[turbofan] Add a --turbo_allocation_folding flag.

Change-Id: If8678bd244a0a4e21b1967c15da80e49643a33ea
Reviewed-on: https://chromium-review.googlesource.com/973001Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52101}

src/compilation-info.h

@@ -55,6 +55,7 @@ class V8_EXPORT_PRIVATE CompilationInfo final {
     kSwitchJumpTableEnabled = 1 << 13,
     kGenerateSpeculationPoisonOnEntry = 1 << 14,
     kPoisonRegisterArguments = 1 << 15,
+    kAllocationFoldingEnabled = 1 << 16
   };
 
   // TODO(mtrofin): investigate if this might be generalized outside wasm, with
@@ -195,6 +196,11 @@ class V8_EXPORT_PRIVATE CompilationInfo final {
     return enabled;
   }
 
+  void MarkAsAllocationFoldingEnabled() { SetFlag(kAllocationFoldingEnabled); }
+  bool is_allocation_folding_enabled() const {
+    return GetFlag(kAllocationFoldingEnabled);
+  }
+
   // Code getters and setters.
 
   void SetCode(Handle<Code> code) { code_ = code; }

src/compiler/memory-optimizer.cc

@@ -16,14 +16,16 @@ namespace internal {
 namespace compiler {
 
 MemoryOptimizer::MemoryOptimizer(JSGraph* jsgraph, Zone* zone,
-                                 LoadPoisoning load_poisoning)
+                                 LoadPoisoning load_poisoning,
+                                 AllocationFolding allocation_folding)
     : jsgraph_(jsgraph),
       empty_state_(AllocationState::Empty(zone)),
       pending_(zone),
       tokens_(zone),
       zone_(zone),
       graph_assembler_(jsgraph, nullptr, nullptr, zone),
-      load_poisoning_(load_poisoning) {}
+      load_poisoning_(load_poisoning),
+      allocation_folding_(allocation_folding) {}
 
 void MemoryOptimizer::Optimize() {
   EnqueueUses(graph()->start(), empty_state());
@@ -172,7 +174,8 @@ void MemoryOptimizer::VisitAllocateRaw(Node* node,
   Int32Matcher m(size);
   if (m.HasValue() && m.Value() < kMaxRegularHeapObjectSize) {
     int32_t const object_size = m.Value();
-    if (state->size() <= kMaxRegularHeapObjectSize - object_size &&
+    if (allocation_folding_ == AllocationFolding::kDoAllocationFolding &&
+        state->size() <= kMaxRegularHeapObjectSize - object_size &&
         state->group()->pretenure() == pretenure) {
       // We can fold this Allocate {node} into the allocation {group}
       // represented by the given {state}. Compute the upper bound for

src/compiler/memory-optimizer.h

@@ -31,7 +31,10 @@ typedef uint32_t NodeId;
 // implicitly.
 class MemoryOptimizer final {
  public:
-  MemoryOptimizer(JSGraph* jsgraph, Zone* zone, LoadPoisoning load_poisoning);
+  enum class AllocationFolding { kDoAllocationFolding, kDontAllocationFolding };
+
+  MemoryOptimizer(JSGraph* jsgraph, Zone* zone, LoadPoisoning load_poisoning,
+                  AllocationFolding allocation_folding);
   ~MemoryOptimizer() {}
 
   void Optimize();
@@ -143,6 +146,7 @@ class MemoryOptimizer final {
   Zone* const zone_;
   GraphAssembler graph_assembler_;
   LoadPoisoning load_poisoning_;
+  AllocationFolding allocation_folding_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(MemoryOptimizer);
 };

src/compiler/pipeline.cc

@@ -805,6 +805,9 @@ PipelineCompilationJob::Status PipelineCompilationJob::PrepareJobImpl(
   if (FLAG_branch_load_poisoning) {
     compilation_info()->MarkAsPoisonLoads();
   }
+  if (FLAG_turbo_allocation_folding) {
+    compilation_info()->MarkAsAllocationFoldingEnabled();
+  }
   if (compilation_info()->closure()->feedback_cell()->map() ==
       isolate->heap()->one_closure_cell_map()) {
     compilation_info()->MarkAsFunctionContextSpecializing();
@@ -1446,10 +1449,13 @@ struct MemoryOptimizationPhase {
     trimmer.TrimGraph(roots.begin(), roots.end());
 
     // Optimize allocations and load/store operations.
-    MemoryOptimizer optimizer(data->jsgraph(), temp_zone,
-                              data->info()->is_poison_loads()
-                                  ? LoadPoisoning::kDoPoison
-                                  : LoadPoisoning::kDontPoison);
+    MemoryOptimizer optimizer(
+        data->jsgraph(), temp_zone,
+        data->info()->is_poison_loads() ? LoadPoisoning::kDoPoison
+                                        : LoadPoisoning::kDontPoison,
+        data->info()->is_allocation_folding_enabled()
+            ? MemoryOptimizer::AllocationFolding::kDoAllocationFolding
+            : MemoryOptimizer::AllocationFolding::kDontAllocationFolding);
     optimizer.Optimize();
   }
 };

src/flag-definitions.h

@@ -478,6 +478,7 @@ DEFINE_BOOL(turbo_loop_peeling, true, "Turbofan loop peeling")
 DEFINE_BOOL(turbo_loop_variable, true, "Turbofan loop variable optimization")
 DEFINE_BOOL(turbo_cf_optimization, true, "optimize control flow in TurboFan")
 DEFINE_BOOL(turbo_escape, true, "enable escape analysis")
+DEFINE_BOOL(turbo_allocation_folding, true, "Turbofan allocation folding")
 DEFINE_BOOL(turbo_instruction_scheduling, false,
             "enable instruction scheduling in TurboFan")
 DEFINE_BOOL(turbo_stress_instruction_scheduling, false,