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Bit width analysis for avoiding conversion of bitops to unsigned.

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R=sigmund@google.com

Committed: a5443a8b0d

Review URL: https://codereview.chromium.org/2507883002 .

Reverted:  08854375b3
This commit is contained in:
Stephen Adams 2016-11-18 10:26:07 -08:00
parent 5595b9698d
commit 7b12dbc665
1 changed files with 52 additions and 0 deletions
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52
pkg/compiler/lib/src/ssa/codegen.dart
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@ -2,6 +2,7 @@
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
import 'dart:math' as math;
import '../common.dart';
import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
import '../common/tasks.dart' show CompilerTask;
@ -210,8 +211,59 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
return false;
}
// Returns the number of bits occupied by the value computed by [instruction].
// Returns `32` if the value is negative or does not fit in a smaller number
// of bits.
int bitWidth(HInstruction instruction) {
const int MAX = 32;
int constant(HInstruction insn) {
if (insn is HConstant && insn.isConstantInteger()) {
IntConstantValue constant = insn.constant;
return constant.primitiveValue;
}
return null;
}
if (instruction.isConstantInteger()) {
int value = constant(instruction);
if (value < 0) return MAX;
if (value > ((1 << 31) - 1)) return MAX;
return value.bitLength;
}
if (instruction is HBitAnd) {
return math.min(bitWidth(instruction.left), bitWidth(instruction.right));
}
if (instruction is HBitOr || instruction is HBitXor) {
HBinaryBitOp bitOp = instruction;
int leftWidth = bitWidth(bitOp.left);
if (leftWidth == MAX) return MAX;
return math.max(leftWidth, bitWidth(bitOp.right));
}
if (instruction is HShiftLeft) {
int shiftCount = constant(instruction.right);
if (shiftCount == null || shiftCount < 0 || shiftCount > 31) return MAX;
int leftWidth = bitWidth(instruction.left);
int width = leftWidth + shiftCount;
return math.min(width, MAX);
}
if (instruction is HShiftRight) {
int shiftCount = constant(instruction.right);
if (shiftCount == null || shiftCount < 0 || shiftCount > 31) return MAX;
int leftWidth = bitWidth(instruction.left);
if (leftWidth >= MAX) return MAX;
return math.max(leftWidth - shiftCount, 0);
}
if (instruction is HAdd) {
return math.min(
1 + math.max(bitWidth(instruction.left), bitWidth(instruction.right)),
MAX);
}
return MAX;
}
bool requiresUintConversion(instruction) {
if (instruction.isUInt31(compiler)) return false;
if (bitWidth(instruction) <= 31) return false;
// If the result of a bit-operation is only used by other bit
// operations, we do not have to convert to an unsigned integer.
return hasNonBitOpUser(instruction, new Set<HPhi>());