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[SCEV] Generalize (C * A /u C) -> A fold to (C1 * A /u C2) -> C1/C2 * A. #157159

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Merged
merged 4 commits into from
Sep 9, 2025
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[SCEV] Generalize (C * A /u C) -> A fold to (C1 * A /u C2) -> C1/C2 * A. #157159

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962be5d
[SCEV] Generalize (C * A /u C) -> A fold to (C1 * A /u C2) -> C1/C2 * A.
fhahn Sep 5, 2025
2d050bf
!fixup update test
fhahn Sep 8, 2025
19ae5af
!fixup use logBase2.
fhahn Sep 8, 2025
81b23c3
!fixup address comments, thanks!
fhahn Sep 9, 2025
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15 changes: 9 additions & 6 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3216,13 +3216,16 @@ const SCEV *ScalarEvolution::getMulExpr(SmallVectorImpl<const SCEV *> &Ops,
};
}

// Try to fold (C * D /u C) -> D, if C is a power-of-2 and D is a multiple
// of C.
// Try to fold (C1 * D /u C2) -> C1/C2 * D, if C1 and C2 are powers-of-2,
// D is a multiple of C2, and C1 is a multiple of C1.
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Should probably say C1 is a multiple of C2?

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Yep, I adjusted this in a follow-up PR (#157555), but can also fix separately.

const SCEV *D;
if (match(Ops[1], m_scev_UDiv(m_SCEV(D), m_scev_Specific(LHSC))) &&
LHSC->getAPInt().isPowerOf2() &&
LHSC->getAPInt().logBase2() <= getMinTrailingZeros(D)) {
return D;
const SCEVConstant *C2;
const APInt &LHSV = LHSC->getAPInt();
if (LHSV.isPowerOf2() &&
match(Ops[1], m_scev_UDiv(m_SCEV(D), m_SCEVConstant(C2))) &&
C2->getAPInt().isPowerOf2() && LHSV.uge(C2->getAPInt()) &&
LHSV.logBase2() <= getMinTrailingZeros(D)) {
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FWIW, the actual underlying API for getMinTrailingZeros() is getConstantMultiple(), so this is probably easy to generalize to the non-pow2 case.

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Yep, initially I was checking for multiples directly, but started with just the power-of-2 cases, then extend it further

return getMulExpr(getUDivExpr(LHSC, C2), D);
}
}
}
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2 changes: 1 addition & 1 deletion llvm/test/Analysis/ScalarEvolution/mul-udiv-folds.ll
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Expand Up @@ -23,7 +23,7 @@ define void @udiv4_and_udiv2(i1 %c, ptr %A) {
; CHECK-NEXT: %gep.16 = getelementptr i16, ptr %A, i64 %iv
; CHECK-NEXT: --> {((2 * ((zext i32 %start to i64) /u 4))<nuw><nsw> + %A),+,2}<%loop> U: full-set S: full-set Exits: ((zext i32 %start to i64) + %A) LoopDispositions: { %loop: Computable }
; CHECK-NEXT: %gep.32 = getelementptr i32, ptr %A, i64 %iv
; CHECK-NEXT: --> {((zext i32 %start to i64) + %A),+,4}<%loop> U: full-set S: full-set Exits: ((zext i32 %start to i64) + (4 * ((zext i32 %start to i64) /u 2))<nuw><nsw> + (-4 * ((zext i32 %start to i64) /u 4))<nsw> + %A) LoopDispositions: { %loop: Computable }
; CHECK-NEXT: --> {((zext i32 %start to i64) + %A),+,4}<%loop> U: full-set S: full-set Exits: ((3 * (zext i32 %start to i64))<nuw><nsw> + (-4 * ((zext i32 %start to i64) /u 4))<nsw> + %A) LoopDispositions: { %loop: Computable }
; CHECK-NEXT: %gep.40 = getelementptr <{ i32, i8 }>, ptr %A, i64 %iv
; CHECK-NEXT: --> {((5 * ((zext i32 %start to i64) /u 4))<nuw><nsw> + %A),+,5}<%loop> U: full-set S: full-set Exits: ((5 * ((zext i32 %start to i64) /u 2))<nuw><nsw> + %A) LoopDispositions: { %loop: Computable }
; CHECK-NEXT: %gep.48 = getelementptr <{ i32, i16 }>, ptr %A, i64 %iv
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