Limit the number of passive segments to work around a Chrome bug (WebAssembly#2613)

Chrome is currently decoding the segment indices as signed numbers, so
some ranges of indices greater than 63 do not work. As a temporary
workaround, limit the number of segments produced by MemoryPacking to
63 when bulk-memory is enabled.

Author: tlively

src/passes/MemoryPacking.cpp

@@ -87,6 +87,11 @@ Expression* makeShiftedMemorySize(Builder& builder) {
 struct MemoryPacking : public Pass {
   size_t dropStateGlobalCount = 0;
 
+  // FIXME: Chrome has a bug decoding section indices that prevents it from
+  // using more than 63. Just use WebLimitations::MaxDataSegments once this is
+  // fixed. See https://bugs.chromium.org/p/v8/issues/detail?id=10151.
+  uint32_t maxSegments;
+
   void run(PassRunner* runner, Module* module) override;
   void optimizeBulkMemoryOps(PassRunner* runner, Module* module);
   void getSegmentReferrers(Module* module, std::vector<Referrers>& referrers);
@@ -116,6 +121,9 @@ void MemoryPacking::run(PassRunner* runner, Module* module) {
     return;
   }
 
+  maxSegments = module->features.hasBulkMemory()
+                  ? 63
+                  : uint32_t(WebLimitations::MaxDataSegments);
   auto& segments = module->memory.segments;
 
   // For each segment, a list of bulk memory instructions that refer to it
@@ -436,7 +444,7 @@ void MemoryPacking::createSplitSegments(Builder& builder,
         offset = segment.offset;
       }
     }
-    if (WebLimitations::MaxDataSegments <= packed.size() + segmentsRemaining) {
+    if (maxSegments <= packed.size() + segmentsRemaining) {
       // Give up splitting and merge all remaining ranges except end zeroes
       auto lastNonzero = ranges.end() - 1;
       if (lastNonzero->isZero) {

src/wasm-binary.h

@@ -47,7 +47,7 @@ enum {
 
 // wasm VMs on the web have decided to impose some limits on what they
 // accept
-enum WebLimitations {
+enum WebLimitations : uint32_t {
   MaxDataSegments = 100 * 1000,
   MaxFunctionBodySize = 128 * 1024,
   MaxFunctionLocals = 50 * 1000