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Bug 916539 - ARM: Integer multiplication by a constant negative power of two is incorrectly optimized to a shift operation. r=mjrosenb

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This commit is contained in:
Douglas Crosher 2013-09-15 21:53:58 +10:00
parent a7190bdd2f
commit 173be546af
1 changed files with 35 additions and 32 deletions
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67
js/src/jit/arm/CodeGenerator-arm.cpp
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@ -420,41 +420,44 @@ CodeGeneratorARM::visitMulI(LMulI *ins)
break;
default: {
bool handled = false;
if (!mul->canOverflow()) {
// If it cannot overflow, we can do lots of optimizations
Register src = ToRegister(lhs);
uint32_t shift = FloorLog2(constant);
uint32_t rest = constant - (1 << shift);
// See if the constant has one bit set, meaning it can be encoded as a bitshift
if ((1 << shift) == constant) {
masm.ma_lsl(Imm32(shift), src, ToRegister(dest));
handled = true;
} else {
// If the constant cannot be encoded as (1<<C1), see if it can be encoded as
// (1<<C1) | (1<<C2), which can be computed using an add and a shift
uint32_t shift_rest = FloorLog2(rest);
if ((1u << shift_rest) == rest) {
masm.as_add(ToRegister(dest), src, lsl(src, shift-shift_rest));
if (shift_rest != 0)
masm.ma_lsl(Imm32(shift_rest), ToRegister(dest), ToRegister(dest));
if (constant > 0) {
// Try shift and add sequences for a positive constant.
if (!mul->canOverflow()) {
// If it cannot overflow, we can do lots of optimizations
Register src = ToRegister(lhs);
uint32_t shift = FloorLog2(constant);
uint32_t rest = constant - (1 << shift);
// See if the constant has one bit set, meaning it can be encoded as a bitshift
if ((1 << shift) == constant) {
masm.ma_lsl(Imm32(shift), src, ToRegister(dest));
handled = true;
} else {
// If the constant cannot be encoded as (1<<C1), see if it can be encoded as
// (1<<C1) | (1<<C2), which can be computed using an add and a shift
uint32_t shift_rest = FloorLog2(rest);
if ((1u << shift_rest) == rest) {
masm.as_add(ToRegister(dest), src, lsl(src, shift-shift_rest));
if (shift_rest != 0)
masm.ma_lsl(Imm32(shift_rest), ToRegister(dest), ToRegister(dest));
handled = true;
}
}
} else if (ToRegister(lhs) != ToRegister(dest)) {
// To stay on the safe side, only optimize things that are a
// power of 2.
uint32_t shift = FloorLog2(constant);
if ((1 << shift) == constant) {
// dest = lhs * pow(2,shift)
masm.ma_lsl(Imm32(shift), ToRegister(lhs), ToRegister(dest));
// At runtime, check (lhs == dest >> shift), if this does not hold,
// some bits were lost due to overflow, and the computation should
// be resumed as a double.
masm.as_cmp(ToRegister(lhs), asr(ToRegister(dest), shift));
c = Assembler::NotEqual;
handled = true;
}
}
} else if (ToRegister(lhs) != ToRegister(dest)) {
// To stay on the safe side, only optimize things that are a
// power of 2.
uint32_t shift = FloorLog2(constant);
if ((1 << shift) == constant) {
// dest = lhs * pow(2,shift)
masm.ma_lsl(Imm32(shift), ToRegister(lhs), ToRegister(dest));
// At runtime, check (lhs == dest >> shift), if this does not hold,
// some bits were lost due to overflow, and the computation should
// be resumed as a double.
masm.as_cmp(ToRegister(lhs), asr(ToRegister(dest), shift));
c = Assembler::NotEqual;
handled = true;
}
}
if (!handled) {