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[RISCV] Don't vectorize for loops with small trip count #132176

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Mar 28, 2025
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[RISCV] Don't vectorize for loops with small trip count #132176

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10 changes: 10 additions & 0 deletions llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,12 @@ static cl::opt<unsigned> SLPMaxVF(
"exclusively by SLP vectorizer."),
cl::Hidden);

static cl::opt<unsigned>
RVVMinTripCount("riscv-v-min-trip-count",
cl::desc("Set the lower bound of a trip count to decide on "
"vectorization while tail-folding."),
cl::init(5), cl::Hidden);

InstructionCost
RISCVTTIImpl::getRISCVInstructionCost(ArrayRef<unsigned> OpCodes, MVT VT,
TTI::TargetCostKind CostKind) {
Expand Down Expand Up @@ -2598,6 +2604,10 @@ unsigned RISCVTTIImpl::getMaximumVF(unsigned ElemWidth, unsigned Opcode) const {
return std::max<unsigned>(1U, RegWidth.getFixedValue() / ElemWidth);
}

unsigned RISCVTTIImpl::getMinTripCountTailFoldingThreshold() const {
return RVVMinTripCount;
}

TTI::AddressingModeKind
RISCVTTIImpl::getPreferredAddressingMode(const Loop *L,
ScalarEvolution *SE) const {
Expand Down
2 changes: 2 additions & 0 deletions llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h
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Original file line number Diff line number Diff line change
Expand Up @@ -388,6 +388,8 @@ class RISCVTTIImpl : public BasicTTIImplBase<RISCVTTIImpl> {

bool enableInterleavedAccessVectorization() { return true; }

unsigned getMinTripCountTailFoldingThreshold() const;

