[wasm-split] Remove dead globals (WebAssembly#8505)

Currently dead module items are not split and just end up remaining in
the primary module. Usually a user runs DCE before or after the
splitting, and the goal of wasm-split is not DCE, so from the
optimization perspective it shouldn't be a problem.

But after WebAssembly#8441, this can be a problem because a dead global's
initializer can refer to another global that is moved to a secondary
module:
```wast
;; Primary
(global.get $dead i32 (global.get $a))

;; Secondary
(global $a i32 (...))
```

This PR just removes those dead globals. We leave it and do some post
processing to make it work but that's more complicated, or we can move
it to the same secondary module but this requires scanning of the
reverse mapping. Removing it seems the simplest. We can remove dead
items for other module items (memories, tables, and tags) but it is not
necessary for wasm-split to run, and they can be handled later in DCE.

Fixes
WebAssembly#8442 (comment).

Author: aheejin

src/ir/module-splitting.cpp

@@ -1182,8 +1182,18 @@ void ModuleSplitter::shareImportableItems() {
 
     auto usingSecondaries =
       getUsingSecondaries(global->name, &UsedNames::globals);
-    bool usedInPrimary = primaryUsed.globals.count(global->name);
-    if (!usedInPrimary && usingSecondaries.size() == 1) {
+    bool inPrimary = primaryUsed.globals.count(global->name);
+
+    if (!inPrimary && usingSecondaries.empty()) {
+      // It's not used anywhere, so delete it. Unlike other unused module items
+      // (memories, tables, and tags) that can just sit in the primary module
+      // and later be DCE'ed by another pass, we should remove it here, because
+      // an unused global can contain an initialier that refers to another
+      // global that will be moved to a secondary module, like
+      // (global $unused i32 (global.get $a)) // $a is moved to a secondary
+      globalsToRemove.push_back(global->name);
+
+    } else if (!inPrimary && usingSecondaries.size() == 1) {
       // We are moving this global to this secondary module
       auto* secondary = usingSecondaries[0];
       auto* secondaryGlobal = ModuleUtils::copyGlobal(global.get(), *secondary);
@@ -1213,6 +1223,7 @@ void ModuleSplitter::shareImportableItems() {
           // function.
         }
       }
+
     } else { // We are NOT moving this global to the secondary module
       if (global->init) {
         for (auto* ref : FindAll<RefFunc>(global->init).list) {

test/example/module-splitting.txt

@@ -22,7 +22,6 @@ Keeping: <none>
 After:
 (module
  (type $0 (func (param i32)))
- (global $glob (mut i32) (i32.const 7))
  (memory $mem 3 42 shared)
  (table $tab 3 42 funcref)
  (tag $e (type $0) (param i32))
@@ -46,7 +45,6 @@ After:
  (type $0 (func (param i32)))
  (import "env" "mem" (memory $mem 3 42 shared))
  (import "env" "tab" (table $tab 3 42 funcref))
- (import "env" "glob" (global $glob (mut i32)))
  (import "env" "e" (tag $e (type $0) (param i32)))
 )
 Secondary:

test/lit/wasm-split/ref.func.wast

@@ -45,6 +45,12 @@
  ;; PRIMARY-NEXT:  (drop
  ;; PRIMARY-NEXT:   (ref.func $trampoline_second)
  ;; PRIMARY-NEXT:  )
+ ;; PRIMARY-NEXT:  (drop
+ ;; PRIMARY-NEXT:   (global.get $glob1)
+ ;; PRIMARY-NEXT:  )
+ ;; PRIMARY-NEXT:  (drop
+ ;; PRIMARY-NEXT:   (global.get $glob2)
+ ;; PRIMARY-NEXT:  )
  ;; PRIMARY-NEXT: )
  (func $prime
   (drop
@@ -53,6 +59,12 @@
   (drop
    (ref.func $second)
   )
+  (drop
+   (global.get $glob1)
+  )
+  (drop
+   (global.get $glob2)
+  )
  )
 
  ;; SECONDARY:      (type $0 (func))

test/lit/wasm-split/transitive-globals.wast

@@ -34,6 +34,11 @@
 
   ;; SECONDARY:      (global $d i32 (global.get $e))
 
+  ;; This dead global is referring to a global ($a) that's moved to the
+  ;; secondary module. This should be deleted.
+  ;; PRIMARY-NOT:  (global (global $dead i32 (global.get $a))
+  (global $dead i32 (global.get $a))
+
   ;; PRIMARY:      (func $keep
   ;; PRIMARY-NEXT:  (drop
   ;; PRIMARY-NEXT:   (global.get $e)