[serializer] make snapshot reproducible.

R=ulan@chromium.org
BUG=v8:4886
LOG=N

Review URL: https://codereview.chromium.org/1859063003

Cr-Commit-Position: refs/heads/master@{#35266}

src/heap/heap.cc

@@ -5369,6 +5369,10 @@ void Heap::SetStackLimits() {
       (isolate_->stack_guard()->real_jslimit() & ~kSmiTagMask) | kSmiTag);
 }
 
+void Heap::ClearStackLimits() {
+  roots_[kStackLimitRootIndex] = Smi::FromInt(0);
+  roots_[kRealStackLimitRootIndex] = Smi::FromInt(0);
+}
 
 void Heap::PrintAlloctionsHash() {
   uint32_t hash = StringHasher::GetHashCore(raw_allocations_hash_);

src/heap/heap.h

@@ -1004,6 +1004,10 @@ class Heap {
   // jslimit_/real_jslimit_ variable in the StackGuard.
   void SetStackLimits();
 
+  // The stack limit is thread-dependent. To be able to reproduce the same
+  // snapshot blob, we need to reset it before serializing.
+  void ClearStackLimits();
+
   // Generated code can treat direct references to this root as constant.
   bool RootCanBeTreatedAsConstant(RootListIndex root_index);
 

src/snapshot/mksnapshot.cc

@@ -139,7 +139,8 @@ int main(int argc, char** argv) {
   // By default, log code create information in the snapshot.
   i::FLAG_log_code = true;
   i::FLAG_logfile_per_isolate = false;
-
+  // Make mksnapshot runs predictable to create reproducible snapshots.
+  i::FLAG_predictable = true;
   // Print the usage if an error occurs when parsing the command line
   // flags or if the help flag is set.
   int result = i::FlagList::SetFlagsFromCommandLine(&argc, argv, true);

src/snapshot/startup-serializer.cc

@@ -113,7 +113,12 @@ void StartupSerializer::SerializeStrongReferences() {
   CHECK(HasNotExceededFirstPageOfEachSpace());
   serializing_immortal_immovables_roots_ = false;
   // Visit the rest of the strong roots.
+  // Clear the stack limits to make the snapshot reproducible.
+  // Reset it again afterwards.
+  isolate->heap()->ClearStackLimits();
   isolate->heap()->IterateSmiRoots(this);
+  isolate->heap()->SetStackLimits();
+
   isolate->heap()->IterateStrongRoots(this,
                                       VISIT_ONLY_STRONG_FOR_SERIALIZATION);
 }