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

base/bits.h

Index: base/bits.h
diff --git a/base/bits.h b/base/bits.h
index a3a59d1dfad0d4c2992b1a6d0abd9a0c4fc2b1aa..301a0353f0f3e617e25f61921a31fe4e23a1ab62 100644
--- a/base/bits.h
+++ b/base/bits.h
@@ -1,4 +1,4 @@
-// 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.
 
@@ -10,8 +10,13 @@
 #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 {
 
@@ -49,6 +54,58 @@ inline size_t Align(size_t size, size_t alignment) {
   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 these's clearly a return value that makes sense, and even

[dcheng, 2016/11/17 20:06:25]

these's?

I wonder if this comment meant to write "there's"?

(I know this is an inherited comment)

[palmer, 2016/11/17 20:18:27]

Done.

+// though nascent processor clz instructions have defined behaviour for 0.

[dcheng, 2016/11/17 20:06:25]

Not sure if it's still nascent ;-)

[palmer, 2016/11/17 20:18:27]

Done.

+// 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