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Bug 976110 - Part 0: Optimize division and modulus by negative powers of two; r=sunfish

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This commit is contained in:
Mauricio Collares Neto 2014-04-19 10:37:49 -07:00
parent a80cd06599
commit 0528a958fb
4 changed files with 73 additions and 31 deletions
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19
js/src/jit-test/tests/basic/testDivModWithIntMin.js Normal file
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@ -0,0 +1,19 @@
var intMin = -2147483648;
assertEq(intMin % (-2147483648), -0);
assertEq(intMin % (-3), -2);
assertEq(intMin % (-1), -0);
assertEq(intMin % 1, -0);
assertEq(intMin % 3, -2);
assertEq(intMin % 10, -8);
assertEq(intMin % 2147483647, -1);
assertEq((-2147483648) % (-2147483648), -0);
for (var i = -10; i <= 10; ++i)
assertEq(i % (-2147483648), i);
assertEq(2147483647 % (-2147483648), 2147483647);
assertEq((-2147483648) / (-2147483648), 1);
assertEq(0 / (-2147483648), -0);
assertEq((-2147483648) / (-1), 2147483648);
assertEq((-0) / (-2147483648), 0);

43
js/src/jit/shared/CodeGenerator-x86-shared.cpp
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@ -869,14 +869,23 @@ CodeGeneratorX86Shared::visitDivPowTwoI(LDivPowTwoI *ins)
{
Register lhs = ToRegister(ins->numerator());
mozilla::DebugOnly<Register> output = ToRegister(ins->output());
int32_t shift = ins->shift();
bool negativeDivisor = ins->negativeDivisor();
MDiv *mir = ins->mir();
// We use defineReuseInput so these should always be the same, which is
// convenient since all of our instructions here are two-address.
JS_ASSERT(lhs == output);
if (!mir->isTruncated() && negativeDivisor) {
// 0 divided by a negative number must return a double.
masm.testl(lhs, lhs);
if (!bailoutIf(Assembler::Zero, ins->snapshot()))
return false;
}
if (shift != 0) {
MDiv *mir = ins->mir();
if (!mir->isTruncated()) {
// If the remainder is != 0, bailout since this must be a double.
masm.testl(lhs, Imm32(UINT32_MAX >> (32 - shift)));
@ -884,24 +893,26 @@ CodeGeneratorX86Shared::visitDivPowTwoI(LDivPowTwoI *ins)
return false;
}
if (!mir->canBeNegativeDividend()) {
// Numerator is unsigned, so needs no adjusting. Do the shift.
masm.sarl(Imm32(shift), lhs);
return true;
}
// Adjust the value so that shifting produces a correctly rounded result
// when the numerator is negative. See 10-1 "Signed Division by a Known
// Power of 2" in Henry S. Warren, Jr.'s Hacker's Delight.
if (mir->canBeNegativeDividend()) {
Register lhsCopy = ToRegister(ins->numeratorCopy());
JS_ASSERT(lhsCopy != lhs);
if (shift > 1)
masm.sarl(Imm32(31), lhs);
masm.shrl(Imm32(32 - shift), lhs);
masm.addl(lhsCopy, lhs);
}
// Do the shift.
masm.sarl(Imm32(shift), lhs);
if (negativeDivisor)
masm.negl(lhs);
} else if (shift == 0 && negativeDivisor) {
// INT32_MIN / -1 overflows.
masm.negl(lhs);
if (!mir->isTruncated() && !bailoutIf(Assembler::Overflow, ins->snapshot()))
return false;
}
return true;
@ -1012,7 +1023,7 @@ CodeGeneratorX86Shared::visitModPowTwoI(LModPowTwoI *ins)
masm.branchTest32(Assembler::Signed, lhs, lhs, &negative);
}
masm.andl(Imm32((1 << shift) - 1), lhs);
masm.andl(Imm32((uint32_t(1) << shift) - 1), lhs);
if (ins->mir()->canBeNegativeDividend()) {
Label done;
@ -1020,11 +1031,19 @@ CodeGeneratorX86Shared::visitModPowTwoI(LModPowTwoI *ins)
// Negative numbers need a negate, bitmask, negate
masm.bind(&negative);
// visitModI has an overflow check here to catch INT_MIN % -1, but
// here the rhs is a power of 2, and cannot be -1, so the check is not generated.
// Unlike in the visitModI case, we are not computing the mod by means of a
// division. Therefore, the divisor = -1 case isn't problematic (the andl
// always returns 0, which is what we expect).
//
// The negl instruction overflows if lhs == INT32_MIN, but this is also not
// a problem: shift is at most 31, and so the andl also always returns 0.
masm.negl(lhs);
masm.andl(Imm32((1 << shift) - 1), lhs);
masm.andl(Imm32((uint32_t(1) << shift) - 1), lhs);
masm.negl(lhs);
// Since a%b has the same sign as b, and a is negative in this branch,
// an answer of 0 means the correct result is actually -0. Bail out.
if (!ins->mir()->isTruncated() && !bailoutIf(Assembler::Zero, ins->snapshot()))
return false;
masm.bind(&done);

