doubly-linked free-lists for thread caches and page heaps

third_party/tcmalloc/chromium/src/common.h

Index: third_party/tcmalloc/chromium/src/common.h
diff --git a/third_party/tcmalloc/chromium/src/common.h b/third_party/tcmalloc/chromium/src/common.h
index a3df8de02fd0056b750e096d0d74eb5477cde6bb..5ba3883057cab48310691616d3c45e045a2b430d 100644
--- a/third_party/tcmalloc/chromium/src/common.h
+++ b/third_party/tcmalloc/chromium/src/common.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2008, Google Inc.
+// Copyright (c) 2011, Google Inc.
 // All rights reserved.
 // 
 // Redistribution and use in source and binary forms, with or without
@@ -31,7 +31,6 @@
 // Author: Sanjay Ghemawat <opensource@google.com>
 //
 // Common definitions for tcmalloc code.
-
 #ifndef TCMALLOC_COMMON_H_
 #define TCMALLOC_COMMON_H_
 
@@ -40,6 +39,7 @@
 #ifdef HAVE_STDINT_H
 #include <stdint.h>                     // for uintptr_t, uint64_t
 #endif
+#include "free_list.h"  // for SIZE_CLASS macros
 #include "internal_logging.h"  // for ASSERT, etc
 
 // Type that can hold a page number
@@ -175,7 +175,11 @@ class SizeMap {
     const bool big = (s > kMaxSmallSize);
     const int add_amount = big ? (127 + (120<<7)) : 7;
     const int shift_amount = big ? 7 : 3;
-    return (s + add_amount) >> shift_amount;
+    const int size_class = (s + add_amount) >> shift_amount;
+    if (HAS_64_BIT_POINTERS && s > 0 &&
+        size_class < FIRST_NON_0_SIZE_CLASS)
+      return FIRST_NON_0_SIZE_CLASS;

[jar (doing other things), 2011/08/26 18:56:29]

I haven't measured it... but I've been told that compilers optimize for the
if-true case, and assume that is most likely. The if-false case generally
requires a branch (and is slower).  As a result, I liked the code you had in the
last version better (since this seems to be a heavilly optimized piece of code).
 Possibly even better would be to test last for for the (rarer) s == 0 case. 
The code would then read:

if (!HAS_64_BIT_POINTERS ||
    (size_class > FIRST_NON_0_SIZE_CLASS) || (s == 0))
  return size_class;
return FIRST_NON_0_SIZE_CLASS;

[bxx, 2011/08/26 21:45:10]

Done.

+    return size_class;
   }
 
   int NumMoveSize(size_t size);