[x64] Eliminate redundant code in RecordWriteStub

Do conditional jump based on the second instruction rather than
generate similar assembly code twice.

Bug: 
Change-Id: I8c10f3415f213f88d8222a92671e7104c65befdc
Reviewed-on: https://chromium-review.googlesource.com/634631
Commit-Queue: Jing Bao <jing.bao@intel.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47844}

src/x64/code-stubs-x64.cc

@@ -975,16 +975,17 @@ void StoreBufferOverflowStub::GenerateFixedRegStubsAheadOfTime(
 // we keep the GC informed.  The word in the object where the value has been
 // written is in the address register.
 void RecordWriteStub::Generate(MacroAssembler* masm) {
-  Label skip_to_incremental_noncompacting;
-  Label skip_to_incremental_compacting;
+  Label skip_to_incremental;
+  Label second_instr;
 
   // The first two instructions are generated with labels so as to get the
   // offset fixed up correctly by the bind(Label*) call.  We patch it back and
   // forth between a compare instructions (a nop in this position) and the
   // real branch when we start and stop incremental heap marking.
   // See RecordWriteStub::Patch for details.
-  __ jmp(&skip_to_incremental_noncompacting, Label::kNear);
-  __ jmp(&skip_to_incremental_compacting, Label::kFar);
+  __ jmp(&skip_to_incremental, Label::kNear);
+  __ bind(&second_instr);
+  __ jmp(&skip_to_incremental, Label::kNear);
 
   if (remembered_set_action() == EMIT_REMEMBERED_SET) {
     __ RememberedSetHelper(object(), address(), value(), save_fp_regs_mode(),
@@ -993,20 +994,18 @@ void RecordWriteStub::Generate(MacroAssembler* masm) {
     __ ret(0);
   }
 
-  __ bind(&skip_to_incremental_noncompacting);
-  GenerateIncremental(masm, INCREMENTAL);
+  __ bind(&skip_to_incremental);
 
-  __ bind(&skip_to_incremental_compacting);
-  GenerateIncremental(masm, INCREMENTAL_COMPACTION);
+  GenerateIncremental(masm, &second_instr);
 
   // Initial mode of the stub is expected to be STORE_BUFFER_ONLY.
   // Will be checked in IncrementalMarking::ActivateGeneratedStub.
   masm->set_byte_at(0, kTwoByteNopInstruction);
-  masm->set_byte_at(2, kFiveByteNopInstruction);
+  masm->set_byte_at(2, kTwoByteNopInstruction);
 }
 
-
-void RecordWriteStub::GenerateIncremental(MacroAssembler* masm, Mode mode) {
+void RecordWriteStub::GenerateIncremental(MacroAssembler* masm,
+                                          Label* second_instr) {
   regs_.Save(masm);
 
   if (remembered_set_action() == EMIT_REMEMBERED_SET) {
@@ -1023,7 +1022,8 @@ void RecordWriteStub::GenerateIncremental(MacroAssembler* masm, Mode mode) {
     // First notify the incremental marker if necessary, then update the
     // remembered set.
     CheckNeedsToInformIncrementalMarker(
-        masm, kUpdateRememberedSetOnNoNeedToInformIncrementalMarker, mode);
+        masm, kUpdateRememberedSetOnNoNeedToInformIncrementalMarker,
+        second_instr);
     InformIncrementalMarker(masm);
     regs_.Restore(masm);
     __ RememberedSetHelper(object(), address(), value(), save_fp_regs_mode(),
@@ -1033,7 +1033,7 @@ void RecordWriteStub::GenerateIncremental(MacroAssembler* masm, Mode mode) {
   }
 
   CheckNeedsToInformIncrementalMarker(
-      masm, kReturnOnNoNeedToInformIncrementalMarker, mode);
+      masm, kReturnOnNoNeedToInformIncrementalMarker, second_instr);
   InformIncrementalMarker(masm);
   regs_.Restore(masm);
   __ ret(0);
@@ -1067,9 +1067,8 @@ void RecordWriteStub::Activate(Code* code) {
 }
 
 void RecordWriteStub::CheckNeedsToInformIncrementalMarker(
-    MacroAssembler* masm,
-    OnNoNeedToInformIncrementalMarker on_no_need,
-    Mode mode) {
+    MacroAssembler* masm, OnNoNeedToInformIncrementalMarker on_no_need,
+    Label* second_instr) {
   Label need_incremental;
   Label need_incremental_pop_object;
 
