dbce27047e
Arithmetic right shifting is *not* division in two's complement representation, only in one's complement. So we convert to one's complement, shift, and go back to two's complement. By permutating the last steps, one can get efficient branch-free code. This insight comes from the paleozoic era of computer science, see the paper from 1976: Guy Lewis Steele Jr.: "Arithmetic Shifting Considered Harmful" ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-378.pdf This results in better and more correct code than our previous "neg/shift/neg" dance. LOG=y BUG=v8:3151 R=bmeurer@chromium.org Review URL: https://codereview.chromium.org/166793002 git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19434 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
99 lines
2.7 KiB
JavaScript
99 lines
2.7 KiB
JavaScript
// Copyright 2012 the V8 project authors. All rights reserved.
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// Redistribution and use in source and binary forms, with or without
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// modification, are permitted provided that the following conditions are
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// met:
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//
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// * Redistributions of source code must retain the above copyright
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// notice, this list of conditions and the following disclaimer.
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// * Redistributions in binary form must reproduce the above
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// copyright notice, this list of conditions and the following
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// disclaimer in the documentation and/or other materials provided
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// with the distribution.
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// * Neither the name of Google Inc. nor the names of its
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// contributors may be used to endorse or promote products derived
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// from this software without specific prior written permission.
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//
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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// Flags: --allow-natives-syntax
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function divp4(x) {
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return x / 4;
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}
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divp4(8);
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divp4(8);
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%OptimizeFunctionOnNextCall(divp4);
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assertEquals(2, divp4(8));
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assertEquals(0.5, divp4(2));
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function divn4(x) {
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return x / (-4);
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}
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divn4(8);
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divn4(8);
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%OptimizeFunctionOnNextCall(divn4);
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assertEquals(-2, divn4(8));
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// Check for (0 / -x)
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assertEquals(-0, divn4(0));
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// Check for (kMinInt / -1)
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function divn1(x) {
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return x / (-1);
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}
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var two_31 = 1 << 31;
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divn1(2);
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divn1(2);
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%OptimizeFunctionOnNextCall(divn1);
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assertEquals(-2, divn1(2));
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assertEquals(-two_31, divn1(two_31));
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//Check for truncating to int32 case
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function divp4t(x) {
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return (x / 4) | 0;
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}
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divp4t(8);
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divp4t(8);
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%OptimizeFunctionOnNextCall(divp4t);
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assertEquals(-1, divp4t(-5));
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assertEquals(1, divp4t(5));
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assertOptimized(divp4t);
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function divn4t(x) {
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return (x / -4) | 0;
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}
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divn4t(8);
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divn4t(8);
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%OptimizeFunctionOnNextCall(divn4t);
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assertEquals(1, divn4t(-5));
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assertEquals(-1, divn4t(5));
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assertOptimized(divn4t);
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// Check kMinInt case.
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function div_by_two(x) {
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return (x / 2) | 0;
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}
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div_by_two(12);
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div_by_two(34);
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%OptimizeFunctionOnNextCall(div_by_two);
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div_by_two(56);
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assertEquals(-(1 << 30), div_by_two(1 << 31));
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