling convention in the case of floating-point arguments that are passed in integer registers.
• AArch64: the default function alignment was incorrect, causing a warning from the LLVM assembler.
• Pick the correct archiver to build .a library archives (#380).
• x86 32 bits: make sure functions returning structs and unions return the correct pointer in register EAX (#377).
• PowerPC, ARM, AArch64: updated the registers destroyed by asm pseudo-instructions and built-in functions.
• Remove spurious error on initialization of a local struct containing a flexible array member.
• Fixed bug in emulation of assignment to a volatile bit-field (#395).

The clightgen tool:

• Move the $ notation for Clight identifiers to scope clight_scope and submodule ClightNotations, to avoid clashes with Ltac2's use of $ (#392).

Coq development:

• Compatibility with Coq 8.12.2, 8.13.0, 8.13.1, 8.13.2.
• Compatibility with Menhir 20210419 and up.
• Oldest Coq version supported is now 8.9.0.
• Use the lia tactic instead of omega.
• Updated the Flocq library to version 3.4.0.

Licensing and distribution:

• Dual-licensed source files are now distributed under the LGPL version 2.1 (plus the Inria non-commercial license) instead of the GPL version 2 (plus the Inria non-commercial license).

Release 3.8, 2020-11-16

New features:

• Support _Static_assert from ISO C11.
• Support __builtin_constant_p from GCC and Clang.
• New port: x86 64 bits Windows with the Cygwin 64 environment. (configure with target x86_64-cygwin).
• The following built-in functions are now available for all ports: __builtin_sqrt, __builtin_fabsf, and all variants of __builtin_clz and __builtin_ctz.
• Added __builtin_fmin and __builtin_fmax for AArch64.

Removed features:

• The x86 32 bits port is no longer supported under macOS.

Compiler internals:

• Simpler translation of CompCert C casts used for their effects but not for their values.
• Known builtins whose results are unused are eliminated earlier.
• Improved error reporting for ++ and -- applied to pointers to incomplete types.
• Improved error reporting for redefinitions and implicit definitions of built-in functions.
• Added formal semantics for some PowerPC built-ins.

The clightgen tool:

• New -canonical-idents mode, selected by default, to change the way C identifiers are encoded as CompCert idents (positive numbers). In -canonical-idents mode, a fixed one-to-one encoding is used so that the same identifier occurring in different compilation units encodes to the same number.
• The -short-idents flag restores the previous encoding where C identifiers are consecutively numbered in order of appearance, causing the same identifier to have different numbers in different compilation units.
• Removed the automatic translation of annotation builtins to Coq logical assertions, which was never used and possibly confusing.

Coq and OCaml development:

• Compatibility with Coq 8.12.1, 8.12.0, 8.11.2, 8.11.1.
• Can use already-installed Flocq and MenhirLib libraries instead of their local copies (options -use-external-Flocq and -use-external-MenhirLib to the configure script).
• Automatically build to OCaml bytecode on platforms where OCaml native-code compilation is not available.
• Install the compcert.config summary of configuration choices in the same directory as the Coq development.
• Updated the list of dual-licensed source files.

Release 3.7, 2020-03-31

ISO C conformance:

• Functions declared extern then implemented inline remain extern
• The type of a wide char constant is wchar_t, not int
• Support vertical tabs and treat them as whitespace
• Define the semantics of free(NULL)

Bug fixing:

• Take sign into account for conversions from 32-bit integers to 64-bit pointers
• PowerPC: more precise determination of small data accesses
• AArch64: when addressing global variables, check for correct alignment
• PowerPC, ARM: double rounding error in int64->float32 conversions

ABI conformance:

• x86, AArch64: re-normalize values of small integer types returned by function calls
• PowerPC: float arguments passed on stack are passed in 64-bit format
• RISC-V: use the new ELF psABI instead of the old ABI from ISA 2.1

Usability and diagnostics:

• Unknown builtin functions trigger a specific error message
• Improved error messages

Coq formalization:

• Revised modeling of the PowerPC/EREF isel instruction
• Weaker ec_readonly condition over external calls (permissions can be dropped on read-only locations)

Coq and OCaml development:

• Compatibility with Coq version 8.10.1, 8.10.2, 8.11.0
• Compatibility with OCaml 4.10 and up
• Compatibility with Menhir 20200123 and up
• Coq versions prior to 8.8.0 are no longer supported
• OCaml versions prior to 4.05.0 are no longer supported

Release 3.6, 2019-09-17

New features and optimizations:

• New port targeting the AArch64 architecture: ARMv8 in 64-bit mode.
• New optimization: if-conversion. Some if/else statements and a ? b : c conditional expressions are compiled to branchless conditional move instructions, when supported by the target processor
• New optimization flag: -Obranchless, to favor the generation of branchless instruction sequences, even if probably slower than branches.
• Built-in functions can now be given a formal semantics within CompCert, instead of being treated as I/O interactions. Currently, __builtin_fsqrt and __builtin_bswap* have semantics.
• Extend constant propagation and CSE optimizations to built-in functions that have known semantics.
• New "polymorphic" built-in function: __builtin_sel(a,b,c). Similar to a ? b : c but b and c are always evaluated, and a branchless conditional move instruction is produced if possible.
• x86 64 bits: faster, branchless instruction sequences are produced for conversions between double and unsigned int.
• __builtin_bswap64 is now available for all platforms.

Usability and diagnostics:

• Improved the DWARF debug information generated in -g mode.
• Added options -fcommon and -fno-common to control the generation of "common" declarations for uninitialized global.
• Check for reserved keywords _Complex and _Imaginary.
• Reject function declarations with multiple void parameters.
• Define macros __COMPCERT_MAJOR__, __COMPCERT_MINOR__, and __COMPCERT_VERSION__ with CompCert's version number. (#284)
• Prepend $(DESTDIR) to the installation target. (#169)
• Extended inline asm: print register names according to the types of the corresponding arguments (e.g. for x86_64, %eax if int and %rax if long).

Bug fixing:

• Introduce distinct scopes for iteration and selection statements, as required by ISO C99.
• Handle dependencies in sequences of declarations (e.g. int * x, sz = sizeof(x);). (#267)
• Corrected the check for overflow in integer literals.
• On x86, __builtin_fma was producing wrong code in some cases.
• float arguments to __builtin_annot and __builtin_ais_annot were uselessly promoted to type double.

Coq formalization and development:

• Improved C parser based on Menhir version 20190626: fewer run-time checks, faster validation, no axioms. (#276)
• Compatibility with Coq versions 8.9.1 and 8.10.0.
• Compatibility with OCaml versions 4.08.0 and 4.08.1.
• Updated to Flocq version 3.1.
• Revised the construction of NaN payloads in processor descriptions so as to accommodate FMA.
• Removed some definitions and lemmas from lib/Coqlib.v, using Coq's standard library instead.

