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[SCEV] Try to re-use pointer LCSSA phis when expanding SCEVs. #147824

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[SCEV] Try to re-use pointer LCSSA phis when expanding SCEVs. #147824

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72b71ca
[SCEV] Try to re-use pointer LCSSA phis when expanding SCEVs.
fhahn Jul 4, 2025
4b53050
!fixup add comment
fhahn Jul 22, 2025
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27 changes: 23 additions & 4 deletions llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1224,18 +1224,37 @@ Value *SCEVExpander::expandAddRecExprLiterally(const SCEVAddRecExpr *S) {
}

Value *SCEVExpander::tryToReuseLCSSAPhi(const SCEVAddRecExpr *S) {
Type *STy = S->getType();
const Loop *L = S->getLoop();
BasicBlock *EB = L->getExitBlock();
if (!EB || !EB->getSinglePredecessor() ||
!SE.DT.dominates(EB, Builder.GetInsertBlock()))
return nullptr;

for (auto &PN : EB->phis()) {
if (!SE.isSCEVable(PN.getType()) || PN.getType() != S->getType())
if (!SE.isSCEVable(PN.getType()))
continue;
auto *ExitV = SE.getSCEV(&PN);
if (S == ExitV)
return &PN;
auto *ExitSCEV = SE.getSCEV(&PN);
Type *PhiTy = PN.getType();
if (STy->isIntegerTy() && PhiTy->isPointerTy())
ExitSCEV = SE.getPtrToIntExpr(ExitSCEV, STy);
else if (S->getType() != PN.getType())
continue;

// Check if we can re-use the existing PN, by adjusting it with an expanded
// offset, if the offset is simpler (for now just checks if it is
// AddRec-free).
const SCEV *Diff = SE.getMinusSCEV(S, ExitSCEV);
if (isa<SCEVCouldNotCompute>(Diff) ||
SCEVExprContains(Diff,
[](const SCEV *S) { return isa<SCEVAddRecExpr>(S); }))
continue;
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Can we limit this to just constant offsets (and use computeConstantDifference)? This check excludes one particularly bad case, but other complex expansions may also be non-profitable.


Value *DiffV = expand(Diff);
Value *BaseV = &PN;
if (DiffV->getType()->isIntegerTy() && PhiTy->isPointerTy())
BaseV = Builder.CreatePtrToInt(BaseV, DiffV->getType());
return Builder.CreateAdd(BaseV, DiffV);
}

