Improvement to SmiLexicalCompare

src/misc-intrinsics.h

+// Copyright 2011 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef V8_MISC_INTRINSICS_H_
+#define V8_MISC_INTRINSICS_H_
+
+#include "../include/v8.h"
+#include "globals.h"
+
+namespace v8 {
+namespace internal {
+
+// Returns the index of the leading 1 bit, counting the least significant bit at
+// index 0.  (1 << IntegerLog2(x)) is a mask for the most significant bit of x.
+// Result is undefined if input is zero.
+int IntegerLog2(uint32_t value);
+
+} }  // namespace v8::internal

[Erik Corry, 2011/06/25 11:12:03]

Instead of closing the name space here and then using explicit name spaces on
the definitions below you can just close it at the end.

[sra1, 2011/06/25 19:31:33]

Done.  Originally I was expecting to have to #include other files, like
atomicops.

+
+
+#if defined(__GNUC__)
+
+inline int v8::internal::IntegerLog2(uint32_t value) {
+  return 31 - __builtin_clz(value);
+}
+
+#elif defined(_MSC_VER)
+
+#pragma intrinsic(_BitScanReverse)
+
+inline int v8::internal::IntegerLog2(uint32_t value) {
+  unsigned long result;
+  _BitScanReverse(&result, value);
+  return result;
+}
+
+#else
+
+// Default version using regular operations. Code taken from:
+// http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
+inline int v8::internal::IntegerLog2(uint32_t value) {
+  int result, shift;
+  shift = (value > 0xFFFF) << 4;  value >>= shift;   result = shift;

[Erik Corry, 2011/06/25 11:12:03]

This code won't pass presubmit.py.  See
http://code.google.com/p/v8/wiki/Contributing
Also, we don't normally allow booleans to be used in a numeric way.  Please
replace with ?:

+  shift = (value > 0xFF) << 3;    value >>= shift;   result |= shift;
+  shift = (value > 0xF) << 2;     value >>= shift;   result |= shift;
+  shift = (value > 0x3) << 1;     value >>= shift;   result |= shift;
+  result |= (value >> 1);
+  return result;
+}
+#endif
+
+#endif  // V8_MISC_INTRINSICS_H_