The clightgen tool:

• Fix normalization of Clight switch statements. (#285)
• Add more tracing options: -dprepro, -dall. (#298)
• Fix the output of -dclight. (#314)

Release 3.5, 2019-02-27

Bug fixing:

• Modeling error in PowerPC ISA: how register 0 is interpreted when used as base register for indexed load/stores. The code generated by CompCert was correct, but was proved correct against the wrong specification.
• Modeling error in x86 ISA: how flag ZF is set by floating-point comparisons. Here as well, the code generated by CompCert was correct, but was proved correct against the wrong specification.
• Revised handling of attributes so that they behave more like in GCC and Clang. CompCert now distinguishes between attributes that attach to names (variables, fields, typedefs, structs and unions) and attributes that attach to objects (variables). In particular, the aligned(N) attribute now attaches to names, while the _Alignas(N) modifier still attaches to objects. This fixes issue 256.
• Issue with NULL as argument to a variadic function on 64-bit platforms (issue 265)
• Macro __bool_true_false_are_defined was missing from <stdbool.h> (issue 266)

Coq development:

• Can now be entirely rechecked using coqchk (contributed by Vincent Laporte)
• Support Coq version 8.9.0
• Avoid using "refine mode" when defining Instance (contributed by Maxime Dénès)
• Do not support Menhir versions more recent than 20181113, because they will introduce an incompatibility with this CompCert release.

New feature:

• PowerPC port: add __builtin_isel (conditional move) at types int64, uint64, and _Bool.

Release 3.4, 2018-09-17

Bug fixing:

• Redefinition of a typedef in a different scope was wrongly rejected.
• Attach _Alignas(N) and __attribute((aligned(N))) to names instead of types, so that _Alignas(16) int * p means "16-aligned pointer to int", not "pointer to 16-aligned int".
• For packed structs, fix a discrepancy between the size and alignment computed during elaboration and those computed by the verified front-end after expansion.
• Honor qualified array types in function parameters: if a parameter is declared as e.g. int t[const 4], it is now treated as int * const t in the function body, not int * t like before.
• Reject __builtin_offsetof(struct s, f) if f is a bit-field.
• Wrong parsing of attributes having multiple arguments such as __attribute((packed(A,B,C))).
• If __builtin_ais_annot is followed immediately by a label (e.g. a loop head), add a nop instruction to separate the annotation from the label.
• Wrong parsing of the command-line options -u <symbol> and -iquote.
• PowerPC in hybrid 32/64 bit mode: reject %Q and %R register specifications in inline assembly code, since 64-bit integer arguments are not split in two registers.
• x86 64-bit mode: wrong expansion of __builtin_clzl and builtin_ctzl (issue #127).

New checks for ISO C conformance:

• Removed support for _Alignof(expr), which is not C11; only _Alignof(ty) is part of C11.
• Reject occurrences of _Alignas in places that are not allowed by C11, e.g. in typedef. __attribute((aligned(N))) can be used instead.
• Reject occurrences of restrict in places that are not allowed by C99 and C11.
• Reject structs composed of a single flexible array struct { ty []; }.
• Check that qualified array types such as int t[const 4] occur only as function parameters, but nowhere else.
• In function definitions, reject function parameters that have no names.

New warnings:

• Warn for flexible array types ty[] in places where they do not make sense.
• Warn for inline (not static inline) functions that declare non-constant static variables.
• Optionally warn if the alignment of an object is reduced below its natural alignment because of a _Alignas qualifier or an aligned attribute, or a packed attribute.
• Warn for tentative static definitions with an incomplete type, e.g. static int x[];.
• The warning about uses of C11 features is now off by default.

Semantic preservation proof:

• Model the fact that external functions can destroy caller-save registers and Outgoing stack slots; adapt the proofs accordingly.

Coq and OCaml development:

• Support Coq versions 8.8.1 and 8.8.2.
• Support OCaml versions 4.7.0 and up.
• Support Menhir versions 20180530 and up.

Others:

• Improved error handling in "configure" script (issue #244)
• clightgen adds configuration information to the generated .v file (issue #226)

Release 3.3, 2018-05-30

New features:

• Introduced the __builtin_ais_annot built-in function to communicate source-level annotations to AbsInt's a3 tool suite via a special section in object and executable files.
• Improved C11 support: define the C11 conditional feature macros; define the max_align_t type in stddef.h.
• PowerPC 64-bit port: new built-in functions for 64-bit load-store with byte reversal and for 64-bit integer multiply high.
• x86 64 bits: add support for BSD.

Bug fixing:

• Wrong code generated for unions containing several bit fields.
• Internal compiler errors for some initializers for structs and unions containing bit-fields, and for anonymous members of unions.
• Missing error reporting for <integer> - <ptr> subtraction, causing an internal retyping error later during compilation.
• String literals are l-values.
• String literals have array types, not pointer types.
• Array sizes >= 2^32 were handled incorrectly on 64-bit platforms.
• Wrong code generated for global variables of size 2^31 bytes or more.
• struct and union arguments to annotation builtins must be passed by reference, regardless of the ABI calling conventions.
• e1, e2 has pointer type if e2 has array type.
• x86 64 bits: in "symbol + ofs" addressing modes, the offset "ofs" must be limited to [-2^24, 2^24) otherwise linking can fail.

New or improved diagnostics (errors and warnings):

• Warn for comparison of a pointer to a complete type and a pointer to an incomplete type.
• More checks on variables declared in for loops: not static, not extern, not function types.
• Reject empty declarations in K&R functions.
• Reject arrays of incomplete types.
• Reject duplicate case or default statements within a switch.
• Reject case and default statements outside a switch.
• Check that typedef declares a name and doesn't contain _Noreturn.
• Function parameters are in the same scope as function local variables.
• More comprehensive constant-ness checks for initializers of global or static local variables.
• Make sure an enum cannot have the same tag as a struct or an union.
• More checks on where the auto storage class can be used.
• Accept empty enum declaration after nonempty enum definition.
• Reject pointers to incomplete types in ptr - ptr subtraction.
• When defining a function, take attributes (_Noreturn, etc) from earlier declarations of the function into account.
• Better check for multiple definitions of functions or global variables.
• Reject illegal initializations of aggregates such as char c[4] = 42;.
• Reject designated initializers where a member of a composite type is re-initialized after the composite has been initialized as a whole.
• Reject casts to struct/union types.
• Reject sizeof(e) where e designates a bit-field member of a struct or union.
• e1, e2 is not a compile-time constant expression even if e1 and e2 are.
• main function must not be inline
• Warn for functions declared extern after having been defined.
• Warn for function declarations after function definitions when the declaration has more attributes than the definition.
• Warn for assignment of a volatile struct to a non-volatile struct.
• Warn for main function if declared _Noreturn.

Coq development:

• Added support for Coq versions 8.7.2 and 8.8.0.
• Rewrote Implicit Arguments and Require inside sections, these are obsolete in 8.8.0.
• Upgraded Flocq to version 2.6.1.
• Optionally install the .vo files for reuse by other projects (options -install-coqdev and -coqdevdir to configure script; automatically selected if option -clightgen is given).

Release 3.2, 2018-01-15

Code generation and optimization:

• Inline static functions that are called only once. Can be turned off by setting the noinline attribute on the function.
• More consistent detection and elimination of divisions by 1.
• ARM in Thumb mode: simpler instruction sequence for branch through jump table.
• ARM: support and use the cmn instruction.
• Issue #208: make value analysis of comparisons more conservative for dubious comparisons such as (uintptr_t) &global == 0x1234 which are undefined behavior in CompCert.

