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[InstCombine] Improve folding of mul + icmp
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This diff adds folds for patterns like X * A < B
where A, B are constants and "mul" has either "nsw" or "nuw".
(to address #56563).

Test plan:
1/ ninja check-llvm check-clang
2/ Bootstrapped LLVM/Clang pass tests

Differential revision: https://reviews.llvm.org/D130039
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@alexander-shaposhnikov
alexander-shaposhnikov committed Jul 22, 2022
1 parent 8f0c901 commit 2ebfda2
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Showing 3 changed files with 51 additions and 27 deletions.
40 changes: 38 additions & 2 deletions llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
View file
Expand Up @@ -2002,9 +2002,12 @@ Instruction *InstCombinerImpl::foldICmpMulConstant(ICmpInst &Cmp,
Constant::getNullValue(Mul->getType()));
}

if (MulC->isZero() || !(Mul->hasNoSignedWrap() || Mul->hasNoUnsignedWrap()))
return nullptr;

// If the multiply does not wrap, try to divide the compare constant by the
// multiplication factor.
if (Cmp.isEquality() && !MulC->isZero()) {
if (Cmp.isEquality()) {
// (mul nsw X, MulC) == C --> X == C /s MulC
if (Mul->hasNoSignedWrap() && C.srem(*MulC).isZero()) {
Constant *NewC = ConstantInt::get(Mul->getType(), C.sdiv(*MulC));
Expand All @@ -2017,7 +2020,40 @@ Instruction *InstCombinerImpl::foldICmpMulConstant(ICmpInst &Cmp,
}
}

return nullptr;
Constant *NewC = nullptr;

// FIXME: Add assert that Pred is not equal to ICMP_SGE, ICMP_SLE,
// ICMP_UGE, ICMP_ULE.

if (Mul->hasNoSignedWrap()) {
if (MulC->isNegative()) {
// MININT / -1 --> overflow.
if (C.isMinSignedValue() && MulC->isAllOnes())
return nullptr;
Pred = ICmpInst::getSwappedPredicate(Pred);
}
if (Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_SGE)
NewC = ConstantInt::get(
Mul->getType(),
APIntOps::RoundingSDiv(C, *MulC, APInt::Rounding::UP));
if (Pred == ICmpInst::ICMP_SLE || Pred == ICmpInst::ICMP_SGT)
NewC = ConstantInt::get(
Mul->getType(),
APIntOps::RoundingSDiv(C, *MulC, APInt::Rounding::DOWN));
}

if (Mul->hasNoUnsignedWrap()) {
if (Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_UGE)
NewC = ConstantInt::get(
Mul->getType(),
APIntOps::RoundingUDiv(C, *MulC, APInt::Rounding::UP));
if (Pred == ICmpInst::ICMP_ULE || Pred == ICmpInst::ICMP_UGT)
NewC = ConstantInt::get(
Mul->getType(),
APIntOps::RoundingUDiv(C, *MulC, APInt::Rounding::DOWN));
}

return NewC ? new ICmpInst(Pred, Mul->getOperand(0), NewC) : nullptr;
}

/// Fold icmp (shl 1, Y), C.
Expand Down
2 changes: 1 addition & 1 deletion llvm/test/Analysis/ValueTracking/known-power-of-two-urem.ll
View file
Expand Up @@ -159,7 +159,7 @@ define i64 @known_power_of_two_urem_loop_mul_negative(i64 %size, i64 %a) {
; CHECK-NEXT: [[UREM:%.*]] = urem i64 [[SIZE:%.*]], [[PHI]]
; CHECK-NEXT: [[ADD]] = add nuw i64 [[SUM]], [[UREM]]
; CHECK-NEXT: [[I]] = mul nuw i64 [[PHI]], 3
; CHECK-NEXT: [[ICMP:%.*]] = icmp ult i64 [[I]], 100000000
; CHECK-NEXT: [[ICMP:%.*]] = icmp ult i64 [[PHI]], 33333334
; CHECK-NEXT: br i1 [[ICMP]], label [[FOR_BODY]], label [[FOR_END:%.*]]
; CHECK: for.end:
; CHECK-NEXT: ret i64 [[SUM]]
Expand Down
36 changes: 12 additions & 24 deletions llvm/test/Transforms/InstCombine/icmp-mul.ll
View file
Expand Up @@ -7,8 +7,7 @@ declare void @use(i8)

