RemoveUnusedModuleElements: Make exports skip trampolines (#6026)

If we export a function that just calls another function, we can export that one
instead. Then the one in the middle may be unused,

function foo() {
  return bar();
}

export foo; // can be an export of bar

This saves a few bytes in rare cases, but probably more important is that it saves
the trampoline, so if this is on a hot path, we save a call.

Context: emscripten-core/emscripten#20478 (comment)

In general this is not needed as inlining helps us out by inlining foo() into the
caller (since foo is tiny, that always ends up happening). But exports are a case
the inliner cannot handle, so we do it here.

1 files changed, 5 insertions, 9 deletions

diff --git a/test/lit/passes/extract-function.wast b/test/lit/passes/extract-function.wast
index 9258f8c63..e4b804abe 100644
--- a/test/lit/passes/extract-function.wast
+++ b/test/lit/passes/extract-function.wast
@@ -5,15 +5,6 @@
 ;; RUN: foreach %s %t wasm-opt --extract-function-index --pass-arg=extract-function-index@0 -S -o - | filecheck %s
 
 (module
-  ;; CHECK:      (type $0 (func))
-
-  ;; CHECK:      (import "env" "bar" (func $bar))
-
-  ;; CHECK:      (export "foo" (func $foo))
-
-  ;; CHECK:      (func $foo
-  ;; CHECK-NEXT:  (call $bar)
-  ;; CHECK-NEXT: )
   (func $foo
     (call $bar)
   )
@@ -27,6 +18,11 @@
   )
 )
 
+;; CHECK:      (type $0 (func))
+
+;; CHECK:      (import "env" "bar" (func $bar))
+
+;; CHECK:      (export "foo" (func $bar))
 (module
   ;; Use another function in the table, but the table is not used in the
   ;; extracted function