Usability:

• Resurrected support for the Cygwin x86-32 port, which got lost at release 3.0.
• Support the noinline attribute on C function definitions.
• PowerPC port with Diab toolchain: support -t <target processor> option and pass it to the Diab tools.
• Clightgen tool: add -o option to specify output file.
• Pull request #192: improve the printing of Clight intermediate code so that it looks more like valid C source. (Frédéric Besson)

Bug fixing:

• Issue #P25: make sure sizeof(long double) = sizeof(double) in all contexts.
• Issue #211: wrong scoping for C99 declarations within a for statement.

Coq and Caml development:

• Pull request #191: Support Coq version 8.7.0 and 8.7.1 in addition to Coq 8.6.1. Coq 8.6 (.0) is no longer supported owing to an incompatibility with 8.7.0. (Sigurd Schneider)
• ARM code generator: refactoring of constant expansions and EABI fixups.
• Resynchronized the list of dual-licensed files given in file LICENSE and the copyright headers of the dual-licensed files.

Release 3.1, 2017-08-18

Major improvements:

• New port targeting the RISC-V architecture, in 32- and 64-bit modes.

• Improved support for PowerPC 64 processors: use 64-bit registers and instructions for 64-bit integer arithmetic. Pointers remain 32 bits and the 32-bit ABI is still used.

Code generation and optimization:

• Optimize leaf functions in the PowerPC back-end. (Avoid reloading the return address from the stack.)
• Avoid generating useless conditional branches for empty if/else statements.
• Earlier elimination of redundant &*expr and *&expr addressings.
• Improve utilization of addressing modes for volatile loads and stores.

Usability:

• Add options -finline / -fno-inline to control function inlining.
• Removed the compilation of .cm files written in Cminor concrete syntax.
• More precise warnings about missing function returns.
• clightgen: add option -normalize to avoid memory loads deep inside expressions.

Bug fixing:

• Issue #179: clightgen produces wrong output for switch statements.
• Issue #196: excessive proof times in .v files produced by clightgen.
• Do not generate code for functions with inline specifier that are neither static nor extern, as per ISO C99.
• Some line number information was missing for some goto labels and switch cases.
• Issue #P16: illegal PowerPC asm generated for unsigned division after constant propagation.
• Issue #P18: ARM addressing overflows caused by 1- underestimation of code size, causing mismanagement of constant pool, and 2- large stack frames where return address and back link are at offsets >= 4Kb.
• Pass -no-pie flag to the x86 linker when -pie is the default.

Coq and Caml development:

• Support Coq 8.6.1.
• Improve compatibility with Coq working version.
• Always generate .merlin and _CoqProject files.

Release 3.0.1, 2017-02-14

• Ported to Coq 8.6.

Release 3.0, 2017-02-10

Major improvements:

• Added support for 64-bit target platforms, including pointers that are 64-bit wide, and the ability to use 64-bit integer registers and arithmetic operations. This support does not replace but comes in addition to CompCert's original support for 32-bit target platforms, with 32-bit pointers and emulation of 64-bit integer arithmetic using pairs of 32-bit integers. In terms of C data models, CompCert used to be restricted to the ILP32LL64 model; now it also supports I32LP64 and IL32LLP64.

• The x86 port of CompCert was extended to produce x86-64 bit code in addition to the original x86-32 bit (IA32) code. (This is the first instantiation of the new support for 64-bit targets described above.) Support for x86-64 is currently available for Linux and MacOS X. (Run the configure script with x86_64-linux or x86_64-macosx.) This is an early port: several ABI incompatibilities remain.

Language features:

• Support for anonymous structures and unions as members of structures or unions. (ISO C11, section 6.7.2.1, para 13 and 19.)
• New built-in functions for ARM and PowerPC: __builtin_ctz, __builtin_ctzl, __builtin_ctzll (count trailing zeros, 32 and 64 bits).

Usability:

• Added options -Wxxx and -Wno-xxx (for various values of xxx) to control which warnings are emitted.
• Added options -Werror=xxx and -Wno-error=xxx (for various values of xxx) to control which warnings are treated as errors.
• Support response files where additional command-line arguments can be passed (syntax: @file).
• Improved wording of warning and error messages.
• Improved handling of attributes, distinguishing attributes that apply to types from attributes that apply to names. For example, in __attribute((aligned(8),section("foo"))) int * p; the aligned attribute is attached to type int, while the section attribute is attached to name p.

Code generation:

• Support for ARM target processors in big-endian mode.
• Optimize 64-bit integer division by constants.

Bug fixing:

• Issue #155: on ARM, assembly errors caused by large jump tables for switch statements and overflow in accessing constant pools.
• Issue #151: large inductive definition causes a fatal error in 32-bit versions of Coq.
• Issue #143: handle %lf printf() format in the reference interpreter
• Issue #138: struct declarations in K&R function parameters were ignored.
• Issues #110, #111, #113, #114, #115, #119, #120, #121, #122, #123, #124, #125, #126, #127, #128, #129, #130, #133, #138, #144: various cases of internal errors and failed assertions that should have been proper errors instead.
• For __builtin_memcpy_aligned, size and alignment arguments of 64-bit integer type were causing a fatal error on a 32-bit target.
• ARM and x86 ports: wrong register allocation for some calls to function pointers.

Release 2.7.1, 2016-07-18

• Ported to Coq 8.5pl2.

Bug fixing:

• Fixed a compile-time assertion failure involving builtins taking a 64-bit integer parameter and given an unsigned 32-bit integer argument.
• Updates to the Cminor parser.

Release 2.7, 2016-06-29

Major improvement:

• The proof of semantic preservation now accounts for separate compilation and linking, following the approach of Kang, Kim, Hur, Dreyer and Vafeiadis, "Lightweight verification of separate compilation", POPL 2016. Namely, the proof considers a set of C compilation units, separately compiled to assembly then linked, and shows that the resulting assembly program preserves the semantics of the C program that would be obtained by syntactic linking of the source C compilation units.

Language features:

• Parse the _Noreturn function attribute from ISO C11.
• New standard includes files: <iso646.h> and <stdnoreturn.h> from ISO C11.
• New built-in functions: __builtin_clzl, __builtin_clzll (count leading zeros, 32 and 64 bits) for ARM, IA32 and PowerPC; __builtin_ctz, __builtin_ctzl, __builtin_ctzll (count trailing zeros, 32 and 64 bits) for IA32.

Formal C semantics:

• The semantics of conversions from pointer types to _Bool is fully defined (again).

Usability:

• The generation of DWARF debugging information in -g mode is now supported for ARM and IA32 (in addition to PowerPC).

Coq development:

• Revised the Stacking pass and its proof to make it more extensible later to e.g. 64-bit integer registers.
• Use register pairs in function calling conventions to control more precisely the splitting of 64-bit integer arguments and results into pairs of 32-bit quantities
• Revised the way register conventions are described in Machregs and Conventions.
• Simulation diagrams now live in Prop instead of Type.

OCaml development:

• Code cleanup to remove warnings, support "safe strings" mode, and be fully compatible with OCaml 4.02 and 4.03.
• Cminor parser: support for single-precision FP numbers and operators.

