[wasm-split] Split globals' transitive global dependencies

src/ir/module-splitting.cpp

@@ -45,7 +45,8 @@
 //      instantiation.
 //
 //   8. Export globals, tags, tables, and memories from the primary module and
-//      import them in the secondary modules.
+//      import them in the secondary modules. If possible, move those module
+//      items instead to the secondary modules.
 //
 // Functions can be used or referenced three ways in a WebAssembly module: they
 // can be exported, called, or referenced with ref.func. The above procedure
@@ -583,6 +584,25 @@ Expression* ModuleSplitter::maybeLoadSecondary(Builder& builder,
   return builder.makeSequence(loadSecondary, callIndirect);
 }
 
+// Helper to walk expressions in segments but NOT in globals.
+template<typename Walker>
+static void walkSegments(Walker& walker, Module* module) {
+  walker.setModule(module);
+  for (auto& curr : module->elementSegments) {
+    if (curr->offset) {
+      walker.walk(curr->offset);
+    }
+    for (auto* item : curr->data) {
+      walker.walk(item);
+    }
+  }
+  for (auto& curr : module->dataSegments) {
+    if (curr->offset) {
+      walker.walk(curr->offset);
+    }
+  }
+}
+
 void ModuleSplitter::indirectReferencesToSecondaryFunctions() {
   // Turn references to secondary functions into references to thunks that
   // perform a direct call to the original referent. The direct calls in the
@@ -977,7 +997,19 @@ void ModuleSplitter::shareImportableItems() {
     }
 
     NameCollector collector(used);
-    collector.walkModuleCode(&module);
+    // We shouldn't use collector.walkModuleCode here, because we don't want to
+    // walk global initializers. At this point, all globals are still in the
+    // primary module, so if we walk global initializers here, other globals appearing
+    // in their initializers will all be marked as used in the primary module,
+    // which is not what we want.
+    //
+    // For example, we have (global $a i32 (global.get $b)). Because $a is at
+    // this point still in the primary module, $b will be marked as "used" in
+    // the primary module. But $a can be moved to a secondary module later if it
+    // is used exclusively by that module. Then $b can be also moved, in case it
+    // doesn't have other uses. But if it is marked as "used" in the primary
+    // module, it can't.
+    walkSegments(collector, &module);
     for (auto& segment : module.dataSegments) {
       if (segment->memory.is()) {
         used.memories.insert(segment->memory);
@@ -1009,7 +1041,6 @@ void ModuleSplitter::shareImportableItems() {
           break;
       }
     }
-
     return used;
   };
 
@@ -1019,25 +1050,33 @@ void ModuleSplitter::shareImportableItems() {
     secondaryUsed.push_back(getUsedNames(*secondaryPtr));
   }
 
-  // Compute globals referenced in other globals' initializers. Since globals
-  // can reference other globals, we must ensure that if a global is used in a
-  // module, all its dependencies are also marked as used.
-  auto computeDependentItems = [&](UsedNames& used) {
+  // Compute the transitive closure of globals referenced in other globals'
+  // initializers. Since globals can reference other globals, we must ensure
+  // that if a global is used in a module, all its dependencies are also marked
+  // as used.
+  auto computeTransitiveGlobals = [&](UsedNames& used) {
     std::vector<Name> worklist(used.globals.begin(), used.globals.end());
-    for (auto name : worklist) {
+    std::unordered_set<Name> visited(used.globals.begin(), used.globals.end());
+    while (!worklist.empty()) {
+      Name name = worklist.back();
+      worklist.pop_back();
       // At this point all globals are still in the primary module, so this
       // exists
       auto* global = primary.getGlobal(name);
       if (!global->imported() && global->init) {
         for (auto* get : FindAll<GlobalGet>(global->init).list) {
-          used.globals.insert(get->name);
+          if (visited.insert(get->name).second) {
+            worklist.push_back(get->name);
+            used.globals.insert(get->name);
+          }
         }
       }
     }
   };
 
+  computeTransitiveGlobals(primaryUsed);
   for (auto& used : secondaryUsed) {
-    computeDependentItems(used);
+    computeTransitiveGlobals(used);
   }
 
   // Given a name and module item kind, returns the list of secondary modules
@@ -1127,20 +1166,21 @@ void ModuleSplitter::shareImportableItems() {
       getUsingSecondaries(global->name, &UsedNames::globals);
     bool usedInPrimary = primaryUsed.globals.count(global->name);
     if (!usedInPrimary && usingSecondaries.size() == 1) {
+      // We are moving this global to this secondary module
       auto* secondary = usingSecondaries[0];
       ModuleUtils::copyGlobal(global.get(), *secondary);
       globalsToRemove.push_back(global->name);
       // Import global initializer's ref.func dependences
       if (global->init) {
         for (auto* ref : FindAll<RefFunc>(global->init).list) {
-          // Here, ref->func is either a function the primary module, or a
+          // Here, ref->func is either a function in the primary module, or a
           // trampoline created in indirectReferencesToSecondaryFunctions in
           // case the original function is in one of the secondaries.
           assert(primary.getFunctionOrNull(ref->func));
           exportImportFunction(ref->func, {secondary});
         }
       }
-    } else {
+    } else { // We are NOT moving this global to the secondary module
       for (auto* secondary : usingSecondaries) {
         auto* secondaryGlobal =
           ModuleUtils::copyGlobal(global.get(), *secondary);

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

@@ -3,26 +3,20 @@
 ;; RUN: wasm-dis %t.2.wasm | filecheck %s --check-prefix SECONDARY
 
 ;; Check that transitive dependencies in global initializers are correctly
-;; analyzed and exported from the primary module to the secondary module.
-;; TODO Move $b and $c to the secondary module
+;; analyzed and moved to the secondary module.
 
 (module
-  ;; PRIMARY:      (global $c i32 (i32.const 42))
+  ;; SECONDARY:      (global $c i32 (i32.const 42))
   (global $c i32 (i32.const 42))
 
   ;; $b depends on $c.
-  ;; PRIMARY:      (global $b i32 (global.get $c))
+  ;; SECONDARY:      (global $b i32 (global.get $c))
   (global $b i32 (global.get $c))
 
-  ;; Globals $b is exported to the secondary module
-  ;; PRIMARY:      (export "global" (global $b))
-
-  ;; Globals $b is imported from the primary module
-  ;; SECONDARY:      (import "primary" "global" (global $b i32))
-
-  ;; $a depends on $b. Since $a is exclusively used by the secondary module,
-  ;; it will be moved there. Its dependency $b should be exported from the
-  ;; primary module and imported into the secondary module.
+  ;; $a depends on $b. since $a is exclusively used by the secondary module,
+  ;; it will be moved there. The transitive dependency must ensure that $b (and
+  ;; $c) are moved to the secondary module too, because they are not used in the
+  ;; primary module.
   ;; SECONDARY:      (global $a i32 (global.get $b))
   (global $a i32 (global.get $b))