define i1 @slt_positive_multip_rem_zero(i8 %x) {
; CHECK-LABEL: @slt_positive_multip_rem_zero(
; CHECK-NEXT: [[A:%.*]] = mul nsw i8 [[X:%.*]], 7
; CHECK-NEXT: [[B:%.*]] = icmp slt i8 [[A]], 21
; CHECK-NEXT: [[B:%.*]] = icmp slt i8 [[X:%.*]], 3
; CHECK-NEXT: ret i1 [[B]]
;
%a = mul nsw i8 %x, 7
Expand All @@ -18,8 +17,7 @@ define i1 @slt_positive_multip_rem_zero(i8 %x) {

define i1 @slt_negative_multip_rem_zero(i8 %x) {
; CHECK-LABEL: @slt_negative_multip_rem_zero(
; CHECK-NEXT: [[A:%.*]] = mul nsw i8 [[X:%.*]], -7
; CHECK-NEXT: [[B:%.*]] = icmp slt i8 [[A]], 21
; CHECK-NEXT: [[B:%.*]] = icmp sgt i8 [[X:%.*]], -3
; CHECK-NEXT: ret i1 [[B]]
;
%a = mul nsw i8 %x, -7
Expand All @@ -29,8 +27,7 @@ define i1 @slt_negative_multip_rem_zero(i8 %x) {

define i1 @slt_positive_multip_rem_nz(i8 %x) {
; CHECK-LABEL: @slt_positive_multip_rem_nz(
; CHECK-NEXT: [[A:%.*]] = mul nsw i8 [[X:%.*]], 5
; CHECK-NEXT: [[B:%.*]] = icmp slt i8 [[A]], 21
; CHECK-NEXT: [[B:%.*]] = icmp slt i8 [[X:%.*]], 5
; CHECK-NEXT: ret i1 [[B]]
;
%a = mul nsw i8 %x, 5
Expand All @@ -40,8 +37,7 @@ define i1 @slt_positive_multip_rem_nz(i8 %x) {

define i1 @ult_rem_zero(i8 %x) {
; CHECK-LABEL: @ult_rem_zero(
; CHECK-NEXT: [[A:%.*]] = mul nuw i8 [[X:%.*]], 7
; CHECK-NEXT: [[B:%.*]] = icmp ult i8 [[A]], 21
; CHECK-NEXT: [[B:%.*]] = icmp ult i8 [[X:%.*]], 3
; CHECK-NEXT: ret i1 [[B]]
;
%a = mul nuw i8 %x, 7
Expand All @@ -51,8 +47,7 @@ define i1 @ult_rem_zero(i8 %x) {

define i1 @ult_rem_nz(i8 %x) {
; CHECK-LABEL: @ult_rem_nz(
; CHECK-NEXT: [[A:%.*]] = mul nuw i8 [[X:%.*]], 5
; CHECK-NEXT: [[B:%.*]] = icmp ult i8 [[A]], 21
; CHECK-NEXT: [[B:%.*]] = icmp ult i8 [[X:%.*]], 5
; CHECK-NEXT: ret i1 [[B]]
;
%a = mul nuw i8 %x, 5
Expand All @@ -64,8 +59,7 @@ define i1 @ult_rem_nz(i8 %x) {

