SimplifyGlobals: Fix folding of exports (#7940)

An exported global can have extra uses, which prevent folding its value
into others.

Author: kripken

src/passes/SimplifyGlobals.cpp

@@ -738,10 +738,11 @@ struct SimplifyGlobals : public Pass {
 
       void visitGlobalGet(GlobalGet* curr) {
         // If this is a get of a global with a single get and no sets, then we
-        // can fold that code into here.
+        // can fold that code into here. We must also avoid an export, as it can
+        // have additional gets and sets that we do not see.
         auto name = curr->name;
         auto& info = infos[name];
-        if (info.written == 0 && info.read == 1) {
+        if (info.written == 0 && info.read == 1 && !info.exported) {
           auto* global = wasm.getGlobal(name);
           if (global->init) {
             // Copy that global's code. For simplicity we copy it as we have to

test/lit/passes/simplify-globals-gc.wast

@@ -55,3 +55,34 @@
  )
 )
 
+;; One global reads another, and should contain the same value. We should not
+;; erroneously optimize the global.get to a struct.new, as struct.new generates
+;; a new value each time, and the export allows that difference to be noticed.
+;; TODO: We could flip the export to read $B instead, basically considering it
+;;       a use that we can modify, like global.gets that we already do.
+(module
+ ;; CHECK:      (type $struct (struct))
+ (type $struct (struct))
+
+ ;; CHECK:      (global $A (ref $struct) (struct.new_default $struct))
+ (global $A (ref $struct) (struct.new_default $struct))
+
+ ;; CHECK:      (global $B (ref $struct) (global.get $A))
+ (global $B (ref $struct) (global.get $A))
+
+ ;; CHECK:      (export "A" (global $A))
+ (export "A" (global $A))
+)
+
+;; Without that export, we only use $A once, and can fold it into that use.
+(module
+ ;; CHECK:      (type $struct (struct))
+ (type $struct (struct))
+
+ ;; CHECK:      (global $A (ref $struct) (struct.new_default $struct))
+ (global $A (ref $struct) (struct.new_default $struct))
+
+ ;; CHECK:      (global $B (ref $struct) (struct.new_default $struct))
+ (global $B (ref $struct) (global.get $A))
+)
+