return nullptr;
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11 changes: 4 additions & 7 deletions llvm/test/Transforms/LoopLoadElim/invalidate-laa-after-versioning.ll
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Expand Up @@ -59,19 +59,16 @@ define void @test(ptr %arg, i64 %arg1) {
; CHECK-NEXT: [[GEP_5:%.*]] = getelementptr inbounds double, ptr [[LCSSA_PTR_IV_1]], i64 1
; CHECK-NEXT: br label [[INNER_2:%.*]]
; CHECK: inner.2:
; CHECK-NEXT: [[INDVAR:%.*]] = phi i64 [ [[INDVAR_NEXT:%.*]], [[INNER_2]] ], [ 0, [[INNER_1_EXIT]] ]
; CHECK-NEXT: [[PTR_IV_2:%.*]] = phi ptr [ [[GEP_5]], [[INNER_1_EXIT]] ], [ [[PTR_IV_2_NEXT:%.*]], [[INNER_2]] ]
; CHECK-NEXT: [[PTR_IV_2_NEXT]] = getelementptr inbounds double, ptr [[PTR_IV_2]], i64 1
; CHECK-NEXT: [[INDVAR_NEXT]] = add i64 [[INDVAR]], 1
; CHECK-NEXT: br i1 false, label [[INNER_3_LVER_CHECK:%.*]], label [[INNER_2]]
; CHECK: inner.3.lver.check:
; CHECK-NEXT: [[INDVAR_LCSSA:%.*]] = phi i64 [ [[INDVAR]], [[INNER_2]] ]
; CHECK-NEXT: [[LCSSA_PTR_IV_2:%.*]] = phi ptr [ [[PTR_IV_2]], [[INNER_2]] ]
; CHECK-NEXT: [[GEP_6:%.*]] = getelementptr inbounds double, ptr [[PTR_PHI]], i64 1
; CHECK-NEXT: [[GEP_7:%.*]] = getelementptr inbounds double, ptr [[LCSSA_PTR_IV_2]], i64 1
; CHECK-NEXT: [[TMP0:%.*]] = shl i64 [[INDVAR_LCSSA]], 3
; CHECK-NEXT: [[TMP1:%.*]] = add i64 [[TMP0]], 24
; CHECK-NEXT: [[SCEVGEP3:%.*]] = getelementptr i8, ptr [[LCSSA_PTR_IV_1]], i64 [[TMP1]]
; CHECK-NEXT: [[TMP0:%.*]] = ptrtoint ptr [[LCSSA_PTR_IV_2]] to i64
; CHECK-NEXT: [[TMP1:%.*]] = add i64 [[TMP0]], 16
; CHECK-NEXT: [[SCEVGEP3:%.*]] = inttoptr i64 [[TMP1]] to ptr
; CHECK-NEXT: [[BOUND0:%.*]] = icmp ult ptr [[GEP_7]], [[GEP_1]]
; CHECK-NEXT: [[BOUND1:%.*]] = icmp ult ptr [[PTR_PHI]], [[SCEVGEP3]]
; CHECK-NEXT: [[FOUND_CONFLICT:%.*]] = and i1 [[BOUND0]], [[BOUND1]]
Expand Down Expand Up @@ -104,7 +101,7 @@ define void @test(ptr %arg, i64 %arg1) {
; CHECK-NEXT: br i1 [[C_2]], label [[OUTER_LATCH_LOOPEXIT4:%.*]], label [[INNER_3]]
; CHECK: outer.latch.loopexit:
; CHECK-NEXT: br label [[OUTER_LATCH]]
; CHECK: outer.latch.loopexit4:
; CHECK: outer.latch.loopexit3:
; CHECK-NEXT: br label [[OUTER_LATCH]]
; CHECK: outer.latch:
; CHECK-NEXT: br label [[INNER_1_LVER_CHECK]]
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20 changes: 8 additions & 12 deletions llvm/test/Transforms/LoopVectorize/reuse-lcssa-phi-scev-expansion.ll
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Expand Up @@ -104,27 +104,23 @@ define void @runtime_checks_ptr_inductions(ptr %dst.1, ptr %dst.2, i1 %c) {
; CHECK-LABEL: define void @runtime_checks_ptr_inductions(
; CHECK-SAME: ptr [[DST_1:%.*]], ptr [[DST_2:%.*]], i1 [[C:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: [[DST_11:%.*]] = ptrtoint ptr [[DST_1]] to i64
; CHECK-NEXT: br label %[[LOOP_1:.*]]
; CHECK: [[LOOP_1]]:
; CHECK-NEXT: [[INDVAR:%.*]] = phi i64 [ [[INDVAR_NEXT:%.*]], %[[LOOP_1]] ], [ 0, %[[ENTRY]] ]
; CHECK-NEXT: [[PTR_IV_1:%.*]] = phi ptr [ [[DST_1]], %[[ENTRY]] ], [ [[PTR_IV_1_NEXT:%.*]], %[[LOOP_1]] ]
; CHECK-NEXT: [[CALL:%.*]] = call i32 @val()
