Provide a tagged allocation top pointer.

src/globals.h

 // 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.
 
 #ifndef V8_GLOBALS_H_
 #define V8_GLOBALS_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 286 matching lines @@
 const intptr_t kObjectAlignment = 1 << kObjectAlignmentBits;
 const intptr_t kObjectAlignmentMask = kObjectAlignment - 1;
 
 // Desired alignment for pointers.
 const intptr_t kPointerAlignment = (1 << kPointerSizeLog2);
 const intptr_t kPointerAlignmentMask = kPointerAlignment - 1;
 
 // Desired alignment for double values.
 const intptr_t kDoubleAlignment = 8;
 const intptr_t kDoubleAlignmentMask = kDoubleAlignment - 1;
+const intptr_t kDoubleAlignmentMaskTagged = kDoubleAlignmentMask - 1;
 
 // Desired alignment for 128 bit SIMD values.
 const intptr_t kSimd128Alignment = 16;
 const intptr_t kSimd128AlignmentMask = kSimd128Alignment - 1;
 
 // Desired alignment for generated code is 32 bytes (to improve cache line
 // utilization).
 const int kCodeAlignmentBits = 5;
 const intptr_t kCodeAlignment = 1 << kCodeAlignmentBits;
 const intptr_t kCodeAlignmentMask = kCodeAlignment - 1;
@@ skipping 772 matching lines @@
   return static_cast<uint32_t>(bit_cast<uintptr_t>(address) >>
                                kPointerSizeLog2);
 }
 
 }  // namespace internal
 }  // namespace v8
 
 namespace i = v8::internal;
 
 #endif  // V8_GLOBALS_H_