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[SCEV] Check if predicate is known false for predicated AddRecs. #151134

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merged 3 commits into from
Aug 15, 2025
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[SCEV] Check if predicate is known false for predicated AddRecs. #151134

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d71df7e
[SCEV] Check if predicate is known false for predicated AddRecs.
fhahn Jul 19, 2025
c51df74
!fixup strip unneeded changes
fhahn Aug 12, 2025
c474bbe
!fixup address comments, thanks
fhahn Aug 13, 2025
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23 changes: 23 additions & 0 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -14952,6 +14952,29 @@ const SCEVAddRecExpr *ScalarEvolution::convertSCEVToAddRecWithPredicates(
if (!AddRec)
return nullptr;

// Check if any of the transformed predicates is known to be false. In that
// case, it doesn't make sense to convert to a predicated AddRec, as the
// versioned loop will never execute.
for (const SCEVPredicate *Pred : TransformPreds) {
auto *WrapPred = dyn_cast<SCEVWrapPredicate>(Pred);
if (!WrapPred || WrapPred->getFlags() != SCEVWrapPredicate::IncrementNSSW)
continue;

const SCEVAddRecExpr *AddRecToCheck = WrapPred->getExpr();
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Do we need to check that the addrecs are on the same loop? Or is that already guaranteed?

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I think for all cases the loop should match in practice, but I updated to use the loop from AddRecToCheck, thanks

const SCEV *ExitCount = getBackedgeTakenCount(AddRecToCheck->getLoop());
if (isa<SCEVCouldNotCompute>(ExitCount))
continue;

const SCEV *Step = AddRecToCheck->getStepRecurrence(*this);
if (!Step->isOne())
continue;

ExitCount = getTruncateOrSignExtend(ExitCount, Step->getType());
const SCEV *Add = getAddExpr(AddRecToCheck->getStart(), ExitCount);
if (isKnownPredicate(CmpInst::ICMP_SLT, Add, AddRecToCheck->getStart()))
return nullptr;
}

