cleanup empty ifs

WebAssembly · Nov 26, 2024 · c243013 · c243013
1 parent 0d9d994
commit c243013
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Showing 4 changed files with 27 additions and 63 deletions.
diff --git a/src/passes/CodeFolding.cpp b/src/passes/CodeFolding.cpp
@@ -105,8 +105,7 @@ struct CodeFolding
     Tail(Block* block) : expr(nullptr), block(block), pointer(nullptr) {}
     // For a break
     Tail(Expression* expr, Block* block)
-      : expr(expr), block(block), pointer(nullptr) {
-    }
+      : expr(expr), block(block), pointer(nullptr) {}
     Tail(Expression* expr, Expression** pointer)
       : expr(expr), block(nullptr), pointer(pointer) {}
 
@@ -486,7 +485,20 @@ struct CodeFolding
     // make a block with curr + the merged code
     Builder builder(*getModule());
     auto* block = builder.makeBlock();
-    block->list.push_back(curr);
+    if constexpr (T::SpecificId == Expression::IfId) {
+      // If we've moved all the contents out of both arms of the If, then we can
+      // simplify the output by replacing it entirely with just a drop of the
+      // condition.
+      auto* iff = curr->template cast<If>();
+      if (iff->ifTrue->template cast<Block>()->list.empty() &&
+          iff->ifFalse->template cast<Block>()->list.empty()) {
+        block->list.push_back(builder.makeDrop(iff->condition));
+      } else {
+        block->list.push_back(curr);
+      }
+    } else {
+      block->list.push_back(curr);
+    }
     while (!mergeable.empty()) {
       block->list.push_back(mergeable.back());
       mergeable.pop_back();

diff --git a/test/lit/passes/code-folding-eh-legacy.wast b/test/lit/passes/code-folding-eh-legacy.wast
@@ -352,12 +352,8 @@
   ;; CHECK-NEXT:    )
   ;; CHECK-NEXT:    (block
   ;; CHECK-NEXT:     (block
-  ;; CHECK-NEXT:      (if
+  ;; CHECK-NEXT:      (drop
   ;; CHECK-NEXT:       (local.get $0)
-  ;; CHECK-NEXT:       (then
-  ;; CHECK-NEXT:       )
-  ;; CHECK-NEXT:       (else
-  ;; CHECK-NEXT:       )
   ;; CHECK-NEXT:      )
   ;; CHECK-NEXT:      (nop)
   ;; CHECK-NEXT:      (drop

diff --git a/test/lit/passes/code-folding.wast b/test/lit/passes/code-folding.wast
@@ -95,12 +95,8 @@
  )
 
  ;; CHECK:      (func $negative-zero-b (type $1) (result f32)
- ;; CHECK-NEXT:  (if
+ ;; CHECK-NEXT:  (drop
  ;; CHECK-NEXT:   (i32.const 0)
- ;; CHECK-NEXT:   (then
- ;; CHECK-NEXT:   )
- ;; CHECK-NEXT:   (else
- ;; CHECK-NEXT:   )
  ;; CHECK-NEXT:  )
  ;; CHECK-NEXT:  (f32.const -0)
  ;; CHECK-NEXT: )
@@ -747,12 +743,8 @@
  )
 
  ;; CHECK:      (func $unreachable-if (type $0)
- ;; CHECK-NEXT:  (if
+ ;; CHECK-NEXT:  (drop
  ;; CHECK-NEXT:   (unreachable)
- ;; CHECK-NEXT:   (then
- ;; CHECK-NEXT:   )
- ;; CHECK-NEXT:   (else
- ;; CHECK-NEXT:   )
  ;; CHECK-NEXT:  )
  ;; CHECK-NEXT:  (nop)
  ;; CHECK-NEXT:  (drop
@@ -907,12 +899,8 @@
  ;; CHECK:      (func $refined-type-blocks (type $3) (result anyref)
  ;; CHECK-NEXT:  (select (result anyref)
  ;; CHECK-NEXT:   (block (result anyref)
- ;; CHECK-NEXT:    (if
+ ;; CHECK-NEXT:    (drop
  ;; CHECK-NEXT:     (i32.const 0)
- ;; CHECK-NEXT:     (then
- ;; CHECK-NEXT:     )
- ;; CHECK-NEXT:     (else
- ;; CHECK-NEXT:     )
  ;; CHECK-NEXT:    )
  ;; CHECK-NEXT:    (nop)
  ;; CHECK-NEXT:    (ref.null none)

diff --git a/test/passes/remove-unused-names_code-folding.txt b/test/passes/remove-unused-names_code-folding.txt
@@ -65,12 +65,8 @@
  )
  (func $ifs-blocks
   (block
-   (if
+   (drop
     (i32.const 0)
-    (then
-    )
-    (else
-    )
    )
    (nop)
   )
@@ -119,12 +115,8 @@
  )
  (func $ifs-blocks-big
   (block
-   (if
+   (drop
     (i32.const 0)
-    (then
-    )
-    (else
-    )
    )
    (drop
     (i32.add
@@ -512,12 +504,8 @@
   (block $out
    (block $out2
     (block
-     (if
+     (drop
       (local.get $x)
-      (then
-      )
-      (else
-      )
      )
      (br_if $out
       (local.get $y)
@@ -707,12 +695,8 @@
  (func $mixture
   (block $out
    (block
-    (if
+    (drop
      (i32.const 1)
-     (then
-     )
-     (else
-     )
     )
     (drop
      (i32.const 2)
@@ -759,22 +743,14 @@
   (block
    (block $out3
     (block
-     (if
+     (drop
       (i32.const 1)
-      (then
-      )
-      (else
-      )
      )
      (br $out3)
     )
     (block
-     (if
+     (drop
       (i32.const 1)
-      (then
-      )
-      (else
-      )
      )
      (br $out3)
     )
@@ -1519,12 +1495,8 @@
    )
   )
   (block
-   (if
+   (drop
     (i32.const 0)
-    (then
-    )
-    (else
-    )
    )
    (nop)
   )
@@ -1540,12 +1512,8 @@
   (nop)
   (drop
    (block (result i32)
-    (if
+    (drop
      (unreachable)
-     (then
-     )
-     (else
-     )
     )
     (i32.add
      (i32.const 1)