enum RISCVRegisterClass { GPRRC, FPRRC, VRRC };
unsigned getNumberOfRegisters(unsigned ClassID) const {
switch (ClassID) {
Expand Down
82 changes: 13 additions & 69 deletions llvm/test/Transforms/LoopVectorize/RISCV/low-trip-count.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -45,47 +45,19 @@ for.end: ; preds = %for.body
define void @trip3_i8(ptr noalias nocapture noundef %dst, ptr noalias nocapture noundef readonly %src) #0 {
; CHECK-LABEL: @trip3_i8(
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; CHECK: vector.ph:
; CHECK-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 2
; CHECK-NEXT: [[TMP2:%.*]] = sub i64 [[TMP1]], 1
; CHECK-NEXT: [[N_RND_UP:%.*]] = add i64 3, [[TMP2]]
; CHECK-NEXT: [[N_MOD_VF:%.*]] = urem i64 [[N_RND_UP]], [[TMP1]]
; CHECK-NEXT: [[N_VEC:%.*]] = sub i64 [[N_RND_UP]], [[N_MOD_VF]]
; CHECK-NEXT: [[TMP3:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP4:%.*]] = mul i64 [[TMP3]], 2
; CHECK-NEXT: br label [[VECTOR_BODY:%.*]]
; CHECK: vector.body:
; CHECK-NEXT: [[ACTIVE_LANE_MASK:%.*]] = call <vscale x 2 x i1> @llvm.get.active.lane.mask.nxv2i1.i64(i64 0, i64 3)
; CHECK-NEXT: [[TMP8:%.*]] = getelementptr inbounds i8, ptr [[DST:%.*]], i64 0
; CHECK-NEXT: [[TMP9:%.*]] = getelementptr inbounds i8, ptr [[TMP8]], i32 0
; CHECK-NEXT: [[WIDE_MASKED_LOAD:%.*]] = call <vscale x 2 x i8> @llvm.masked.load.nxv2i8.p0(ptr [[TMP9]], i32 1, <vscale x 2 x i1> [[ACTIVE_LANE_MASK]], <vscale x 2 x i8> poison)
; CHECK-NEXT: [[TMP10:%.*]] = shl <vscale x 2 x i8> [[WIDE_MASKED_LOAD]], splat (i8 1)
; CHECK-NEXT: [[TMP11:%.*]] = getelementptr inbounds i8, ptr [[DST1:%.*]], i64 0
; CHECK-NEXT: [[TMP12:%.*]] = getelementptr inbounds i8, ptr [[TMP11]], i32 0
; CHECK-NEXT: [[WIDE_MASKED_LOAD1:%.*]] = call <vscale x 2 x i8> @llvm.masked.load.nxv2i8.p0(ptr [[TMP12]], i32 1, <vscale x 2 x i1> [[ACTIVE_LANE_MASK]], <vscale x 2 x i8> poison)
; CHECK-NEXT: [[TMP13:%.*]] = add <vscale x 2 x i8> [[TMP10]], [[WIDE_MASKED_LOAD1]]
; CHECK-NEXT: [[TMP14:%.*]] = getelementptr inbounds i8, ptr [[TMP11]], i32 0
; CHECK-NEXT: call void @llvm.masked.store.nxv2i8.p0(<vscale x 2 x i8> [[TMP13]], ptr [[TMP14]], i32 1, <vscale x 2 x i1> [[ACTIVE_LANE_MASK]])
; CHECK-NEXT: br label [[MIDDLE_BLOCK:%.*]]
; CHECK: middle.block:
; CHECK-NEXT: br i1 true, label [[FOR_END:%.*]], label [[SCALAR_PH]]
; CHECK: scalar.ph:
; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; CHECK-NEXT: br label [[FOR_BODY:%.*]]
; CHECK: for.body:
; CHECK-NEXT: [[I_08:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[INC:%.*]], [[FOR_BODY]] ]
; CHECK-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds i8, ptr [[DST]], i64 [[I_08]]
; CHECK-NEXT: [[I_08:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[FOR_BODY]] ]
; CHECK-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds i8, ptr [[DST:%.*]], i64 [[I_08]]
; CHECK-NEXT: [[TMP15:%.*]] = load i8, ptr [[ARRAYIDX]], align 1
; CHECK-NEXT: [[MUL:%.*]] = shl i8 [[TMP15]], 1
; CHECK-NEXT: [[ARRAYIDX1:%.*]] = getelementptr inbounds i8, ptr [[DST1]], i64 [[I_08]]
; CHECK-NEXT: [[ARRAYIDX1:%.*]] = getelementptr inbounds i8, ptr [[DST1:%.*]], i64 [[I_08]]
; CHECK-NEXT: [[TMP16:%.*]] = load i8, ptr [[ARRAYIDX1]], align 1
; CHECK-NEXT: [[ADD:%.*]] = add i8 [[MUL]], [[TMP16]]
; CHECK-NEXT: store i8 [[ADD]], ptr [[ARRAYIDX1]], align 1
; CHECK-NEXT: [[INC]] = add nuw nsw i64 [[I_08]], 1
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i64 [[INC]], 3
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END]], label [[FOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END:%.*]], label [[FOR_BODY]]
; CHECK: for.end:
; CHECK-NEXT: ret void
;
Expand All @@ -112,47 +84,19 @@ for.end: ; preds = %for.body
define void @trip5_i8(ptr noalias nocapture noundef %dst, ptr noalias nocapture noundef readonly %src) #0 {
; CHECK-LABEL: @trip5_i8(
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; CHECK: vector.ph:
; CHECK-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 4
; CHECK-NEXT: [[TMP2:%.*]] = sub i64 [[TMP1]], 1
; CHECK-NEXT: [[N_RND_UP:%.*]] = add i64 5, [[TMP2]]
; CHECK-NEXT: [[N_MOD_VF:%.*]] = urem i64 [[N_RND_UP]], [[TMP1]]
; CHECK-NEXT: [[N_VEC:%.*]] = sub i64 [[N_RND_UP]], [[N_MOD_VF]]
; CHECK-NEXT: [[TMP3:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP4:%.*]] = mul i64 [[TMP3]], 4
; CHECK-NEXT: br label [[VECTOR_BODY:%.*]]
; CHECK: vector.body:
; CHECK-NEXT: [[ACTIVE_LANE_MASK:%.*]] = call <vscale x 4 x i1> @llvm.get.active.lane.mask.nxv4i1.i64(i64 0, i64 5)
; CHECK-NEXT: [[TMP8:%.*]] = getelementptr inbounds i8, ptr [[DST:%.*]], i64 0
; CHECK-NEXT: [[TMP9:%.*]] = getelementptr inbounds i8, ptr [[TMP8]], i32 0
; CHECK-NEXT: [[WIDE_MASKED_LOAD:%.*]] = call <vscale x 4 x i8> @llvm.masked.load.nxv4i8.p0(ptr [[TMP9]], i32 1, <vscale x 4 x i1> [[ACTIVE_LANE_MASK]], <vscale x 4 x i8> poison)
; CHECK-NEXT: [[TMP10:%.*]] = shl <vscale x 4 x i8> [[WIDE_MASKED_LOAD]], splat (i8 1)
; CHECK-NEXT: [[TMP11:%.*]] = getelementptr inbounds i8, ptr [[DST1:%.*]], i64 0