// Since the transformation was successful, we can now transfer the SCEV
// predicates.
Preds.append(TransformPreds.begin(), TransformPreds.end());
Expand Down
63 changes: 54 additions & 9 deletions llvm/test/Transforms/LoopVectorize/first-order-recurrence-dead-instructions.ll
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Expand Up @@ -6,16 +6,61 @@ define i8 @recurrence_phi_with_same_incoming_values_after_simplifications(i8 %fo
; CHECK-LABEL: define i8 @recurrence_phi_with_same_incoming_values_after_simplifications(
; CHECK-SAME: i8 [[FOR_START:%.*]], ptr [[DST:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br i1 false, label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
; CHECK: [[VECTOR_PH]]:
; CHECK-NEXT: [[BROADCAST_SPLATINSERT:%.*]] = insertelement <4 x i8> poison, i8 [[FOR_START]], i64 0
; CHECK-NEXT: [[BROADCAST_SPLAT:%.*]] = shufflevector <4 x i8> [[BROADCAST_SPLATINSERT]], <4 x i8> poison, <4 x i32> zeroinitializer
; CHECK-NEXT: [[TMP0:%.*]] = shufflevector <4 x i8> [[BROADCAST_SPLAT]], <4 x i8> [[BROADCAST_SPLAT]], <4 x i32> <i32 3, i32 4, i32 5, i32 6>
; CHECK-NEXT: br label %[[VECTOR_BODY:.*]]
; CHECK: [[VECTOR_BODY]]:
; CHECK-NEXT: [[INDEX:%.*]] = phi i32 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY]] ]
; CHECK-NEXT: [[OFFSET_IDX:%.*]] = add i32 1, [[INDEX]]
; CHECK-NEXT: [[TMP1:%.*]] = add i32 [[OFFSET_IDX]], 0
; CHECK-NEXT: [[TMP2:%.*]] = add i32 [[OFFSET_IDX]], 1
; CHECK-NEXT: [[TMP3:%.*]] = add i32 [[OFFSET_IDX]], 2
; CHECK-NEXT: [[TMP4:%.*]] = add i32 [[OFFSET_IDX]], 3
; CHECK-NEXT: [[TMP5:%.*]] = add i32 [[OFFSET_IDX]], 4
; CHECK-NEXT: [[TMP6:%.*]] = add i32 [[OFFSET_IDX]], 5
; CHECK-NEXT: [[TMP7:%.*]] = add i32 [[OFFSET_IDX]], 6
; CHECK-NEXT: [[TMP8:%.*]] = add i32 [[OFFSET_IDX]], 7
; CHECK-NEXT: [[TMP9:%.*]] = getelementptr inbounds i8, ptr [[DST]], i32 [[TMP1]]
; CHECK-NEXT: [[TMP10:%.*]] = getelementptr inbounds i8, ptr [[DST]], i32 [[TMP2]]
; CHECK-NEXT: [[TMP11:%.*]] = getelementptr inbounds i8, ptr [[DST]], i32 [[TMP3]]
; CHECK-NEXT: [[TMP12:%.*]] = getelementptr inbounds i8, ptr [[DST]], i32 [[TMP4]]
; CHECK-NEXT: [[TMP13:%.*]] = getelementptr inbounds i8, ptr [[DST]], i32 [[TMP5]]
; CHECK-NEXT: [[TMP14:%.*]] = getelementptr inbounds i8, ptr [[DST]], i32 [[TMP6]]
; CHECK-NEXT: [[TMP15:%.*]] = getelementptr inbounds i8, ptr [[DST]], i32 [[TMP7]]
; CHECK-NEXT: [[TMP16:%.*]] = getelementptr inbounds i8, ptr [[DST]], i32 [[TMP8]]
; CHECK-NEXT: [[TMP17:%.*]] = extractelement <4 x i8> [[TMP0]], i32 0
; CHECK-NEXT: store i8 [[TMP17]], ptr [[TMP9]], align 1
; CHECK-NEXT: [[TMP18:%.*]] = extractelement <4 x i8> [[TMP0]], i32 1
; CHECK-NEXT: store i8 [[TMP18]], ptr [[TMP10]], align 1
; CHECK-NEXT: [[TMP19:%.*]] = extractelement <4 x i8> [[TMP0]], i32 2
; CHECK-NEXT: store i8 [[TMP19]], ptr [[TMP11]], align 1
; CHECK-NEXT: [[TMP20:%.*]] = extractelement <4 x i8> [[TMP0]], i32 3
; CHECK-NEXT: store i8 [[TMP20]], ptr [[TMP12]], align 1
; CHECK-NEXT: store i8 [[TMP17]], ptr [[TMP13]], align 1
; CHECK-NEXT: store i8 [[TMP18]], ptr [[TMP14]], align 1
; CHECK-NEXT: store i8 [[TMP19]], ptr [[TMP15]], align 1
; CHECK-NEXT: store i8 [[TMP20]], ptr [[TMP16]], align 1
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 8
; CHECK-NEXT: [[TMP21:%.*]] = icmp eq i32 [[INDEX_NEXT]], -8
; CHECK-NEXT: br i1 [[TMP21]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
; CHECK: [[MIDDLE_BLOCK]]:
; CHECK-NEXT: br label %[[SCALAR_PH]]
; CHECK: [[SCALAR_PH]]:
; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i32 [ -7, %[[MIDDLE_BLOCK]] ], [ 1, %[[ENTRY]] ]
; CHECK-NEXT: [[SCALAR_RECUR_INIT:%.*]] = phi i8 [ [[FOR_START]], %[[MIDDLE_BLOCK]] ], [ [[FOR_START]], %[[ENTRY]] ]
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 1, %[[ENTRY]] ], [ [[IV_NEXT:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[FOR:%.*]] = phi i8 [ [[FOR_START]], %[[ENTRY]] ], [ [[FOR_NEXT:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ [[BC_RESUME_VAL]], %[[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[FOR:%.*]] = phi i8 [ [[SCALAR_RECUR_INIT]], %[[SCALAR_PH]] ], [ [[FOR_NEXT:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[FOR_NEXT]] = and i8 [[FOR_START]], -1
