wasm2js: don't run coalesce-locals if not optimizing (#2076)

That pass is very slow on unoptimized code (super-linear on the number of locals, which if unoptimized can be massive due to flatten).

1 files changed, 5 insertions, 1 deletions

diff --git a/src/wasm2js.h b/src/wasm2js.h
index 4e5bebb4c..1a5050af4 100644
--- a/src/wasm2js.h
+++ b/src/wasm2js.h
@@ -295,7 +295,11 @@ Ref Wasm2JSBuilder::processWasm(Module* wasm, Name funcName) {
     runner.add("remove-unused-names");
     runner.add("merge-blocks");
     runner.add("simplify-locals-notee-nostructure");
-    runner.add("coalesce-locals");
+    // Coalescing is slow if we didn't run full optimizations earlier, so don't
+    // run it automatically.
+    if (options.optimizeLevel > 0) {
+      runner.add("coalesce-locals");
+    }
     runner.add("reorder-locals");
     runner.add("vacuum");
     runner.add("remove-unused-module-elements");