Bug fixing:

• Some declarations within C expressions were incorrectly ignored (e.g. sizeof(enum e {A})).
• ARM in Thumb mode: incorrect movs instructions involving the stack pointer register were generated.

Release 2.6, 2015-12-21

Usability:

• Generation of full DWARF v2 debugging information in -g mode, including function-local variables. This is fully supported for the PowerPC target with GNU tools or Diab tools. Support for IA32 and ARM is nearly there.
• Production of detailed explanations for syntax errors during parsing. (Exploiting recent work by F. Pottier on the Menhir parser generator.)
• PowerPC port: added many new builtin functions.

Code generation and optimization:

• Support for PowerPC 64-bits (pointers are still 32-bit wide) and Freescale's E5500 variant.
• More prudent alias analysis for operations over pointers that are formally undefined, such as bit masking.
• New pass: Debugvar, to associate debug information to local variables.

Coq development:

• Richer representation of arguments and results to builtin operations.
• As a consequence, annotation builtins no longer need special handling.
• Added EF_debug builtins to transport debugging information throughout the compiler back-end.
• Upgraded the Flocq library to version 2.5.0.

Bug fixing:

• Issue #71: incorrect initialization of an array of wchar_t
• Corrected the handling of bit-fields of type _Bool and width > 1
• Removed copy optimization when returning a struct from a function.
• Full parsing of unprototyped (K&R-style) function definitions. (Before, the parsing was incomplete and would reject some definitions.)

Miscellaneous:

• The cchecklink tool (for a posteriori validation of assembly and linking) was removed. It is replaced by the Valex tool, available from AbsInt.
• Added a command-line option -conf <config file> to select a different compcert.ini configuration file.
• Removed the command-line options -fstruct-passing=<convention> and -fstruct-return=<convention>, more confusing than useful.
• Added a command-line option -fstruct-passing that activates ABI-conformant by-value passing of structs and unions as function arguments or results. If this option is not set, passing a struct/union as function argument is now rejected.
• The -fstruct-return command-line option is deprecated and becomes a synonymous for -fstruct-passing.
• The return type of __builtin_clz is int, as documented, and not unsigned int, as previously implemented.

Release 2.5, 2015-06-12

Language features:

• Extended inline assembly in the style of GCC. (See section 6.5 of the user's manual.) The implementation is not as complete as that of GCC or Clang. In particular, the only constraints supported over operands are r (register), m (memory), and i (integer immediate).

Code generation and optimization:

• Revised translation of || and && to Clight so as to produce well-typed Clight code.
• More prudent value analysis of uninitialized declarations of const global variables.
• Revised handling of "common" global declarations, fixes an issue with uninitialized declarations of const global variables.

Improvements in confidence:

• Formalized the typing rules for CompCert C in Coq and verified a type-checker, which is used to produce the type annotations in CompCert C ASTs, rather than trusting the types produced by the Elab pass.
• Coq proof of correctness for the Unusedglob pass (elimination of unreferenced static global definitions). The Coq AST for compilation units now records which globals are static.
• More careful semantics of comparisons between a non-null pointer and the null pointer. The comparison is undefined if the non-null pointer is out of bounds.

Usability:

• Generation of DWARF v2 debugging information in -g mode. The information describes C types, global variables, functions, but not yet function-local variables. This is currently available only for the PowerPC/Diab target.
• Added command-line options to turn individual optimizations on or off, and a -O0 option to turn them all off.
• Revised handling of arguments to __builtin_annot so that no code is generated for an argument that is a global variable or a local variable whose address is taken.
• In string and character literals, treat illegal escape sequences (e.g. \% or \0) as an error instead of a warning.
• Warn if floating-point literals overflow or underflow when converted to FP numbers.
• In -g -S mode, annotate the generated .s file with comments containing the C source code.
• Recognize and accept more of GCC's alternate keywords, e.g. __signed, __volatile__, etc.
• cchecklink: added option -files-from to read .sdump file names from a file or from standard input.

ABI conformance:

• Improved ABI conformance for passing values of struct or union types as function arguments or results. Full conformance is achieved on IA32/ELF, IA32/MacOSX, PowerPC/EABI, PowerPC/Linux, and ARM/EABI.
• Support the va_arg macro from <stdarg.h> in the case of arguments of struct or union types.

Coq development:

• In the CompCert C and Clight ASTs, struct and union types are now represented by name instead of by structure. A separate environment maps these names to struct/union definitions. This avoids bad algorithmic complexity of operations over structural types.
• Introduce symbol environments (type Senv.t) as a restricted view on global environments (type Genv.t).
• Upgraded the Flocq library to version 2.4.0.

Bug fixing:

• Issue #4: exponential behaviors with deeply-nested struct types.
• Issue #6: mismatch on the definition of wchar_t
• Issue #10: definition of composite type missing from the environment.
• Issue #13: improved handling of wide string literals
• Issue #15: variable-argument functions are not eligible for inlining.
• Issue #19: support empty switch statements
• Issue #20: ABI incompatibility wrt struct passing on IA32.
• Issue #28: missing type decay in __builtin_memcpy_aligned applied to arrays.
• Issue #42: emit error if static definition follows non-static declaration.
• Issue #44: OSX assembler does not recognize .global directive.
• Protect against redefinition of the __i64_xxx helper library functions.
• Revised handling of nonstandard attributes in C type compatibility check.
• Emit an error on "preprocessing numbers" that are invalid numerical literals.
• Added missing check for static redefinition following a non-static declaration.
• Added missing check for redefinition of a typedef as an ordinary identifier within the same scope.

Miscellaneous:

• When preprocessing with gcc or clang, use -std=c99 mode to force C99 conformance.
• Use a Makefile instead of ocamlbuild to compile the OCaml code.

Release 2.4, 2014-09-17

Language features:

• Support C99 compound literals (ISO C99 section 6.5.2.5).
• Support switch statements over an argument of type long long.

Code generation and optimization:

• Revised and improved support for single-precision floating-point arithmetic. Earlier, all FP arithmetic was performed at double precision, with conversions to/from single precision as needed, in particular when loading/storing a single-precision FP number from/to memory. Now, FP operations whose arguments are of type float are performed in single-precision, using the processor's single FP instructions. Fewer conversions between double and single precision are generated.
• Value analysis and constant propagation: more precise treatment of comparisons against an integer constant.

Improvements in confidence:

• Full correctness proofs for the algorithms used in the runtime support library for conversions between 64-bit integers and floating-point numbers.

ARM port:

• Added support for Thumb2 instruction encoding (option -mthumb). Thumb2 is supported on ARMv7 and up, and produces more compact machine code.
• Exploit some VFPv3 instructions when available.
• Built-in function __builtin_cntlz (count leading zeros) renamed __builtin_clz for GCC / Clang compatibility.

PowerPC port:

• Refactored the expansion of built-in functions and pseudo-instructions so that it does not need to be re-done in cchecklink.
• Updated the cchecklink validator accordingly.
• More efficient code generated for volatile accesses to small data areas.
• Built-in function __builtin_cntlz (count leading zeros) renamed __builtin_clz for GCC / Clang compatibility.

IA32 port:

• Added built-in functions __builtin_clz and __builtin_ctz (count leading / trailing zeros).

