Rename failure tag to page owner tag and add proper description.

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 "include/v8stdint.h"
 
 #include "src/base/build_config.h"
@@ skipping 198 matching lines @@
 // Desired alignment for double values.
 const intptr_t kDoubleAlignment = 8;
 const intptr_t kDoubleAlignmentMask = kDoubleAlignment - 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;
 
-// Tag information for Failure.
-// TODO(yangguo): remove this from space owner calculation.
-const int kFailureTag = 3;
-const int kFailureTagSize = 2;
-const intptr_t kFailureTagMask = (1 << kFailureTagSize) - 1;
+// The owner field of a page is tagged with the page header tag. We need that
+// to find out if a slot is part of a large object. If we mask out the lower
+// 0xfffff bits (1M pages), go to the owner offset, and see that this field
+// is tagged with the page header tag, we can just look up the owner.
+// Otherwise, we know that we are somewhere (not within the first 1M) in a
+// large object.
+const int kPageHeaderTag = 3;
+const int kPageHeaderTagSize = 2;
+const intptr_t kPageHeaderTagMask = (1 << kPageHeaderTagSize) - 1;
 
 
 // Zap-value: The value used for zapping dead objects.
 // Should be a recognizable hex value tagged as a failure.
 #ifdef V8_HOST_ARCH_64_BIT
 const Address kZapValue =
     reinterpret_cast<Address>(V8_UINT64_C(0xdeadbeedbeadbeef));
 const Address kHandleZapValue =
     reinterpret_cast<Address>(V8_UINT64_C(0x1baddead0baddeaf));
 const Address kGlobalHandleZapValue =
@@ skipping 493 matching lines @@
 enum MinusZeroMode {
   TREAT_MINUS_ZERO_AS_ZERO,
   FAIL_ON_MINUS_ZERO
 };
 
 } }  // namespace v8::internal
 
 namespace i = v8::internal;
 
 #endif  // V8_GLOBALS_H_