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[InstCombine] visitTrunc - trunc (lshr (sext A), C) --> (ashr A, C) non-uniform support

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This came from @lebedev.ri's suggestion to use m_SpecificInt_ICMP for D88429 - since I was going to change the m_APInt to m_Constant for that patch I thought I would do it for the only other user of the APInt first.

I've added a ConstantExpr::getUMin helper - its trivial to add UMAX/SMIN/SMAX but thought I'd wait until we have use cases.

Differential Revision: https://reviews.llvm.org/D88475
This commit is contained in:
Simon Pilgrim 2020-09-29 14:45:30 +01:00
parent 83da9d3852
commit 11e1b3f795
4 changed files with 22 additions and 15 deletions
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1
include/llvm/IR/Constants.h
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@ -959,6 +959,7 @@ public:
static Constant *getAnd(Constant *C1, Constant *C2);
static Constant *getOr(Constant *C1, Constant *C2);
static Constant *getXor(Constant *C1, Constant *C2);
static Constant *getUMin(Constant *C1, Constant *C2);
static Constant *getShl(Constant *C1, Constant *C2,
bool HasNUW = false, bool HasNSW = false);
static Constant *getLShr(Constant *C1, Constant *C2, bool isExact = false);

5
lib/IR/Constants.cpp
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@ -2560,6 +2560,11 @@ Constant *ConstantExpr::getXor(Constant *C1, Constant *C2) {
return get(Instruction::Xor, C1, C2);
}
Constant *ConstantExpr::getUMin(Constant *C1, Constant *C2) {
Constant *Cmp = ConstantExpr::getICmp(CmpInst::ICMP_ULT, C1, C2);
return getSelect(Cmp, C1, C2);
}
Constant *ConstantExpr::getShl(Constant *C1, Constant *C2,
bool HasNUW, bool HasNSW) {
unsigned Flags = (HasNUW ? OverflowingBinaryOperator::NoUnsignedWrap : 0) |

19
lib/Transforms/InstCombine/InstCombineCasts.cpp
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@ -827,23 +827,30 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
return CastInst::CreateIntegerCast(Shift, DestTy, false);
}
const APInt *C;
if (match(Src, m_LShr(m_SExt(m_Value(A)), m_APInt(C)))) {
Constant *C;
if (match(Src, m_LShr(m_SExt(m_Value(A)), m_Constant(C)))) {
unsigned AWidth = A->getType()->getScalarSizeInBits();
unsigned MaxShiftAmt = SrcWidth - std::max(DestWidth, AWidth);
// If the shift is small enough, all zero bits created by the shift are
// removed by the trunc.
if (C->getZExtValue() <= MaxShiftAmt) {
// TODO: Support passing through undef shift amounts - these currently get
// clamped to MaxAmt.
if (match(C, m_SpecificInt_ICMP(ICmpInst::ICMP_ULE,
APInt(SrcWidth, MaxShiftAmt)))) {
// trunc (lshr (sext A), C) --> ashr A, C
if (A->getType() == DestTy) {
unsigned ShAmt = std::min((unsigned)C->getZExtValue(), DestWidth - 1);
return BinaryOperator::CreateAShr(A, ConstantInt::get(DestTy, ShAmt));
Constant *MaxAmt = ConstantInt::get(SrcTy, DestWidth - 1, false);
Constant *ShAmt = ConstantExpr::getUMin(C, MaxAmt);
ShAmt = ConstantExpr::getTrunc(ShAmt, A->getType());
return BinaryOperator::CreateAShr(A, ShAmt);
}
// The types are mismatched, so create a cast after shifting:
// trunc (lshr (sext A), C) --> sext/trunc (ashr A, C)
if (Src->hasOneUse()) {
unsigned ShAmt = std::min((unsigned)C->getZExtValue(), AWidth - 1);
Constant *MaxAmt = ConstantInt::get(SrcTy, AWidth - 1, false);
Constant *ShAmt = ConstantExpr::getUMin(C, MaxAmt);
ShAmt = ConstantExpr::getTrunc(ShAmt, A->getType());
Value *Shift = Builder.CreateAShr(A, ShAmt);
return CastInst::CreateIntegerCast(Shift, DestTy, true);
}

12
test/Transforms/InstCombine/cast.ll
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@ -1559,9 +1559,7 @@ define <2 x i8> @trunc_lshr_sext_uniform(<2 x i8> %A) {
define <2 x i8> @trunc_lshr_sext_uniform_undef(<2 x i8> %A) {
; ALL-LABEL: @trunc_lshr_sext_uniform_undef(
; ALL-NEXT: [[B:%.*]] = sext <2 x i8> [[A:%.*]] to <2 x i32>
; ALL-NEXT: [[C:%.*]] = lshr <2 x i32> [[B]], <i32 6, i32 undef>
; ALL-NEXT: [[D:%.*]] = trunc <2 x i32> [[C]] to <2 x i8>
; ALL-NEXT: [[D:%.*]] = ashr <2 x i8> [[A:%.*]], <i8 6, i8 7>
; ALL-NEXT: ret <2 x i8> [[D]]
;
%B = sext <2 x i8> %A to <2 x i32>
@ -1572,9 +1570,7 @@ define <2 x i8> @trunc_lshr_sext_uniform_undef(<2 x i8> %A) {
define <2 x i8> @trunc_lshr_sext_nonuniform(<2 x i8> %A) {
; ALL-LABEL: @trunc_lshr_sext_nonuniform(
; ALL-NEXT: [[B:%.*]] = sext <2 x i8> [[A:%.*]] to <2 x i32>
; ALL-NEXT: [[C:%.*]] = lshr <2 x i32> [[B]], <i32 6, i32 2>
; ALL-NEXT: [[D:%.*]] = trunc <2 x i32> [[C]] to <2 x i8>
; ALL-NEXT: [[D:%.*]] = ashr <2 x i8> [[A:%.*]], <i8 6, i8 2>
; ALL-NEXT: ret <2 x i8> [[D]]
;
%B = sext <2 x i8> %A to <2 x i32>
@ -1585,9 +1581,7 @@ define <2 x i8> @trunc_lshr_sext_nonuniform(<2 x i8> %A) {
define <3 x i8> @trunc_lshr_sext_nonuniform_undef(<3 x i8> %A) {
; ALL-LABEL: @trunc_lshr_sext_nonuniform_undef(
; ALL-NEXT: [[B:%.*]] = sext <3 x i8> [[A:%.*]] to <3 x i32>
; ALL-NEXT: [[C:%.*]] = lshr <3 x i32> [[B]], <i32 6, i32 2, i32 undef>
; ALL-NEXT: [[D:%.*]] = trunc <3 x i32> [[C]] to <3 x i8>
; ALL-NEXT: [[D:%.*]] = ashr <3 x i8> [[A:%.*]], <i8 6, i8 2, i8 7>
; ALL-NEXT: ret <3 x i8> [[D]]
;
%B = sext <3 x i8> %A to <3 x i32>