[JS] Add a new OptimizeForJS pass (#4033)

Add a new OptimizeForJS pass which contains rewriting rules specific to JavaScript.

LLVM usually lowers x != 0 && (x & (x - 1)) == 0 (isPowerOf2) to popcnt(x) == 1 which is ok for wasm and other targets but is quite expensive for JavaScript. In this PR we lower the popcnt pattern back to the isPowerOf2 pattern.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/ir/abstract.h b/src/ir/abstract.h
index 2308c47ba..18df7869e 100644
--- a/src/ir/abstract.h
+++ b/src/ir/abstract.h
@@ -29,6 +29,7 @@ enum Op {
   // Unary
   Abs,
   Neg,
+  Popcnt,
   // Binary
   Add,
   Sub,
@@ -81,6 +82,8 @@ inline UnaryOp getUnary(Type type, Op op) {
       switch (op) {
         case EqZ:
           return EqZInt32;
+        case Popcnt:
+          return PopcntInt32;
         default:
           return InvalidUnary;
       }
@@ -90,6 +93,8 @@ inline UnaryOp getUnary(Type type, Op op) {
       switch (op) {
         case EqZ:
           return EqZInt64;
+        case Popcnt:
+          return PopcntInt64;
         default:
           return InvalidUnary;
       }