8
js/src/jit/shared/LIR-x86-shared.h
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@ -47,12 +47,13 @@ class LDivI : public LBinaryMath<1>
class LDivPowTwoI : public LBinaryMath<0>
{
const int32_t shift_;
const bool negativeDivisor_;
public:
LIR_HEADER(DivPowTwoI)
LDivPowTwoI(const LAllocation &lhs, const LAllocation &lhsCopy, int32_t shift)
: shift_(shift)
LDivPowTwoI(const LAllocation &lhs, const LAllocation &lhsCopy, int32_t shift, bool negativeDivisor)
: shift_(shift), negativeDivisor_(negativeDivisor)
{
setOperand(0, lhs);
setOperand(1, lhsCopy);
@ -67,6 +68,9 @@ class LDivPowTwoI : public LBinaryMath<0>
int32_t shift() const {
return shift_;
}
bool negativeDivisor() const {
return negativeDivisor_;
}
MDiv *mir() const {
return mir_->toDiv();
}

22
js/src/jit/shared/Lowering-x86-shared.cpp
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@ -15,6 +15,7 @@
using namespace js;
using namespace js::jit;
using mozilla::Abs;
using mozilla::FloorLog2;
LTableSwitch *
@ -137,22 +138,21 @@ LIRGeneratorX86Shared::lowerDivI(MDiv *div)
if (div->rhs()->isConstant()) {
int32_t rhs = div->rhs()->toConstant()->value().toInt32();
// Check for division by a positive power of two, which is an easy and
// important case to optimize. Note that other optimizations are also
// possible; division by negative powers of two can be optimized in a
// similar manner as positive powers of two, and division by other
// constants can be optimized by a reciprocal multiplication technique.
int32_t shift = FloorLog2(rhs);
if (rhs > 0 && 1 << shift == rhs) {
// Check for division by a power of two, which is an easy and
// important case to optimize. Note that other optimizations
// are also possible: division by other constants can be
// optimized by a reciprocal multiplication technique.
int32_t shift = FloorLog2(Abs(rhs));
if (rhs != 0 && uint32_t(1) << shift == Abs(rhs)) {
LAllocation lhs = useRegisterAtStart(div->lhs());
LDivPowTwoI *lir;
if (!div->canBeNegativeDividend()) {
// Numerator is unsigned, so does not need adjusting.
lir = new(alloc()) LDivPowTwoI(lhs, lhs, shift);
lir = new(alloc()) LDivPowTwoI(lhs, lhs, shift, rhs < 0);
} else {
// Numerator is signed, and needs adjusting, and an extra
// lhs copy register is needed.
lir = new(alloc()) LDivPowTwoI(lhs, useRegister(div->lhs()), shift);
lir = new(alloc()) LDivPowTwoI(lhs, useRegister(div->lhs()), shift, rhs < 0);
}
if (div->fallible() && !assignSnapshot(lir, Bailout_BaselineInfo))
return false;
@ -175,8 +175,8 @@ LIRGeneratorX86Shared::lowerModI(MMod *mod)
if (mod->rhs()->isConstant()) {
int32_t rhs = mod->rhs()->toConstant()->value().toInt32();
int32_t shift = FloorLog2(rhs);
if (rhs > 0 && 1 << shift == rhs) {
int32_t shift = FloorLog2(Abs(rhs));
if (rhs != 0 && uint32_t(1) << shift == Abs(rhs)) {
LModPowTwoI *lir = new(alloc()) LModPowTwoI(useRegisterAtStart(mod->lhs()), shift);
if (mod->fallible() && !assignSnapshot(lir, Bailout_BaselineInfo))
return false;