@@ -1097,24 +1096,22 @@ void RecordWriteStub::CheckNeedsToInformIncrementalMarker(
   // Get the value from the slot.
   __ movp(regs_.scratch0(), Operand(regs_.address(), 0));
 
-  if (mode == INCREMENTAL_COMPACTION) {
   Label ensure_not_white;
-
+  // If second instruction is TwoByteNopInstruction, we're in noncompacting
+  // mode.
+  __ cmpb(Operand(second_instr), Immediate(kTwoByteNopInstruction));
+  __ j(equal, &ensure_not_white, Label::kNear);
   __ CheckPageFlag(regs_.scratch0(),  // Contains value.
                    regs_.scratch1(),  // Scratch.
-                     MemoryChunk::kEvacuationCandidateMask,
-                     zero,
-                     &ensure_not_white,
-                     Label::kNear);
+                   MemoryChunk::kEvacuationCandidateMask, zero,
+                   &ensure_not_white, Label::kNear);
 
   __ CheckPageFlag(regs_.object(),
                    regs_.scratch1(),  // Scratch.
-                     MemoryChunk::kSkipEvacuationSlotsRecordingMask,
-                     zero,
+                   MemoryChunk::kSkipEvacuationSlotsRecordingMask, zero,
                    &need_incremental);
 
   __ bind(&ensure_not_white);
-  }
 
   // We need an extra register for this, so we push the object register
   // temporarily.

src/x64/code-stubs-x64.h

@@ -119,9 +119,6 @@ class RecordWriteStub: public PlatformCodeStub {
   static const byte kTwoByteNopInstruction = 0x3c;  // Cmpb al, #imm8.
   static const byte kTwoByteJumpInstruction = 0xeb;  // Jmp #imm8.
 
-  static const byte kFiveByteNopInstruction = 0x3d;  // Cmpl eax, #imm32.
-  static const byte kFiveByteJumpInstruction = 0xe9;  // Jmp #imm32.
-
   static Mode GetMode(Code* stub) {
     byte first_instruction = stub->instruction_start()[0];
     byte second_instruction = stub->instruction_start()[2];
@@ -132,11 +129,11 @@ class RecordWriteStub: public PlatformCodeStub {
 
     DCHECK(first_instruction == kTwoByteNopInstruction);
 
-    if (second_instruction == kFiveByteJumpInstruction) {
+    if (second_instruction == kTwoByteJumpInstruction) {
       return INCREMENTAL_COMPACTION;
     }
 
-    DCHECK(second_instruction == kFiveByteNopInstruction);
+    DCHECK(second_instruction == kTwoByteNopInstruction);
 
     return STORE_BUFFER_ONLY;
   }
@@ -147,7 +144,7 @@ class RecordWriteStub: public PlatformCodeStub {
         DCHECK(GetMode(stub) == INCREMENTAL ||
                GetMode(stub) == INCREMENTAL_COMPACTION);
         stub->instruction_start()[0] = kTwoByteNopInstruction;
-        stub->instruction_start()[2] = kFiveByteNopInstruction;
+        stub->instruction_start()[2] = kTwoByteNopInstruction;
         break;
       case INCREMENTAL:
         DCHECK(GetMode(stub) == STORE_BUFFER_ONLY);
@@ -156,7 +153,7 @@ class RecordWriteStub: public PlatformCodeStub {
       case INCREMENTAL_COMPACTION:
         DCHECK(GetMode(stub) == STORE_BUFFER_ONLY);
         stub->instruction_start()[0] = kTwoByteNopInstruction;
-        stub->instruction_start()[2] = kFiveByteJumpInstruction;
+        stub->instruction_start()[2] = kTwoByteJumpInstruction;
         break;
     }
     DCHECK(GetMode(stub) == mode);
@@ -297,11 +294,10 @@ class RecordWriteStub: public PlatformCodeStub {
   Major MajorKey() const final { return RecordWrite; }
 
   void Generate(MacroAssembler* masm) override;
-  void GenerateIncremental(MacroAssembler* masm, Mode mode);
+  void GenerateIncremental(MacroAssembler* masm, Label* second_instr);
   void CheckNeedsToInformIncrementalMarker(
-      MacroAssembler* masm,
-      OnNoNeedToInformIncrementalMarker on_no_need,
-      Mode mode);
+      MacroAssembler* masm, OnNoNeedToInformIncrementalMarker on_no_need,
+      Label* second_instr);
   void InformIncrementalMarker(MacroAssembler* masm);
 
   void Activate(Code* code) override;