; CHECK-NEXT: [[IV_NEXT]] = add i32 [[IV]], 1
; CHECK-NEXT: [[GEP_DST:%.*]] = getelementptr inbounds i8, ptr [[DST]], i32 [[IV]]
; CHECK-NEXT: store i8 [[FOR]], ptr [[GEP_DST]], align 1
; CHECK-NEXT: [[EC:%.*]] = icmp eq i32 [[IV_NEXT]], 0
; CHECK-NEXT: br i1 [[EC]], label %[[EXIT:.*]], label %[[LOOP]]
; CHECK-NEXT: br i1 [[EC]], label %[[EXIT:.*]], label %[[LOOP]], !llvm.loop [[LOOP3:![0-9]+]]
; CHECK: [[EXIT]]:
; CHECK-NEXT: [[FOR_NEXT_LCSSA:%.*]] = phi i8 [ [[FOR_NEXT]], %[[LOOP]] ]
; CHECK-NEXT: ret i8 [[FOR_NEXT_LCSSA]]
Expand Down Expand Up @@ -61,7 +106,7 @@ define i32 @sink_after_dead_inst(ptr %A.ptr) {
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 8
; CHECK-NEXT: [[VEC_IND_NEXT]] = add <4 x i16> [[STEP_ADD]], splat (i16 4)
; CHECK-NEXT: [[TMP6:%.*]] = icmp eq i32 [[INDEX_NEXT]], 16
; CHECK-NEXT: br i1 [[TMP6]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
; CHECK-NEXT: br i1 [[TMP6]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP4:![0-9]+]]
; CHECK: [[MIDDLE_BLOCK]]:
; CHECK-NEXT: [[VECTOR_RECUR_EXTRACT_FOR_PHI:%.*]] = extractelement <4 x i32> [[TMP2]], i32 2
; CHECK-NEXT: br label %[[FOR_END:.*]]
Expand All @@ -82,7 +127,7 @@ define i32 @sink_after_dead_inst(ptr %A.ptr) {
; CHECK-NEXT: [[EXT:%.*]] = zext i1 [[B3]] to i32
; CHECK-NEXT: [[A_GEP:%.*]] = getelementptr i32, ptr [[A_PTR]], i16 [[IV]]
; CHECK-NEXT: store i32 0, ptr [[A_GEP]], align 4
; CHECK-NEXT: br i1 [[VEC_DEAD]], label %[[FOR_END]], label %[[LOOP]], !llvm.loop [[LOOP3:![0-9]+]]
; CHECK-NEXT: br i1 [[VEC_DEAD]], label %[[FOR_END]], label %[[LOOP]], !llvm.loop [[LOOP5:![0-9]+]]
; CHECK: [[FOR_END]]:
; CHECK-NEXT: [[FOR_LCSSA:%.*]] = phi i32 [ [[FOR]], %[[LOOP]] ], [ [[VECTOR_RECUR_EXTRACT_FOR_PHI]], %[[MIDDLE_BLOCK]] ]
; CHECK-NEXT: ret i32 [[FOR_LCSSA]]
Expand Down Expand Up @@ -142,7 +187,7 @@ define void @sink_dead_inst(ptr %a) {
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 8
; CHECK-NEXT: [[VEC_IND_NEXT]] = add <4 x i16> [[STEP_ADD]], splat (i16 4)
; CHECK-NEXT: [[TMP12:%.*]] = icmp eq i32 [[INDEX_NEXT]], 40
; CHECK-NEXT: br i1 [[TMP12]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP4:![0-9]+]]
; CHECK-NEXT: br i1 [[TMP12]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP6:![0-9]+]]
; CHECK: [[MIDDLE_BLOCK]]:
; CHECK-NEXT: [[VECTOR_RECUR_EXTRACT:%.*]] = extractelement <4 x i16> [[TMP4]], i32 3
; CHECK-NEXT: [[VECTOR_RECUR_EXTRACT1:%.*]] = extractelement <4 x i32> [[TMP2]], i32 3
Expand All @@ -163,7 +208,7 @@ define void @sink_dead_inst(ptr %a) {
; CHECK-NEXT: [[REC_1_PREV]] = add i16 [[IV_NEXT]], 5
; CHECK-NEXT: [[GEP:%.*]] = getelementptr i16, ptr [[A]], i16 [[IV]]
; CHECK-NEXT: store i16 [[USE_REC_1]], ptr [[GEP]], align 2
; CHECK-NEXT: br i1 [[CMP]], label %[[FOR_END:.*]], label %[[FOR_COND]], !llvm.loop [[LOOP5:![0-9]+]]
; CHECK-NEXT: br i1 [[CMP]], label %[[FOR_END:.*]], label %[[FOR_COND]], !llvm.loop [[LOOP7:![0-9]+]]
; CHECK: [[FOR_END]]:
; CHECK-NEXT: ret void
;
Expand Down Expand Up @@ -205,7 +250,7 @@ define void @unused_recurrence(ptr %a) {
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 8
; CHECK-NEXT: [[VEC_IND_NEXT]] = add <4 x i16> [[STEP_ADD]], splat (i16 4)
; CHECK-NEXT: [[TMP2:%.*]] = icmp eq i32 [[INDEX_NEXT]], 1024
; CHECK-NEXT: br i1 [[TMP2]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP6:![0-9]+]]
; CHECK-NEXT: br i1 [[TMP2]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP8:![0-9]+]]
; CHECK: [[MIDDLE_BLOCK]]:
; CHECK-NEXT: [[VECTOR_RECUR_EXTRACT:%.*]] = extractelement <4 x i16> [[TMP1]], i32 3
; CHECK-NEXT: br label %[[SCALAR_PH]]
Expand All @@ -220,7 +265,7 @@ define void @unused_recurrence(ptr %a) {
; CHECK-NEXT: [[IV_NEXT]] = add i16 [[IV]], 1
; CHECK-NEXT: [[REC_1_PREV]] = add i16 [[IV_NEXT]], 5
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i16 [[IV]], 1000
; CHECK-NEXT: br i1 [[CMP]], label %[[FOR_END:.*]], label %[[FOR_COND]], !llvm.loop [[LOOP7:![0-9]+]]
; CHECK-NEXT: br i1 [[CMP]], label %[[FOR_END:.*]], label %[[FOR_COND]], !llvm.loop [[LOOP9:![0-9]+]]
; CHECK: [[FOR_END]]:
; CHECK-NEXT: ret void
;
Expand Down