Reland "Optimize tuple.extract of gets in BinaryInstWriter" (#5955)

In general, the binary lowering of tuple.extract expects that all the tuple
values are on top of the stack, so it inserts drops and possibly uses a scratch
local to ensure only the extracted value is left. However, when the extracted
tuple expression is a local.get, local.tee, or global.get, it's much more
efficient to change the lowering of the get or tee to ensure that only the
extracted value is on the stack to begin with. Implement that optimization in
the binary writer.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/wasm-stack.h b/src/wasm-stack.h
index 03ec4eef5..1f66212ad 100644
--- a/src/wasm-stack.h
+++ b/src/wasm-stack.h
@@ -148,6 +148,11 @@ private:
   InsertOrderedMap<Type, Index> scratchLocals;
   void countScratchLocals();
   void setScratchLocals();
+
+  // local.get, local.tee, and glboal.get expressions that will be followed by
+  // tuple.extracts. We can optimize these by getting only the local for the
+  // extracted index.
+  std::unordered_map<Expression*, Index> extractedGets;
 };
 
 // Takes binaryen IR and converts it to something else (binary or stack IR)