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[SCEV] Apply loop guards to End in computeMaxBECountForLT #116187

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[SCEV] Apply loop guards to End in computeMaxBECountForLT #116187

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f03b341
[SCEV] Apply loop guards to End computeMaxBECountForLT
lukel97 Nov 14, 2024
8c104fe
Update tests
lukel97 Nov 19, 2024
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4 changes: 2 additions & 2 deletions llvm/include/llvm/Analysis/ScalarEvolution.h
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Expand Up @@ -2218,8 +2218,8 @@ class ScalarEvolution {
/// actually doesn't, or we'd have to immediately execute UB)
/// We *don't* assert these preconditions so please be careful.
const SCEV *computeMaxBECountForLT(const SCEV *Start, const SCEV *Stride,
const SCEV *End, unsigned BitWidth,
bool IsSigned);
const SCEV *End, const Loop *L,
unsigned BitWidth, bool IsSigned);

/// Verify if an linear IV with positive stride can overflow when in a
/// less-than comparison, knowing the invariant term of the comparison,
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19 changes: 10 additions & 9 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Expand Up @@ -12857,11 +12857,10 @@ const SCEV *ScalarEvolution::getUDivCeilSCEV(const SCEV *N, const SCEV *D) {
return getAddExpr(MinNOne, getUDivExpr(NMinusOne, D));
}

const SCEV *ScalarEvolution::computeMaxBECountForLT(const SCEV *Start,
const SCEV *Stride,
const SCEV *End,
unsigned BitWidth,
bool IsSigned) {
const SCEV *
ScalarEvolution::computeMaxBECountForLT(const SCEV *Start, const SCEV *Stride,
const SCEV *End, const Loop *L,
unsigned BitWidth, bool IsSigned) {
// The logic in this function assumes we can represent a positive stride.
// If we can't, the backedge-taken count must be zero.
if (IsSigned && BitWidth == 1)
Expand Down Expand Up @@ -12895,8 +12894,10 @@ const SCEV *ScalarEvolution::computeMaxBECountForLT(const SCEV *Start,
// the case End = RHS of the loop termination condition. This is safe because
// in the other case (End - Start) is zero, leading to a zero maximum backedge
// taken count.
APInt MaxEnd = IsSigned ? APIntOps::smin(getSignedRangeMax(End), Limit)
: APIntOps::umin(getUnsignedRangeMax(End), Limit);
const SCEV *GuardedEnd = applyLoopGuards(End, L);
APInt MaxEnd = IsSigned
? APIntOps::smin(getSignedRangeMax(GuardedEnd), Limit)
: APIntOps::umin(getUnsignedRangeMax(GuardedEnd), Limit);

// MaxBECount = ceil((max(MaxEnd, MinStart) - MinStart) / Stride)
MaxEnd = IsSigned ? APIntOps::smax(MaxEnd, MinStart)
Expand Down Expand Up @@ -13150,7 +13151,7 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
// loop (RHS), and the fact that IV does not overflow (which is
// checked above).
const SCEV *MaxBECount = computeMaxBECountForLT(
Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned);
Start, Stride, RHS, L, getTypeSizeInBits(LHS->getType()), IsSigned);
return ExitLimit(getCouldNotCompute() /* ExactNotTaken */, MaxBECount,
MaxBECount, false /*MaxOrZero*/, Predicates);
}
Expand Down Expand Up @@ -13334,7 +13335,7 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
MaxOrZero = true;
} else {
ConstantMaxBECount = computeMaxBECountForLT(
Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned);
Start, Stride, RHS, L, getTypeSizeInBits(LHS->getType()), IsSigned);
}

if (isa<SCEVCouldNotCompute>(ConstantMaxBECount) &&
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9 changes: 2 additions & 7 deletions llvm/test/Transforms/LoopVectorize/RISCV/vectorize-force-tail-with-evl-known-no-overflow.ll
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Expand Up @@ -4,8 +4,7 @@
; RUN: -prefer-predicate-over-epilogue=predicate-dont-vectorize \
; RUN: -mtriple=riscv64 -mattr=+v -S < %s | FileCheck %s

; TODO: We know the IV will never overflow here so we can skip the overflow
; check
; We know the IV will never overflow here so we can skip the overflow check

define void @trip_count_max_1024(ptr %p, i64 %tc) vscale_range(2, 1024) {
; CHECK-LABEL: define void @trip_count_max_1024(
Expand All @@ -15,11 +14,7 @@ define void @trip_count_max_1024(ptr %p, i64 %tc) vscale_range(2, 1024) {
; CHECK-NEXT: br i1 [[GUARD]], label %[[EXIT:.*]], label %[[LOOP_PREHEADER:.*]]
; CHECK: [[LOOP_PREHEADER]]:
; CHECK-NEXT: [[UMAX:%.*]] = call i64 @llvm.umax.i64(i64 [[TC]], i64 1)
; CHECK-NEXT: [[TMP0:%.*]] = sub i64 -1, [[UMAX]]
; CHECK-NEXT: [[TMP1:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP2:%.*]] = mul i64 [[TMP1]], 2
; CHECK-NEXT: [[TMP3:%.*]] = icmp ult i64 [[TMP0]], [[TMP2]]
; CHECK-NEXT: br i1 [[TMP3]], label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
; CHECK-NEXT: br i1 false, label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
; CHECK: [[VECTOR_PH]]:
; CHECK-NEXT: [[TMP4:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP5:%.*]] = mul i64 [[TMP4]], 2
Expand Down
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