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[InstCombine] Fold srem(X, PowerOf2) == C into (X & Mask) == C for positive C 

This diff extends InstCombinerImpl::foldICmpSRemConstant to handle the cases
srem(X, PowerOf2) == C and
srem(X, PowerOf2) != C
for positive C.
This addresses the issue https://github.com/llvm/llvm-project/issues/54650

Differential revision: https://reviews.llvm.org/D122942

Test plan: make check-all
This commit is contained in:
Alexander Shaposhnikov 2022-04-03 03:57:05 +00:00
parent 911cfcd7f5
commit 6cf10b7e6e
2 changed files with 16 additions and 4 deletions
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16
llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
View File

@ -2335,7 +2335,8 @@ Instruction *InstCombinerImpl::foldICmpSRemConstant(ICmpInst &Cmp,
// constant power-of-2 value:
// (X % pow2C) sgt/slt 0
const ICmpInst::Predicate Pred = Cmp.getPredicate();
if (Pred != ICmpInst::ICMP_SGT && Pred != ICmpInst::ICMP_SLT)
if (Pred != ICmpInst::ICMP_SGT && Pred != ICmpInst::ICMP_SLT &&
Pred != ICmpInst::ICMP_EQ && Pred != ICmpInst::ICMP_NE)
return nullptr;
// TODO: The one-use check is standard because we do not typically want to
@ -2345,7 +2346,15 @@ Instruction *InstCombinerImpl::foldICmpSRemConstant(ICmpInst &Cmp,
return nullptr;
const APInt *DivisorC;
if (!C.isZero() || !match(SRem->getOperand(1), m_Power2(DivisorC)))
if (!match(SRem->getOperand(1), m_Power2(DivisorC)))
return nullptr;
// For cmp_sgt/cmp_slt only zero valued C is handled.
// For cmp_eq/cmp_ne only positive valued C is handled.
if (((Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_SLT) &&
!C.isZero()) ||
((Pred == ICmpInst::ICMP_EQ || Pred == ICmpInst::ICMP_NE) &&
!C.isStrictlyPositive()))
return nullptr;
// Mask off the sign bit and the modulo bits (low-bits).
@ -2354,6 +2363,9 @@ Instruction *InstCombinerImpl::foldICmpSRemConstant(ICmpInst &Cmp,
Constant *MaskC = ConstantInt::get(Ty, SignMask | (*DivisorC - 1));
Value *And = Builder.CreateAnd(SRem->getOperand(0), MaskC);
if (Pred == ICmpInst::ICMP_EQ || Pred == ICmpInst::ICMP_NE)
return new ICmpInst(Pred, And, ConstantInt::get(Ty, C));
// For 'is positive?' check that the sign-bit is clear and at least 1 masked
// bit is set. Example:
// (i8 X % 32) s> 0 --> (X & 159) s> 0

4
llvm/test/Transforms/InstCombine/rem.ll
View File

@ -742,7 +742,7 @@ define i1 @test28(i32 %A) {
define i1 @positive_and_odd_eq(i32 %A) {
; CHECK-LABEL: @positive_and_odd_eq(
; CHECK-NEXT: [[B:%.*]] = srem i32 [[A:%.*]], 2
; CHECK-NEXT: [[B:%.*]] = and i32 [[A:%.*]], -2147483647
; CHECK-NEXT: [[C:%.*]] = icmp eq i32 [[B]], 1
; CHECK-NEXT: ret i1 [[C]]
;
@ -764,7 +764,7 @@ define i1 @negative_and_odd_eq(i32 %A) {
define i1 @positive_and_odd_ne(i32 %A) {
; CHECK-LABEL: @positive_and_odd_ne(
; CHECK-NEXT: [[B:%.*]] = srem i32 [[A:%.*]], 2
; CHECK-NEXT: [[B:%.*]] = and i32 [[A:%.*]], -2147483647
; CHECK-NEXT: [[C:%.*]] = icmp ne i32 [[B]], 1
; CHECK-NEXT: ret i1 [[C]]
;