Coq development:

• The memory model was extended with two new "chunks", Many32 and Many64, that enable storing any 32-bit value or 64-bit value using an abstract, not bit-based encoding, and reloading these values exactly. These new chunks are used to implement saving and restoring callee-save registers that can contain data of unknown types (e.g. float32 or float64) but known sizes.
• Refactored the library of FP arithmetic (lib/Floats.v) to support both 64- and 32-bit floats.

Release 2.3pl2, 2014-05-15

Usability:

• Re-added support for __func__ identifier as per ISO C99.
• Re-added some popular GCC extensions to ISO C99:
  • alternate keywords __restrict, __inline__, etc,
  • support for empty structs and unions
  • support \e escape in char and string literals, meaning ESC
• Do not assume that the preprocessor removed all comments.

Bug fixing:

• Fixed regression on initializers of the form T x[N] = "literal"; where T is a typedef for a character type.
• asm statements were causing syntax errors.
• Better handling of extern and extern inline function definitions.
• Internal error on some octal escape sequences in string literals.
• Parsing of #pragma section directives made more robust and with better error reporting.

Release 2.3, 2014-05-05

Language features:

• Support for C99 designated initializers. (ISO C99 section 6.7.8.)

Improvements in confidence:

• The parser is now formally verified against the ISO C99 grammar plus CompCert's extensions. The verification proves that the parser recognizes exactly the language specified by the grammar, and that the grammar has no ambiguities. For more details, see the paper "Validating LR(1) parsers" by Jacques-Henri Jourdan, François Pottier, and Xavier Leroy, ESOP 2012, http://dx.doi.org/10.1007/978-3-642-28869-2_20
• More theorems proved about float<->integer conversions.

Optimizations:

• Optimize x != 0, x == 0, x != 1, and x == 1 when x is known to be a boolean already, ranging over {0, 1, undef}.
• More systematic constant propagation in pass Selection, lightens the work of later RTL optimisations.
• IA32: recognize and use the not instruction.

Usability:

• Option -timings to print compilation times for various passes.
• Various tweaks in IRC graph coloring to reduce compilation time.
• IA32: add built-in functions for fused multiply-add (require a recent processor with FMA3 extensions).

Improvements in ABI conformance:

• New target platform: ARM with EABI "hard float" calling conventions (armhf in Debian's classification).
• IA32 and ARM: revised handling of "common" variables to conform with ABI.

Bug fixing:

• In -fbitfields emulation: a->f was not properly rewritten if a had "array of structs" type instead of "pointer to struct".
• Moved analysis of single-precision floats from RTLtyping to Machtyping. (RTLtyping was incorrectly rejecting some functions involving single-precision floats.) Simplified LTL semantics and Allocation pass accordingly.
• Assignment to a l-value of volatile float type could cause an internal error in RTLtyping/Machtyping.
• The case __builtin_fabs applied to integers was missing in the C semantics and in C#minor generation.
• Fixed some type annotations on CompCert C expressions. These annotations were incorrect but not in a way that impacted code generation.

Release 2.2, 2014-02-24

Major improvements:

• Two new static analyses are performed on the RTL intermediate form: . Value analysis, tracking constants, some integer range information, and pointer aliasing information. . Neededness analysis, generalizing liveness analysis to individual bits of integer values and to stack memory locations.

• Improved RTL optimizations, exploiting the results of these analyses: . Constant propagation can track constants across memory stores and loads. . Common subexpression elimination exploits nonaliasing information. . Dead code elimination can eliminate useless memory writes and block copies, as well as integer operations that do not change the needed bits. . Redundant cast elimination is now performed globally (at function level) rather than locally on individual expressions.

• Experimental support for defining and calling variable-argument functions, including support for the <stdarg.h> interface. (Option -fvararg-calls, "on" by default.)

Language features:

• In switch statements, default cases can now appear anywhere, not just as the last case.
• Support for incomplete array as last field of a struct, as specified in ISO C 99.
• Support for declarations within for loops, as specified in ISO C 99. (E.g. for (int i = 0; i < 4; i++) ...)
• Revised semantics and implementation of _Alignas(N) attribute to better match those of GCC and Clang.
• Better tolerance for functions declared without prototypes (option -funprototyped, "on" by default).
• On PowerPC, support "far-data" sections (register-relative addressing with 32-bit offsets).

Improvements in ABI conformance:

• For x86/IA32, align struct fields of types double or long long to 4 instead of 8, as prescribed by the x86 ELF ABI.
• For PowerPC and ARM, structs and unions returned as function results are now passed in integer registers if their sizes are small enough (<= 8 bytes for PowerPC, <= 4 bytes for ARM).

Usability:

• Revised parsing of command-line arguments to be closer to GCC and Clang. In particular, ccomp -c foo.c -o obj/foo.o now works as expected, instead of ignoring the -o option as in earlier CompCert versions.
• Recognize input files ending in .i and .p as C source files that must not be preprocessed.
• Warn for uses of the following GCC extensions to ISO C: zero-sized arrays, empty structs/unions, empty initializer braces.
• Option -fno-fpu to prevent the use of FP registers for some integer operations such as block copies. (Replaces the previous -fno-sse option which was x86/IA32-specific, and extends it to PowerPC and ARM.)
• Option -drtl to record the RTL intermediate representation at every stage of optimization. (Replaces -dtailcall, -dinlining, -dconstprop, and -dcse.)
• Add CompCert version number and command-line arguments as comments in the generated assembly files.

Other performance improvements:

• Recognize __builtin_fabs as a primitive unary operator instead of a built-in function, enabling more optimizations.
• PowerPC: shorter code generated for &global_variable + expr.

Improvements in compilation times:

• More efficient implementation of Kildall's dataflow equation solver, reduces size of worklist and nomber of times a node is visited.
• Better OCaml GC settings significantly reduce compilation times for very large source functions.

Bug fixing:

• Fixed incorrect hypothesis on __builtin_write{16,32}_reversed.
• Fixed syntax error in attribute((packed)).
• Emit clean compile-time error for switch over a value of 64-bit integer type (currently not supported).
• Recognize source files with .i or .p extension as C sources that should not be preprocessed.

Coq development:

• Removed propositional extensionality axiom (prop_ext).
• Suppressed the Mfloat64al32 memory_chunk, no longer needed.

Release 2.1, 2013-10-28

Language semantics:

• More precise modeling of not-a-numbers (NaNs) in floating-point arithmetic.
• The CompCert C language is now defined with reference to ISO C99 instead of ISO C90 ("ANSI C") as before. This affects mostly the wording of the reference manual. However, the parsing of integer constants and character constants was revised to follow the ISO C99 standard.

Language features:

• Support for _Alignas(N) attribute from ISO C 2011.
• Revised implementation of packed structs, taking advantage of _Alignas.
• Suppressed the pragma packed, replaced by a struct-level attribute __packed__(params) or __attribute__(packed(params)).
• Fixed typing rules for __builtin_annot() to avoid casting arguments of small integer or FP types.

Performance improvements:

• Optimize integer divisions by positive constants, turning them into multiply-high and shifts.
• Optimize floating-point divisions by powers of 2, turning them into multiplications.
• Optimize x * 2.0 and 2.0 * x into x + x.
• PowerPC: more efficient implementation of division on 64-bit integers.

