Free one bit in Map by removing unused retaining counter.

src/x64/builtins-x64.cc

 // Copyright 2012 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.
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/code-factory.h"
 #include "src/codegen.h"
 #include "src/deoptimizer.h"
 #include "src/full-codegen/full-codegen.h"
@@ skipping 167 matching lines @@
 
         // Fill all the in-object properties with the appropriate filler.
         // rbx: JSObject (tagged)
         // rcx: First in-object property of JSObject (not tagged)
         __ LoadRoot(r11, Heap::kUndefinedValueRootIndex);
 
         if (!is_api_function) {
           Label no_inobject_slack_tracking;
 
           // The code below relies on these assumptions.
-          STATIC_ASSERT(Map::Counter::kShift + Map::Counter::kSize == 32);
+          STATIC_ASSERT(Map::kNoSlackTracking == 0);
+          STATIC_ASSERT(Map::ConstructionCounter::kNext == 32);
           // Check if slack tracking is enabled.
           __ movl(rsi, FieldOperand(rax, Map::kBitField3Offset));
-          __ shrl(rsi, Immediate(Map::Counter::kShift));
-          __ cmpl(rsi, Immediate(Map::kSlackTrackingCounterEnd));
-          __ j(less, &no_inobject_slack_tracking);
+          __ shrl(rsi, Immediate(Map::ConstructionCounter::kShift));
+          __ j(zero, &no_inobject_slack_tracking);  // Map::kNoSlackTracking
           __ Push(rsi);  // Save allocation count value.
           // Decrease generous allocation count.
           __ subl(FieldOperand(rax, Map::kBitField3Offset),
-                  Immediate(1 << Map::Counter::kShift));
+                  Immediate(1 << Map::ConstructionCounter::kShift));
 
           // Allocate object with a slack.
           __ movzxbp(rsi, FieldOperand(rax, Map::kUnusedPropertyFieldsOffset));
           __ negp(rsi);
           __ leap(rsi, Operand(r9, rsi, times_pointer_size, 0));
           // rsi: offset of first field after pre-allocated fields
           if (FLAG_debug_code) {
             __ cmpp(rcx, rsi);
             __ Assert(less_equal,
                       kUnexpectedNumberOfPreAllocatedPropertyFields);
@@ skipping 1826 matching lines @@
   __ ret(0);
 }
 
 
 #undef __
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64