nearbyhint: Fix for ffast-math #549
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The fix on 45cad81 wasn't working on Clang. On ffast-math the compiler is free to assume that "x + v -v = x". 45cad81 was workarounding this fact by storing "x + v" on a volatile variable. For Clang this wasn't enough to stop optimizing, as it correctly detected that the variable is local-scope, so no one can take a reference to it. This commit reworks the fix by defining a function to do the operation and disabling optimizations on that function for all supported compilers (and those using the same frontend). For non-supported compilers an #error is emitted, as the workaround wasn't safe enough. It could even break between compiler versions. This avoids potentially weird behaviour on the future.
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See commit text.
This fixes:
#548
Notice that I wasn't able to verify it on Visual Studio, just g++ and clang.