Bug fixing:

• Fixed compile-time error when assigning a long long RHS to a bitfield.
• Avoid double rounding issues in conversion from 64-bit integers to single-precision floats.

Miscellaneous:

• Minor simplifications in the generic solvers for dataflow analysis.
• Small improvements in compilation times for the register allocation pass.
• MacOS X port updated to the latest XCode (version 5.0).

Release 2.0, 2013-06-21

Major improvements:

• Support for C types long long and unsigned long long, that is, 64-bit integers. Regarding arithmetic operations on 64-bit integers, . simple operations are expanded in-line and proved correct; . more complex operations (division, modulus, conversions to/from floats) call into library functions written in assembly, heavily tested but not yet proved correct.

• The register allocator was completely rewritten to use an "end-to-end" translation validation approach, using a validation algorithm described in the paper "Validating register allocation and spilling" by Silvain Rideau and Xavier Leroy, Compiler Construction 2010, http://dx.doi.org/10.1007/978-3-642-11970-5_13 This validation-based approach enables better register allocation, esp: . live-range splitting is implemented . two-address operations are treated more efficiently . no need to reserve processor registers for spilling and reloading. The improvements in quality of generated code is significant for IA32 (because of its paucity of registers) but less so for ARM and PowerPC.

• Preliminary support for debugging information. The -g flag causes DWARF debugging information to be generated for line numbers and stack structure (Call Frame Information). With a debugger like GDB, this makes it possible to step through the code, put breakpoints by line number, and print stack backtraces. However, no information is generated yet for C type definitions nor for variables; therefore, it is not possible to print the values of variables.

Improvements in ABI conformance:

• For IA32 and ARM, function arguments of type float (single-precision FP) were incorrectly passed as double.
• For PowerPC, fixed alignment of double and long long arguments passed on stack.

Improvements in code generation:

• More aggressive common subexpression elimination across some builtin function calls, esp. annotations.

Improvements in compiler usability:

• Option -fno-taillcalls to turn off tail-call elimination. (Some static analysis tools are confused by this optimization.)
• Reduced stack usage of the compiler by rewriting some key functions in tail-recursive style.
• Reduced memory requirements of constant propagation pass by forgetting compile-time approximations of dead variables.
• More careful elaboration of C struct and union types into CompCert C types, avoiding behaviors exponential in the nesting of structs.

Bug fixing:

• Fixed parsing of labeled statements inside switch constructs, which were causing syntax errors.
• The -> operator applied to an array type was causing a type error.
• Nested conditional expressions a ? (b ? c : d) : e were causing a compile-time error if c, d and e had different types.

Coq development:

• Adapted the memory model to the needs of the VST project at Princeton: . Memory block identifiers are now of type positive instead of Z . Strengthened invariants in the definition of memory injections and the specification of external calls.
• The LTL intermediate language is now a CFG of basic blocks.
• Suppressed the LTLin intermediate language, no longer used.

Release 1.13, 2013-03-12

Language semantics:

• Comparisons involving pointers "one past" the end of a block are now defined. (They used to be undefined behavior.) (Contributed by Robbert Krebbers).

Language features:

• Arguments to __builtin_annot() that are compile-time constants are now replaced by their (integer or float) value in the annotation generated in the assembly file.

Improvements in performance:

• ARM and PowerPC ports: more efficient access to function parameters that are passed on the call stack.
• ARM port; slightly better code generated for some indirect memory accesses.

Bug fixing:

• Fixed a bug in the reference interpreter in -all mode causing some reductions to be incorrectly merged.
• Wrong parsing of hexadecimal floating-point literals 0xMMMMpEEE.

Improvements in usability:

• Better error and warning messages for declarations of variables of size >= 2^32 bits.
• Reference interpreter: more efficient exploration of states in -all mode.

Coq development:

• More efficient implementation of machine integers (module Integers) taking advantage of bitwise operations defined in ZArith in Coq 8.4.
• Revised handling of return addresses in the Mach language and the Stacking and Asmgen passes.
• A number of definitions that were opaque for no good reason are now properly transparent.

Release 1.12.1, 2013-01-29

Ported to Coq 8.4pl1. Otherwise functionally identical to release 1.12.

Release 1.12, 2013-01-11

Improvements in confidence:

• Floating-point literals are now parsed and converted to IEEE-754 binary FP numbers using a provably-correct conversion function implemented on top of the Flocq library.

Language semantics:

• Comparison between function pointers is now correctly defined in the semantics of CompCert C (it was previously undefined behavior, by mistake).
• Bit-fields of enum type are now treated as either unsigned or signed, whichever is able to represent all values of the enum. (Previously: always signed.)
• The && and || operators are now primitive in CompCert C and are given explicit semantic rules, instead of being expressed in terms of _ ? _ : _ as in previous CompCert releases.
• Added a Ebuiltin expression form (invocation of built-in function) to CompCert C, and a Sbuiltin statement form to Clight. Used it to simplify the encoding of annotations, memcpy, and volatile memory accesses.

Performance improvements:

• Better code generated for && and || operators.
• More aggressive elimination of conditional branches during constant propagation, taking better advantage of inferred constants.

Language features:

• By popular demand, asm statements for inline assembly are now supported if the flag -finline-asm is set. Use with extreme caution, as the semantic preservation proof assumes these statements have no effect on the processor state.

Internal simplifications and reorganization:

• Clight, Csharpminor, Cminor: suppressed the Econdition conditional expressions, no longer useful.
• Clight: a single loop form, the three C loops are derived forms.
• Clight: volatile memory accesses are materialized as builtin operations.
• Clight: removed dependencies on CompCert C syntax and semantics.
• New pass SimplLocals over Clight that replaces local scalar variables whose address is never taken by temporary, nonadressable variables. (This used to be done in Cminorgen.)
• Csharpminor: simplified semantics.
• Cminor: suppressed the Oboolval and Onotbool operators, which can be expressed in terms of Ocmpu at no performance costs.
• All languages: programs are now presented as a list of global definitions (of functions or variables) instead of two lists, one for functions and the other for variables.

Other changes:

• For compatibility with other C compilers, output files are now generated in the current directory, by default; output file name can be controlled with the -o option, somewhat like with GCC.
• Reference interpreter: better handling of volatile memory accesses.
• IA32/MacOS X: now supports referencing global variables defined in shared libraries; old hack for stdio is no longer needed.
• PowerPC/MacOS X: this port was removed, as recent version of MacOS X no longer support PowerPC.

Release 1.11, 2012-07-13

Improvements in confidence:

• Floating-point numbers and arithmetic operations, previously axiomatized, are now implemented and proved correct in Coq, using the Flocq library of S. Boldo and G. Melquiond.

Language semantics:

• In accordance with ISO C standards, the signed division min_int / -1 and the signed remainder min_int % -1 (where min_int is the smallest representable signed integer) now have undefined semantics and are treated as "going wrong" behaviors. (Previously, they were defined with results min_int and 0 respectively, but this behavior requires unnatural code to be generated on IA32 and PowerPC.)