define i1 @sgt_positive_multip_rem_zero(i8 %x) {
; CHECK-LABEL: @sgt_positive_multip_rem_zero(
; CHECK-NEXT: [[A:%.*]] = mul nsw i8 [[X:%.*]], 7
; CHECK-NEXT: [[B:%.*]] = icmp sgt i8 [[A]], 21
; CHECK-NEXT: [[B:%.*]] = icmp sgt i8 [[X:%.*]], 3
; CHECK-NEXT: ret i1 [[B]]
;
%a = mul nsw i8 %x, 7
Expand All @@ -75,8 +69,7 @@ define i1 @sgt_positive_multip_rem_zero(i8 %x) {

define i1 @sgt_negative_multip_rem_zero(i8 %x) {
; CHECK-LABEL: @sgt_negative_multip_rem_zero(
; CHECK-NEXT: [[A:%.*]] = mul nsw i8 [[X:%.*]], -7
; CHECK-NEXT: [[B:%.*]] = icmp sgt i8 [[A]], 21
; CHECK-NEXT: [[B:%.*]] = icmp slt i8 [[X:%.*]], -3
; CHECK-NEXT: ret i1 [[B]]
;
%a = mul nsw i8 %x, -7
Expand All @@ -86,8 +79,7 @@ define i1 @sgt_negative_multip_rem_zero(i8 %x) {

define i1 @sgt_positive_multip_rem_nz(i8 %x) {
; CHECK-LABEL: @sgt_positive_multip_rem_nz(
; CHECK-NEXT: [[A:%.*]] = mul nsw i8 [[X:%.*]], 5
; CHECK-NEXT: [[B:%.*]] = icmp sgt i8 [[A]], 21
; CHECK-NEXT: [[B:%.*]] = icmp sgt i8 [[X:%.*]], 4
; CHECK-NEXT: ret i1 [[B]]
;
%a = mul nsw i8 %x, 5
Expand All @@ -97,8 +89,7 @@ define i1 @sgt_positive_multip_rem_nz(i8 %x) {

define i1 @ugt_rem_zero(i8 %x) {
; CHECK-LABEL: @ugt_rem_zero(
; CHECK-NEXT: [[A:%.*]] = mul nuw i8 [[X:%.*]], 7
; CHECK-NEXT: [[B:%.*]] = icmp ugt i8 [[A]], 21
; CHECK-NEXT: [[B:%.*]] = icmp ugt i8 [[X:%.*]], 3
; CHECK-NEXT: ret i1 [[B]]
;
%a = mul nuw i8 %x, 7
Expand All @@ -108,8 +99,7 @@ define i1 @ugt_rem_zero(i8 %x) {

define i1 @ugt_rem_nz(i8 %x) {
; CHECK-LABEL: @ugt_rem_nz(
; CHECK-NEXT: [[A:%.*]] = mul nuw i8 [[X:%.*]], 5
; CHECK-NEXT: [[B:%.*]] = icmp ugt i8 [[A]], 21
; CHECK-NEXT: [[B:%.*]] = icmp ugt i8 [[X:%.*]], 4
; CHECK-NEXT: ret i1 [[B]]
;
%a = mul nuw i8 %x, 5
Expand Down Expand Up @@ -866,13 +856,11 @@ define i1 @splat_mul_known_lz(i32 %x) {
ret i1 %r
}

; Negative test - the 33rd bit could be set.
; The 33rd bit can only be set when MSB of x is set.

define i1 @splat_mul_unknown_lz(i32 %x) {
; CHECK-LABEL: @splat_mul_unknown_lz(
; CHECK-NEXT: [[Z:%.*]] = zext i32 [[X:%.*]] to i128
; CHECK-NEXT: [[M:%.*]] = mul nuw nsw i128 [[Z]], 18446744078004518913
; CHECK-NEXT: [[R:%.*]] = icmp ult i128 [[M]], 39614081257132168796771975168
; CHECK-NEXT: [[R:%.*]] = icmp sgt i32 [[X:%.*]], -1
; CHECK-NEXT: ret i1 [[R]]
;
%z = zext i32 %x to i128
Expand Down

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