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[SCEV] Better preserve wrapping info in SimplifyICmpOperands for UGE. #144404

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Merged
merged 2 commits into from
Jun 17, 2025
Merged

[SCEV] Better preserve wrapping info in SimplifyICmpOperands for UGE. #144404

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f70bb3d
[SCEV] Better preserve wrapping info in SimplifyICmpOperands for UGE.
fhahn Jun 16, 2025
3baa310
!fixup address comments, thanks
fhahn Jun 17, 2025
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11 changes: 10 additions & 1 deletion llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -10892,7 +10892,12 @@ bool ScalarEvolution::SimplifyICmpOperands(CmpPredicate &Pred, const SCEV *&LHS,
}
break;
case ICmpInst::ICMP_UGE:
if (!getUnsignedRangeMin(RHS).isMinValue()) {
// If RHS is an op we can fold the -1, try that first.
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Suggested change
// If RHS is an op we can fold the -1, try that first.
// If RHS is an op we can fold the -1, try that first.
// Otherwise prefer LHS to preserve the nuw flag.

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Added thanks

// Otherwise prefer LHS to preserve the nuw flag.
if ((isa<SCEVConstant>(RHS) ||
(isa<SCEVAddExpr, SCEVAddRecExpr>(RHS) &&
isa<SCEVConstant>(cast<SCEVNAryExpr>(RHS)->getOperand(0)))) &&
!getUnsignedRangeMin(RHS).isMinValue()) {
RHS = getAddExpr(getConstant(RHS->getType(), (uint64_t)-1, true), RHS);
Pred = ICmpInst::ICMP_UGT;
Changed = true;
Expand All @@ -10901,6 +10906,10 @@ bool ScalarEvolution::SimplifyICmpOperands(CmpPredicate &Pred, const SCEV *&LHS,
SCEV::FlagNUW);
Pred = ICmpInst::ICMP_UGT;
Changed = true;
} else if (!getUnsignedRangeMin(RHS).isMinValue()) {
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I really don't like the code structure here, but I don't have a good suggestion on improving it either...

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Same here, but I also didn't find anything better :(

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@antoniofrighetto antoniofrighetto Jun 17, 2025 •
edited
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@nikic Not sure if this could be an acceptable alternative: https://gist.github.com/antoniofrighetto/83c188d5ad0a4988eec8d4c30684dafe. It started relatively well, it didn't really end up as good as originally envisioned (NFCI, as RHS is checked first for UGE, might not want this as we probably should preserve the flag).

RHS = getAddExpr(getConstant(RHS->getType(), (uint64_t)-1, true), RHS);
Pred = ICmpInst::ICMP_UGT;
Changed = true;
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Does unconditionally swapping the cases cause regressions?

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Yep, we would loose flags in test_simplifycompare_rhs_constant and test_simplifycompare_rhs_addrec, but gain some in test_simplifycompare_rhs_not_constant2

The current version doesn't have any regression on a large test-set of C/C++ workloads.

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llvm-opt-benchmark results are available here: https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2452/files

}
break;
default:
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9 changes: 3 additions & 6 deletions llvm/test/Transforms/IndVarSimplify/simplify-icmp-operands-order.ll
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Expand Up @@ -53,15 +53,12 @@ loop.latch:

define void @test_simplifycompare_rhs_not_constant1() {
; CHECK-LABEL: define void @test_simplifycompare_rhs_not_constant1() {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: [[P:%.*]] = alloca i64, align 8
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
; CHECK-NEXT: [[PTR_IV:%.*]] = phi ptr [ [[P]], %[[ENTRY]] ], [ [[PTR_IV_NEXT:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[PTR_IV_NEXT]] = getelementptr i8, ptr [[PTR_IV]], i64 -8
; CHECK-NEXT: call void @use(ptr [[PTR_IV]])
; CHECK-NEXT: [[EC:%.*]] = icmp ult ptr [[PTR_IV_NEXT]], [[P]]
; CHECK-NEXT: br i1 [[EC]], label %[[EXIT:.*]], label %[[LOOP]]
; CHECK-NEXT: call void @use(ptr [[P]])
; CHECK-NEXT: br i1 true, label %[[EXIT:.*]], label %[[LOOP]]
; CHECK: [[EXIT]]:
; CHECK-NEXT: ret void
;
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