Performance improvements:

• Function inlining is now implemented. The functions that are inlined are those declared inline in the C source, provided they are not recursive.
• Constant propagation is now able to propagate the initial values of const global variables.
• Added option -ffloat-const-prop to control the propagation of floating-point constants; see user's manual for documentation.
• Common subexpression elimination can now eliminate memory loads following a memory store at the same location.
• ARM: make use of the fcmpzd and fmdrr instructions.

New tool:

• The "cchecklink" tool performs a posteriori validation of the assembling and linking phases. It is available for PowerPC-EABI only. It takes as inputs an ELF-PowerPC executable as produced by the linker, as well as .sdump files (abstract assembly) as produced by ccomp -sdump, and checks that the executable contains properly-assembled and linked code and data corresponding to those produced by CompCert.

Other changes:

• Elimination of static functions and static global variables that are not referenced in the generated code.
• The memory model was enriched with "max" permissions in addition to "current" permissions, to better reason over "const" blocks and already-deallocated blocks.
• More efficient implementation of the memory model, resulting in faster interpretation of source files by ccomp -interp.
• Added option -falign-functions to control alignment of function code.

Release 1.10, 2012-03-13

Improvements in confidence:

• CompCert C now natively supports volatile types. Its semantics fully specifies the meaning of volatile memory accesses. The translation of volatile accesses to built-in function invocations is now proved correct.
• CompCert C now natively supports assignment between composite types (structs or unions), passing composite types by value as function parameters, and other instances of using composites as r-values, with the exception of returning composites by value from a function. (The latter remains emulated, using the -fstruct-return option.)
• PowerPC: removed the -fmadd option, not semantically-preserving in the strict sense.

Language features:

• Support for _Bool type from ISO C99.
• Support for _Alignof(ty) operator from ISO C 2011 and __alignof__(ty), __alignof__(expr) from GCC.

Performance improvements:

• Improvements in instruction selection, especially for integer casts and their combinations with bitwise operations.
• Shorter, more efficient code generated for accessing volatile global variables.
• Better code generated for the && and || operators.
• More aggressive common subexpression elimination (CSE) of memory loads.
• Improved register allocation for invocations of built-ins, especially for annotations.
• In Cminor and down, make safe operators non-strict: they return Vundef instead of getting stuck. This enables more optimizations.
• Cast optimization is no longer performed by a separate pass over RTL, but equivalent optimization is done during Cminor generation and during instruction selection.

Other improvements:

• PowerPC/EABI: uninitialized global variables now go in common (bss) section.
• PowerPC: work around limited excursion of conditional branch instructions.
• PowerPC: added __builtin_fnmadd() and __builtin_fnmsub().
• Reference interpreter: better printing of pointer values and locations.
• Added command-line options -Wp, -Wa, -Wl, to pass specific options to the preprocessor, assembler, or linker, respectively.
• More complete Cminor parser and printer (contributed by Andrew Tolmach).

Release 1.9.1, 2011-11-28

Bug fixes:

• Initialization of a char array by a short string literal was wrongly rejected
• Incorrect handling of volatile arrays.
• IA32 code generator: make sure that min_int / -1 does not cause a machine trap.

Improvements:

• Added language option -flongdouble to treat long double like double.
• The reference interpreter (ccomp -interp) now supports 2-argument main functions (int main(int, char **)).
• Improved but still very experimental emulation of packed structs (-fpacked-structs)
• Coq->Caml extraction: extract Coq pairs to Caml pairs and Coq characters to Caml char type.

Release 1.9, 2011-08-22

• The reduction semantics of CompCert C was made executable and turned into a reference interpreter for CompCert C, enabling animation of the semantics. (Thanks to Brian Campbell for suggesting this approach.) Usage is: ccomp -interp [options] source.c Options include: -trace to print a detailed trace of reduction steps -random to randomize execution order -all to explore all possible execution orders in parallel

• Revised and strengthened the top-level statements of semantic preservation. In particular, we now show:

  • backward simulation for the whole compiler without assuming a deterministic external world;
  • if the source program goes wrong after performing some I/O, the compiled code performs at least these I/O before continuing with an arbitrary behavior.

• Fixed two omissions in the semantics of CompCert C (reported by Brian Campbell):

  • Functions calls through a function pointer had undefined semantics.
  • Conditional expressions e1 ? e2 : e3 where e2 and e3 have different types were missing a cast to their common type.

• Support for "read-modify-write" operations over volatiles (such as e++ or --e or e |= 1 where e has volatile type) through a new presimplification (flag -fvolatile-rmw, "on" by default).

• New optimization pass: Redundant Reload Elimination, which fixes up inefficiencies introduced during the Reload pass. On x86, it increases performance by up to 10%. On PowerPC and ARM, the effect is negligible.

• Revised handling of annotation statements. Now they come in two forms:

  1. __builtin_annot("format", x1, ..., xN) (arbitrarily many arguments; no code generated, even if some of the xi's were spilled; no return value)
  2. __builtin_annot_intval("format", x1) (one integer argument, reloaded in a register if needed, returned as result).

• Related clean-ups in the handling of external functions and compiler built-ins. In particular, __builtin_memcpy is now fully specified.

• ARM code generator was ported to the new ABI (EABI in ARM parlance, armel in Debian parlance), using VFD instructions for floating-point. (Successfully tested on a Trimslice platform running Ubuntu 11.04.)

• IA32 code generator: . Added -fno-sse option to prevent generation of SSE instructions for memory copy operations. . More realistic modeling of the ST0 (top-of-FP-stack) register and of floating-point compare and branch.

• PowerPC code generator: more efficient instruction sequences generated for insertion in a bit field and for some comparisons against 0.

Release 1.8.2, 2011-05-24

• Support for aligned and section attributes on global variables, e.g. __attribute__((aligned(16))) int x;

• Experimental emulation of packed structs (flag -fpacked-structs).

• Pointer comparisons now treated as unsigned comparisons (previously: signed). This fixes an issue with arrays straddling the 0x8000_0000 boundary. Consequently, the ofs part of pointer values Vptr b ofs is now treated as unsigned (previously: signed).

• Elimination of unreferenced labels now performed by a separate pass (backend/CleanupLabels.v) and proved correct.

• Stacking pass revised: supports more flexible layout of the stack frame; two-step proof (Stackingproof + Machabstr2concr) merged into one single proof (Stackingproof).

• The requirement that pointers be valid in pointer comparisons was pushed through all intermediate languages of the back-end (previously: requirement present only up to Csharpminor).

• Emulation of assignment between structs and between unions was simplified and made more efficient, thanks to a better implementation of __builtin_memcpy.

• Improvements to the compiler driver: . -E option now prints preprocessed result to standard output instead of saving it in a .i file . support for .s (assembly) and .S (assembly to be preprocessed) input files

Release 1.8.1, 2011-03-14

• Adapted to Coq 8.3pl1.

• Reduced compilation times through several algorithmic improvements (contributed by A. Pilkiewicz).

• In the various semantics, allow float-to-int conversions to fail (if the float argument is outside the range of representable ints).

• Initialization of global C variables made more robust and proved correct.

