Code folding (#1076)

Adds a pass that folds code, i.e. merges it when possible. See details in comment in the pass implementation cpp.

This is enabled by default in -Os and -Oz. Seems risky to enable anywhere else, as it does add branches - likely predictable ones so maybe no slowdown, but still some risk.

Code size numbers:

wasm-backend: 196331
+ binaryen -Os (before): 182598
+ binaryen -Os (with folding): 181943

asm2wasm -Os (before): 172463
asm2wasm -Os (with folding): 168774

So this reduces wasm-backend output by an additional 0.5% than it could before. Mainly this is because the wasm backend already has code folding, whereas on asm2wasm output, where we didn't have folding before, this saves over 2%. The 0.5% improvement on the wasm backend's output might be because this can fold more types of code than LLVM can (it can fold nested control flow, in particular).

1 files changed, 4 insertions, 0 deletions

diff --git a/src/mixed_arena.h b/src/mixed_arena.h
index af585092e..47e718454 100644
--- a/src/mixed_arena.h
+++ b/src/mixed_arena.h
@@ -172,6 +172,10 @@ public:
     return usedElements;
   }
 
+  bool empty() const {
+    return size() == 0;
+  }
+
   void resize(size_t size) {
     if (size > allocatedElements) {
       reallocate(size);