Move wtf/BitwiseOperations.h into base/bits.h.

base/bits.h

-// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file defines some bit utilities.
 
 #ifndef BASE_BITS_H_
 #define BASE_BITS_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
+#include "base/compiler_specific.h"
 #include "base/logging.h"
 
+#if defined(COMPILER_MSVC)
+#include <intrin.h>
+#endif
+
 namespace base {
 namespace bits {
 
 // Returns the integer i such as 2^i <= n < 2^(i+1)
 inline int Log2Floor(uint32_t n) {
   if (n == 0)
     return -1;
   int log = 0;
   uint32_t value = n;
   for (int i = 4; i >= 0; --i) {
@@ skipping 17 matching lines @@
     return 1 + Log2Floor(n - 1);
   }
 }
 
 // Round up |size| to a multiple of alignment, which must be a power of two.
 inline size_t Align(size_t size, size_t alignment) {
   DCHECK_EQ(alignment & (alignment - 1), 0u);
   return (size + alignment - 1) & ~(alignment - 1);
 }
 
+// These functions count the number of leading zeros in a binary value, starting
+// with the most significant bit. C does not have an operator to do this, but
+// fortunately the various compilers have built-ins that map to fast underlying
+// processor instructions.
+#if defined(COMPILER_MSVC)
+
+ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
+  unsigned long index;
+  return LIKELY(_BitScanReverse(&index, x)) ? (31 - index) : 32;
+}
+
+#if defined(ARCH_CPU_64_BITS)
+
+// MSVC only supplies _BitScanForward64 when building for a 64-bit target.
+ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
+  unsigned long index;
+  return LIKELY(_BitScanReverse64(&index, x)) ? (63 - index) : 64;
+}
+
+#endif
+
+#elif defined(COMPILER_GCC)
+
+// This is very annoying. __builtin_clz has undefined behaviour for an input of
+// 0, even though there's clearly a return value that makes sense, and even
+// though some processor clz instructions have defined behaviour for 0. We could
+// drop to raw __asm__ to do better, but we'll avoid doing that unless we see
+// proof that we need to.
+ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
+  return LIKELY(x) ? __builtin_clz(x) : 32;
+}
+
+ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
+  return LIKELY(x) ? __builtin_clzll(x) : 64;
+}
+
+#endif
+
+#if defined(ARCH_CPU_64_BITS)
+
+ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
+  return CountLeadingZeroBits64(x);
+}
+
+#else
+
+ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
+  return CountLeadingZeroBits32(x);
+}
+
+#endif
+
 }  // namespace bits
 }  // namespace base
 
 #endif  // BASE_BITS_H_