• ABI conformance improved:

  • the char type is now signed for x86, remains unsigned for PowerPC and ARM
  • placement of bit-fields now follows SVR4 conventions (affects PowerPC)

• Bug fixes in the C pre-simplifier:

  • nontermination with some recursive struct types
  • issues with zero-width bit fields
  • elimination of struct assignments duplicating some volatile accesses

Release 1.8, 2010-09-21

• The input language to the proved part of the compiler is no longer Clight but CompCert C: a larger subset of the C language supporting in particular side-effects within expressions. The transformations that pull side effects out of expressions and materialize implicit casts, formerly performed by untrusted Caml code, are now fully proved in Coq.

• New port targeting Intel/AMD x86 processors. Generates 32-bit x86 code using SSE2 extensions for floating-point arithmetic. Works under Linux, MacOS X, and the Cygwin environment for Windows. CompCert's compilation strategy is not a very good match for the x86 architecture, therefore the performance of the generated code is not as good as for the PowerPC port, but still usable. (About 75% of the performance of gcc -O1 for x86, compared with more than 90% for PowerPC.)

• More faithful semantics for volatile accesses: . volatile reads and writes from a volatile global variable are treated like input and output system calls (respectively), bypassing the memory model entirely; . volatile reads and writes from other locations are treated like regular loads and stores.

• Introduced __builtin_memcpy() and __builtin_memcpy_words(), use them instead of memcpy() to compile struct and union assignments.

• Introduced __builtin_annotation() to transmit assertions from the source program all the way to the generated assembly code.

• Elimination of some useless casts around &, | and ^ bitwise operators.

• Produce fewer "moves" during RTL generation. This speeds up the rest of compilation and slightly improves the result of register allocation when register pressure is high.

• Improvements in register allocation: . Implemented a spilling heuristic during register allocation. This heuristic reduces significantly the amount of spill code generated when register pressure is high. . More coalescing between low-pressure and high-pressure variables. . Aggressive coalescing between pairs of spilled variables.

• Fixed some bugs in the emulation of bit fields.

Release 1.7.1, 2010-04-13

Bug fixes in the new C pre-simplifier:

• Missing cast on return value for some functions
• Incorrect simplification of some uses of || and &&
• Nontermination in the presence of a bit field of size exactly 32 bits.
• Global initializers for structs containing bit fields.
• Wrong type in volatile reads from variables of type unsigned int.

Small improvements to the PowerPC port:

• Added __builtin_trap() built-in function.
• Support for #pragma reserve_register (EABI)
• Less aggressive alignment of global variables.
• Generate .type and .size directives (EABI).

Release 1.7, 2010-03-31

• New implementation of the C type-checker, simplifier, and translation to Clight. Compared with the previous CIL-based solution, the new implementation is more modular and supports more optional simplifications.

• More features of the C language are handled by expansion during translation to Clight: . assignment between structs and unions (option -fstruct-assign) . passing structs and union by value (option -fstruct-passing) . bit-fields in structs (option -fbitfields)

• The volatile modifier is now honored. Volatile accesses are represented in Clight by calls to built-in functions, which are preserved throughout the compilation chain, then turned into processor loads and stores at the end.

• Generic support for C built-in functions. These predefined external functions give access to special instructions of the processor. See powerpc/CBuiltins.ml for the list of PowerPC built-in functions.

• The memory model now exposes the bit-level in-memory representation of integers and floats. This strengthens the semantic preservation theorem: we now prove that C code that directly manipulates these bit-level representations (e.g. via a union between floats and integers) is correctly compiled.

• The memory model now supports fine-grained access control to individual bytes of a memory block. This feature is currently unused in the compiler proofs, but will facilitate connections with separation logics later.

• External functions are now allowed to read and modify memory. The semantic preservation proofs were strengthened accordingly. In particular, this enables the malloc() and free() C library functions to be modeled as external functions in a provably correct manner.

• Minor improvements in the handling of global environments and the construction of the initial memory state.

• Bug fixes in the handling of #pragma section and #pragma set_section.

• The C test suite was enriched and restructured.

Release 1.6, 2010-01-13

• Support Clight initializers of the form int * x = &y;.

• Fixed spurious compile-time error on Clight initializers of the form const enum E x[2] = { E_1, E_2 };.

• Produce informative error message if a return without argument occurs in a non-void function, or if a return with an argument occurs in a void function.

• Preliminary support for #pragma section and #pragma set_section.

• Preliminary support for small data areas in PowerPC code generator.

• Back-end: added support for jump tables; used them to compile dense switch statements.

• PowerPC code generator: force conversion to single precision before doing a "store single float" instruction.

Release 1.5, 2009-08-28

• Support for goto in the source language Clight.

• Added small-step semantics for Clight.

• Traces for diverging executions are now uniquely defined; tightened semantic preservation results accordingly.

• Emulated assignments between structures (during the C to Clight initial translation).

• Fixed spurious compile-time error on Clight statements of the form x = f(...); where x is a global variable.

• Fixed spurious compile-time error on Clight initializers where the initial value is the result of a floating-point computation (e.g. double x = 3.14159 / 2;).

• Simplified the interface of the generic dataflow solver.

• Reduced running time and memory requirements for the constant propagation pass.

• Improved the implementation of George and Appel's graph coloring heuristic: runs faster, produces better results.

• Revised the implementation of branch tunneling.

• Improved modularization between processor-dependent and processor-independent parts.

Release 1.4.1, 2009-06-05

• Adapted to Coq 8.2-1. No changes in functionality.

Release 1.4, 2009-04-20

• Modularized the processor dependencies in the back-end.

• Three target architectures are now supported: PowerPC / MacOS X (most mature) PowerPC / EABI & Linux (getting stable) ARM / Linux EABI (still experimental)

• Added alignment constraints to the memory model.

• Clight: added support for conditional expressions (a ? b : c); removed support for array accesses a[i], now a derived form.

• C front-end: honor static modifiers on globals.

• New optimization over RTL: turning calls into tail calls when possible.

• Instruction selection pass: elimination of redundant casts following a memory load of a small memory quantity.

• Linearization pass: improved the linearization heuristic.

• Reloading pass: more economical use of temporaries.

• Back-end: removed "alloc heap" instruction; removed pointer validity checks in pointer comparisons.

Release 1.3, 2008-08-11

• Added goto and labeled statements to Cminor. Extended RTLgen and its proof accordingly.

• Introduced small-step transition semantics for Cminor; used it in proof of RTLgen pass; proved consistency of Cminor big-step semantics w.r.t. transition semantics.

• Revised division of labor between the Allocation pass and the Reload pass. The semantics of LTL and LTLin no longer need to anticipate the passing of arguments through the conventional locations.

• Cleaned up Stacking pass: the positions of the back link and of the return address in the stack frame are no longer hard-wired in the Mach semantics.

• Added operator to convert from float to unsigned int; used it in C front-end

• Added flag -fmadd to control recognition of fused multiply-add and -sub

• Semantics of pointer-pointer comparison in Clight was incomplete: pointers within different blocks can now be compared using == or !=

• Addition integer + pointer is now supported in Clight.

• Improved instruction selection for complex conditions involving || and &&.

• Improved translation of Cminor switch statements to RTL decision trees.

• Fixed error in C parser and simplifier related to for loops with complex expressions as condition.

• More benchmark programs in test/

Release 1.2, 2008-04-03

• First public release