MemoryPacking: Properly notice zeroFilledMemory (#3306)

We can only pack memory if we know it is zero-filled before us.

2 files changed, 20 insertions, 10 deletions

diff --git a/src/passes/MemoryPacking.cpp b/src/passes/MemoryPacking.cpp
index d4d75e6d6..b6ee7acf9 100644
--- a/src/passes/MemoryPacking.cpp
+++ b/src/passes/MemoryPacking.cpp
@@ -97,7 +97,7 @@ struct MemoryPacking : public Pass {
   uint32_t maxSegments;
 
   void run(PassRunner* runner, Module* module) override;
-  bool canOptimize(const std::vector<Memory::Segment>& segments);
+  bool canOptimize(const Memory& memory, const PassOptions& passOptions);
   void optimizeBulkMemoryOps(PassRunner* runner, Module* module);
   void getSegmentReferrers(Module* module, std::vector<Referrers>& referrers);
   void dropUnusedSegments(std::vector<Memory::Segment>& segments,
@@ -122,13 +122,7 @@ struct MemoryPacking : public Pass {
 };
 
 void MemoryPacking::run(PassRunner* runner, Module* module) {
-  if (!module->memory.exists) {
-    return;
-  }
-
-  auto& segments = module->memory.segments;
-
-  if (!canOptimize(segments)) {
+  if (!canOptimize(module->memory, runner->options)) {
     return;
   }
 
@@ -136,6 +130,8 @@ void MemoryPacking::run(PassRunner* runner, Module* module) {
                   ? 63
                   : uint32_t(WebLimitations::MaxDataSegments);
 
+  auto& segments = module->memory.segments;
+
   // For each segment, a list of bulk memory instructions that refer to it
   std::vector<Referrers> referrers(segments.size());
 
@@ -182,7 +178,21 @@ void MemoryPacking::run(PassRunner* runner, Module* module) {
   }
 }
 
-bool MemoryPacking::canOptimize(const std::vector<Memory::Segment>& segments) {
+bool MemoryPacking::canOptimize(const Memory& memory,
+                                const PassOptions& passOptions) {
+  if (!memory.exists) {
+    return false;
+  }
+
+  // We must optimize under the assumption that memory has been initialized to
+  // zero. That is the case for a memory declared in the module, but for a
+  // memory that is imported, we must be told that it is zero-initialized.
+  if (memory.imported() && !passOptions.zeroFilledMemory) {
+    return false;
+  }
+
+  auto& segments = memory.segments;
+
   // One segment is always ok to optimize, as it does not have the potential
   // problems handled below.
   if (segments.size() <= 1) {
diff --git a/src/wasm.h b/src/wasm.h
index feaa51e40..2f655fa7f 100644
--- a/src/wasm.h
+++ b/src/wasm.h
@@ -1391,7 +1391,7 @@ struct Importable {
   // If these are set, then this is an import, as module.base
   Name module, base;
 
-  bool imported() { return module.is(); }
+  bool imported() const { return module.is(); }
 
   void setName(Name name_, bool hasExplicitName_) {
     name = name_;