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[SCEV] Try to prove no-wrap for AddRecs via BTC. #131538

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[SCEV] Try to prove no-wrap for AddRecs via BTC. #131538

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d52fbb7
[SCEV] Try to prove no-wrap for AddRecs via BTC
fhahn Mar 20, 2025
8676bce
!fixup address latest comments, thanks
fhahn Jul 21, 2025
126ac2d
!fixup remove check
fhahn Jul 23, 2025
5628fe4
!fixup remove signed case for now
fhahn Jul 30, 2025
db07e4a
!fixup remove unused Add
fhahn Jul 30, 2025
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33 changes: 33 additions & 0 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5090,6 +5090,33 @@ ScalarEvolution::proveNoWrapViaConstantRanges(const SCEVAddRecExpr *AR) {
return Result;
}

/// Return true if \p AR is known to not wrap via the loop's backedge-taken
/// count.
static SCEV::NoWrapFlags proveNoWrapViaBTC(const SCEVAddRecExpr *AR,
ScalarEvolution &SE) {
SCEV::NoWrapFlags Result = SCEV::FlagAnyWrap;
if (AR->hasNoUnsignedWrap() && AR->hasNoSignedWrap())
return Result;

const Loop *L = AR->getLoop();
const SCEV *BTC = SE.getBackedgeTakenCount(L);
if (isa<SCEVCouldNotCompute>(BTC) || !AR->getStepRecurrence(SE)->isOne())
return Result;

Type *WTy = SE.getWiderType(AR->getType(), BTC->getType());
// If AR's type is wider than BTC, we can zero extend BTC, otherwise bail out.
if (WTy != AR->getType() || !WTy->isIntegerTy())
return Result;

const SCEV *ExtBTC = SE.getNoopOrZeroExtend(BTC, WTy);
// AR has a step of 1, it is NUW/NSW if Start + BTC >= Start.
if (!AR->hasNoUnsignedWrap() &&
SE.willNotOverflow(Instruction::Add, false, AR->getStart(), ExtBTC))
Result = ScalarEvolution::setFlags(Result, SCEV::FlagNUW);

return Result;
}

SCEV::NoWrapFlags
ScalarEvolution::proveNoSignedWrapViaInduction(const SCEVAddRecExpr *AR) {
SCEV::NoWrapFlags Result = AR->getNoWrapFlags();
Expand Down Expand Up @@ -5750,6 +5777,9 @@ const SCEV *ScalarEvolution::createSimpleAffineAddRec(PHINode *PN,
setNoWrapFlags(const_cast<SCEVAddRecExpr *>(AR),
(SCEV::NoWrapFlags)(AR->getNoWrapFlags() |
proveNoWrapViaConstantRanges(AR)));
setNoWrapFlags(const_cast<SCEVAddRecExpr *>(AR),
(SCEV::NoWrapFlags)(AR->getNoWrapFlags() |
proveNoWrapViaBTC(AR, *this)));
}

// We can add Flags to the post-inc expression only if we
Expand Down Expand Up @@ -5881,6 +5911,9 @@ const SCEV *ScalarEvolution::createAddRecFromPHI(PHINode *PN) {
setNoWrapFlags(const_cast<SCEVAddRecExpr *>(AR),
(SCEV::NoWrapFlags)(AR->getNoWrapFlags() |
proveNoWrapViaConstantRanges(AR)));
setNoWrapFlags(const_cast<SCEVAddRecExpr *>(AR),
(SCEV::NoWrapFlags)(AR->getNoWrapFlags() |
proveNoWrapViaBTC(AR, *this)));
}

// We can add Flags to the post-inc expression only if we
Expand Down
2 changes: 1 addition & 1 deletion llvm/test/Analysis/ScalarEvolution/addrec-computed-during-addrec-calculation.ll
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Expand Up @@ -20,7 +20,7 @@ define void @test(ptr %p) {
; CHECK-NEXT: %iv2.ext = sext i32 %iv2 to i64
; CHECK-NEXT: --> (sext i32 {%iv,+,1}<%loop2> to i64) U: [-2147483648,2147483648) S: [-2147483648,2147483648) Exits: <<Unknown>> LoopDispositions: { %loop.header: Variant, %loop2: Computable, %loop3: Invariant }
; CHECK-NEXT: %iv3 = phi i64 [ %iv2.ext, %loop2.end ], [ %iv3.next, %loop3 ]
; CHECK-NEXT: --> {(sext i32 {%iv,+,1}<%loop2> to i64),+,1}<nsw><%loop3> U: [-2147483648,2147483648) S: [-2147483648,2147483648) Exits: (sext i32 {%iv,+,1}<%loop2> to i64) LoopDispositions: { %loop3: Computable, %loop.header: Variant }
; CHECK-NEXT: --> {(sext i32 {%iv,+,1}<%loop2> to i64),+,1}<nuw><nsw><%loop3> U: [-2147483648,2147483648) S: [-2147483648,2147483648) Exits: (sext i32 {%iv,+,1}<%loop2> to i64) LoopDispositions: { %loop3: Computable, %loop.header: Variant }
; CHECK-NEXT: %iv3.next = add nsw i64 %iv3, 1
; CHECK-NEXT: --> {(1 + (sext i32 {%iv,+,1}<%loop2> to i64))<nsw>,+,1}<nsw><%loop3> U: [-2147483647,2147483649) S: [-2147483647,2147483649) Exits: (1 + (sext i32 {%iv,+,1}<%loop2> to i64))<nsw> LoopDispositions: { %loop3: Computable, %loop.header: Variant }
; CHECK-NEXT: %iv.next = trunc i64 %iv3 to i32
Expand Down
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