[stubs] Micro optimizations to CodeAssember's allocation path

src/code-stub-assembler.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/code-stub-assembler.h"
 #include "src/code-factory.h"
 
 namespace v8 {
 namespace internal {
 
@@ skipping 304 matching lines @@
                                               Node* top_address,
                                               Node* limit_address) {
   Node* top = Load(MachineType::Pointer(), top_address);
   Node* limit = Load(MachineType::Pointer(), limit_address);
 
   // If there's not enough space, call the runtime.
   Variable result(this, MachineRepresentation::kTagged);
   Label runtime_call(this, Label::kDeferred), no_runtime_call(this);
   Label merge_runtime(this, &result);
 
-  Branch(IntPtrLessThan(IntPtrSub(limit, top), size_in_bytes), &runtime_call,
+  Node* new_top = IntPtrAdd(top, size_in_bytes);
+  Branch(UintPtrGreaterThanOrEqual(new_top, limit), &runtime_call,
          &no_runtime_call);
 
   Bind(&runtime_call);
   // AllocateInTargetSpace does not use the context.
   Node* context = IntPtrConstant(0);
   Node* runtime_flags = SmiTag(Int32Constant(
       AllocateDoubleAlignFlag::encode(false) |
       AllocateTargetSpace::encode(flags & kPretenured
                                       ? AllocationSpace::OLD_SPACE
                                       : AllocationSpace::NEW_SPACE)));
   Node* runtime_result = CallRuntime(Runtime::kAllocateInTargetSpace, context,
                                      SmiTag(size_in_bytes), runtime_flags);
   result.Bind(runtime_result);
   Goto(&merge_runtime);
 
   // When there is enough space, return `top' and bump it up.
   Bind(&no_runtime_call);
   Node* no_runtime_result = top;
   StoreNoWriteBarrier(MachineType::PointerRepresentation(), top_address,
-                      IntPtrAdd(top, size_in_bytes));
+                      new_top);
   no_runtime_result = BitcastWordToTagged(
       IntPtrAdd(no_runtime_result, IntPtrConstant(kHeapObjectTag)));
   result.Bind(no_runtime_result);
   Goto(&merge_runtime);
 
   Bind(&merge_runtime);
   return result.value();
 }
 
 Node* CodeStubAssembler::AllocateRawAligned(Node* size_in_bytes,
@@ skipping 778 matching lines @@
 }
 
 Node* CodeStubAssembler::BitFieldDecode(Node* word32, uint32_t shift,
                                         uint32_t mask) {
   return Word32Shr(Word32And(word32, Int32Constant(mask)),
                    Int32Constant(shift));
 }
 
 }  // namespace internal
 }  // namespace v8