; CHECK-NEXT: [[TMP12:%.*]] = getelementptr inbounds i8, ptr [[TMP11]], i32 0
; CHECK-NEXT: [[WIDE_MASKED_LOAD1:%.*]] = call <vscale x 4 x i8> @llvm.masked.load.nxv4i8.p0(ptr [[TMP12]], i32 1, <vscale x 4 x i1> [[ACTIVE_LANE_MASK]], <vscale x 4 x i8> poison)
; CHECK-NEXT: [[TMP13:%.*]] = add <vscale x 4 x i8> [[TMP10]], [[WIDE_MASKED_LOAD1]]
; CHECK-NEXT: [[TMP14:%.*]] = getelementptr inbounds i8, ptr [[TMP11]], i32 0
; CHECK-NEXT: call void @llvm.masked.store.nxv4i8.p0(<vscale x 4 x i8> [[TMP13]], ptr [[TMP14]], i32 1, <vscale x 4 x i1> [[ACTIVE_LANE_MASK]])
; CHECK-NEXT: br label [[MIDDLE_BLOCK:%.*]]
; CHECK: middle.block:
; CHECK-NEXT: br i1 true, label [[FOR_END:%.*]], label [[SCALAR_PH]]
; CHECK: scalar.ph:
; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; CHECK-NEXT: br label [[FOR_BODY:%.*]]
; CHECK: for.body:
; CHECK-NEXT: [[I_08:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[INC:%.*]], [[FOR_BODY]] ]
; CHECK-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds i8, ptr [[DST]], i64 [[I_08]]
; CHECK-NEXT: [[I_08:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[FOR_BODY]] ]
; CHECK-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds i8, ptr [[DST:%.*]], i64 [[I_08]]
; CHECK-NEXT: [[TMP15:%.*]] = load i8, ptr [[ARRAYIDX]], align 1
; CHECK-NEXT: [[MUL:%.*]] = shl i8 [[TMP15]], 1
; CHECK-NEXT: [[ARRAYIDX1:%.*]] = getelementptr inbounds i8, ptr [[DST1]], i64 [[I_08]]
; CHECK-NEXT: [[ARRAYIDX1:%.*]] = getelementptr inbounds i8, ptr [[DST1:%.*]], i64 [[I_08]]
; CHECK-NEXT: [[TMP16:%.*]] = load i8, ptr [[ARRAYIDX1]], align 1
; CHECK-NEXT: [[ADD:%.*]] = add i8 [[MUL]], [[TMP16]]
; CHECK-NEXT: store i8 [[ADD]], ptr [[ARRAYIDX1]], align 1
; CHECK-NEXT: [[INC]] = add nuw nsw i64 [[I_08]], 1
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i64 [[INC]], 5
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END]], label [[FOR_BODY]], !llvm.loop [[LOOP3:![0-9]+]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END:%.*]], label [[FOR_BODY]]
; CHECK: for.end:
; CHECK-NEXT: ret void
;
Expand Down Expand Up @@ -219,7 +163,7 @@ define void @trip8_i8(ptr noalias nocapture noundef %dst, ptr noalias nocapture
; CHECK-NEXT: store i8 [[ADD]], ptr [[ARRAYIDX1]], align 1
; CHECK-NEXT: [[INC]] = add nuw nsw i64 [[I_08]], 1
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i64 [[INC]], 8
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END]], label [[FOR_BODY]], !llvm.loop [[LOOP4:![0-9]+]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END]], label [[FOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
; CHECK: for.end:
; CHECK-NEXT: ret void
;
Expand Down Expand Up @@ -277,7 +221,7 @@ define void @trip16_i8(ptr noalias nocapture noundef %dst, ptr noalias nocapture
; CHECK-NEXT: store i8 [[ADD]], ptr [[ARRAYIDX1]], align 1
; CHECK-NEXT: [[INC]] = add nuw nsw i64 [[I_08]], 1
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i64 [[INC]], 16
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END]], label [[FOR_BODY]], !llvm.loop [[LOOP5:![0-9]+]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END]], label [[FOR_BODY]], !llvm.loop [[LOOP3:![0-9]+]]
; CHECK: for.end:
; CHECK-NEXT: ret void
;
Expand Down Expand Up @@ -336,7 +280,7 @@ define void @trip32_i8(ptr noalias nocapture noundef %dst, ptr noalias nocapture
; CHECK-NEXT: store i8 [[ADD]], ptr [[ARRAYIDX1]], align 1
; CHECK-NEXT: [[INC]] = add nuw nsw i64 [[I_08]], 1
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i64 [[INC]], 32
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END]], label [[FOR_BODY]], !llvm.loop [[LOOP6:![0-9]+]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END]], label [[FOR_BODY]], !llvm.loop [[LOOP4:![0-9]+]]
; CHECK: for.end:
; CHECK-NEXT: ret void
;
Expand Down Expand Up @@ -379,7 +323,7 @@ define void @trip24_i8(ptr noalias nocapture noundef %dst, ptr noalias nocapture
; CHECK-NEXT: store <8 x i8> [[TMP6]], ptr [[TMP5]], align 1
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 8
; CHECK-NEXT: [[TMP7:%.*]] = icmp eq i64 [[INDEX_NEXT]], 24
; CHECK-NEXT: br i1 [[TMP7]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP7:![0-9]+]]
; CHECK-NEXT: br i1 [[TMP7]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP5:![0-9]+]]
; CHECK: middle.block:
; CHECK-NEXT: br i1 true, label [[FOR_END:%.*]], label [[SCALAR_PH]]
; CHECK: scalar.ph:
Expand All @@ -396,7 +340,7 @@ define void @trip24_i8(ptr noalias nocapture noundef %dst, ptr noalias nocapture
; CHECK-NEXT: store i8 [[ADD]], ptr [[ARRAYIDX1]], align 1
; CHECK-NEXT: [[INC]] = add nuw nsw i64 [[I_08]], 1
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i64 [[INC]], 24
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END]], label [[FOR_BODY]], !llvm.loop [[LOOP8:![0-9]+]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END]], label [[FOR_BODY]], !llvm.loop [[LOOP6:![0-9]+]]
; CHECK: for.end:
; CHECK-NEXT: ret void
;
Expand Down
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