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[InstCombine] allow ashr/lshr demanded bits folds with splat constants 

llvm-svn: 300888
This commit is contained in:
Sanjay Patel 2017-04-20 20:59:02 +00:00
parent 3106fc476c
commit fb5b3e773a
3 changed files with 19 additions and 18 deletions
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25
llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
View File

@ -503,9 +503,9 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
KnownZero.setLowBits(ShiftAmt);
}
break;
case Instruction::LShr:
// For a logical shift right
if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
case Instruction::LShr: {
const APInt *SA;
if (match(I->getOperand(1), m_APInt(SA))) {
uint64_t ShiftAmt = SA->getLimitedValue(BitWidth-1);
// Unsigned shift right.
@ -526,7 +526,8 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
KnownZero.setHighBits(ShiftAmt); // high bits known zero.
}
break;
case Instruction::AShr:
}
case Instruction::AShr: {
// If this is an arithmetic shift right and only the low-bit is set, we can
// always convert this into a logical shr, even if the shift amount is
// variable. The low bit of the shift cannot be an input sign bit unless
@ -543,12 +544,13 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
if (DemandedMask.isSignMask())
return I->getOperand(0);
if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
const APInt *SA;
if (match(I->getOperand(1), m_APInt(SA))) {
uint32_t ShiftAmt = SA->getLimitedValue(BitWidth-1);
// Signed shift right.
APInt DemandedMaskIn(DemandedMask.shl(ShiftAmt));
// If any of the "high bits" are demanded, we should set the sign bit as
// If any of the high bits are demanded, we should set the sign bit as
// demanded.
if (DemandedMask.countLeadingZeros() <= ShiftAmt)
DemandedMaskIn.setSignBit();
@ -561,6 +563,7 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
if (SimplifyDemandedBits(I, 0, DemandedMaskIn, KnownZero, KnownOne,
Depth + 1))
return I;
assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?");
// Compute the new bits that are at the top now.
APInt HighBits(APInt::getHighBitsSet(BitWidth, ShiftAmt));
@ -576,16 +579,16 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
// are demanded, turn this into an unsigned shift right.
if (BitWidth <= ShiftAmt || KnownZero[BitWidth-ShiftAmt-1] ||
(HighBits & ~DemandedMask) == HighBits) {
// Perform the logical shift right.
BinaryOperator *NewVal = BinaryOperator::CreateLShr(I->getOperand(0),
SA, I->getName());
NewVal->setIsExact(cast<BinaryOperator>(I)->isExact());
return InsertNewInstWith(NewVal, *I);
BinaryOperator *LShr = BinaryOperator::CreateLShr(I->getOperand(0),
I->getOperand(1));
LShr->setIsExact(cast<BinaryOperator>(I)->isExact());
return InsertNewInstWith(LShr, *I);
} else if ((KnownOne & SignMask) != 0) { // New bits are known one.
KnownOne |= HighBits;
}
}
break;
}
case Instruction::SRem:
if (ConstantInt *Rem = dyn_cast<ConstantInt>(I->getOperand(1))) {
// X % -1 demands all the bits because we don't want to introduce

6
llvm/test/Transforms/InstCombine/shift.ll
View File

@ -1270,8 +1270,7 @@ define <2 x i64> @test_64_splat_vec(<2 x i32> %t) {
define <2 x i8> @ashr_demanded_bits_splat(<2 x i8> %x) {
; CHECK-LABEL: @ashr_demanded_bits_splat(
; CHECK-NEXT: [[AND:%.*]] = and <2 x i8> %x, <i8 -128, i8 -128>
; CHECK-NEXT: [[SHR:%.*]] = ashr exact <2 x i8> [[AND]], <i8 7, i8 7>
; CHECK-NEXT: [[SHR:%.*]] = ashr <2 x i8> %x, <i8 7, i8 7>
; CHECK-NEXT: ret <2 x i8> [[SHR]]
;
%and = and <2 x i8> %x, <i8 128, i8 128>
@ -1281,8 +1280,7 @@ define <2 x i8> @ashr_demanded_bits_splat(<2 x i8> %x) {
define <2 x i8> @lshr_demanded_bits_splat(<2 x i8> %x) {
; CHECK-LABEL: @lshr_demanded_bits_splat(
; CHECK-NEXT: [[AND:%.*]] = and <2 x i8> %x, <i8 -128, i8 -128>
; CHECK-NEXT: [[SHR:%.*]] = lshr exact <2 x i8> [[AND]], <i8 7, i8 7>
; CHECK-NEXT: [[SHR:%.*]] = lshr <2 x i8> %x, <i8 7, i8 7>
; CHECK-NEXT: ret <2 x i8> [[SHR]]
;
%and = and <2 x i8> %x, <i8 128, i8 128>

6
llvm/test/Transforms/InstCombine/vector-casts.ll
View File

@ -15,9 +15,9 @@ define <2 x i1> @test1(<2 x i64> %a) {
; The ashr turns into an lshr.
define <2 x i64> @test2(<2 x i64> %a) {
; CHECK-LABEL: @test2(
; CHECK-NEXT: [[B:%.*]] = and <2 x i64> %a, <i64 65535, i64 65535>
; CHECK-NEXT: [[T:%.*]] = lshr <2 x i64> [[B]], <i64 1, i64 1>
; CHECK-NEXT: ret <2 x i64> [[T]]
; CHECK-NEXT: [[B:%.*]] = and <2 x i64> %a, <i64 65534, i64 65534>
; CHECK-NEXT: [[TMP1:%.*]] = lshr exact <2 x i64> [[B]], <i64 1, i64 1>
; CHECK-NEXT: ret <2 x i64> [[TMP1]]
;
%b = and <2 x i64> %a, <i64 65535, i64 65535>
%t = ashr <2 x i64> %b, <i64 1, i64 1>