; CHECK-NEXT: [[SEL_DST:%.*]] = select i1 [[C]], ptr [[DST_1]], ptr [[DST_2]]
; CHECK-NEXT: [[PTR_IV_1_NEXT]] = getelementptr i8, ptr [[PTR_IV_1]], i64 1
; CHECK-NEXT: [[EC_1:%.*]] = icmp eq i32 [[CALL]], 0
; CHECK-NEXT: [[INDVAR_NEXT]] = add i64 [[INDVAR]], 1
; CHECK-NEXT: br i1 [[EC_1]], label %[[LOOP_2_HEADER_PREHEADER:.*]], label %[[LOOP_1]]
; CHECK: [[LOOP_2_HEADER_PREHEADER]]:
; CHECK-NEXT: [[SEL_DST_LCSSA2:%.*]] = phi ptr [ [[SEL_DST]], %[[LOOP_1]] ]
; CHECK-NEXT: [[INDVAR_LCSSA:%.*]] = phi i64 [ [[INDVAR]], %[[LOOP_1]] ]
; CHECK-NEXT: [[SEL_DST_LCSSA1:%.*]] = phi ptr [ [[SEL_DST]], %[[LOOP_1]] ]
; CHECK-NEXT: [[PTR_IV_1_LCSSA:%.*]] = phi ptr [ [[PTR_IV_1]], %[[LOOP_1]] ]
; CHECK-NEXT: [[SEL_DST_LCSSA:%.*]] = phi ptr [ [[SEL_DST]], %[[LOOP_1]] ]
; CHECK-NEXT: [[SEL_DST_LCSSA23:%.*]] = ptrtoint ptr [[SEL_DST_LCSSA2]] to i64
; CHECK-NEXT: [[SEL_DST_LCSSA12:%.*]] = ptrtoint ptr [[SEL_DST_LCSSA1]] to i64
; CHECK-NEXT: br i1 false, label %[[SCALAR_PH:.*]], label %[[VECTOR_MEMCHECK:.*]]
; CHECK: [[VECTOR_MEMCHECK]]:
; CHECK-NEXT: [[TMP0:%.*]] = add i64 [[INDVAR_LCSSA]], [[DST_11]]
; CHECK-NEXT: [[TMP1:%.*]] = sub i64 [[TMP0]], [[SEL_DST_LCSSA23]]
; CHECK-NEXT: [[TMP0:%.*]] = ptrtoint ptr [[PTR_IV_1_LCSSA]] to i64
; CHECK-NEXT: [[TMP1:%.*]] = sub i64 [[TMP0]], [[SEL_DST_LCSSA12]]
; CHECK-NEXT: [[DIFF_CHECK:%.*]] = icmp ult i64 [[TMP1]], 2
; CHECK-NEXT: br i1 [[DIFF_CHECK]], label %[[SCALAR_PH]], label %[[VECTOR_PH:.*]]
; CHECK: [[VECTOR_PH]]:
Expand All @@ -146,13 +142,13 @@ define void @runtime_checks_ptr_inductions(ptr %dst.1, ptr %dst.2, i1 %c) {
; CHECK-NEXT: br label %[[SCALAR_PH]]
; CHECK: [[SCALAR_PH]]:
; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i32 [ 1023, %[[MIDDLE_BLOCK]] ], [ 1, %[[LOOP_2_HEADER_PREHEADER]] ], [ 1, %[[VECTOR_MEMCHECK]] ]
; CHECK-NEXT: [[BC_RESUME_VAL5:%.*]] = phi ptr [ [[TMP2]], %[[MIDDLE_BLOCK]] ], [ [[PTR_IV_1_LCSSA]], %[[LOOP_2_HEADER_PREHEADER]] ], [ [[PTR_IV_1_LCSSA]], %[[VECTOR_MEMCHECK]] ]
; CHECK-NEXT: [[BC_RESUME_VAL6:%.*]] = phi ptr [ [[TMP3]], %[[MIDDLE_BLOCK]] ], [ [[SEL_DST_LCSSA]], %[[LOOP_2_HEADER_PREHEADER]] ], [ [[SEL_DST_LCSSA]], %[[VECTOR_MEMCHECK]] ]
; CHECK-NEXT: [[BC_RESUME_VAL4:%.*]] = phi ptr [ [[TMP2]], %[[MIDDLE_BLOCK]] ], [ [[PTR_IV_1_LCSSA]], %[[LOOP_2_HEADER_PREHEADER]] ], [ [[PTR_IV_1_LCSSA]], %[[VECTOR_MEMCHECK]] ]
; CHECK-NEXT: [[BC_RESUME_VAL5:%.*]] = phi ptr [ [[TMP3]], %[[MIDDLE_BLOCK]] ], [ [[SEL_DST_LCSSA]], %[[LOOP_2_HEADER_PREHEADER]] ], [ [[SEL_DST_LCSSA]], %[[VECTOR_MEMCHECK]] ]
; CHECK-NEXT: br label %[[LOOP_2_HEADER:.*]]
; CHECK: [[LOOP_2_HEADER]]:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ [[DEC7:%.*]], %[[LOOP_2_LATCH:.*]] ], [ [[BC_RESUME_VAL]], %[[SCALAR_PH]] ]
; CHECK-NEXT: [[PTR_IV_2:%.*]] = phi ptr [ [[PTR_IV_2_NEXT:%.*]], %[[LOOP_2_LATCH]] ], [ [[BC_RESUME_VAL5]], %[[SCALAR_PH]] ]
; CHECK-NEXT: [[PTR_IV_3:%.*]] = phi ptr [ [[PTR_IV_3_NEXT:%.*]], %[[LOOP_2_LATCH]] ], [ [[BC_RESUME_VAL6]], %[[SCALAR_PH]] ]
; CHECK-NEXT: [[PTR_IV_2:%.*]] = phi ptr [ [[PTR_IV_2_NEXT:%.*]], %[[LOOP_2_LATCH]] ], [ [[BC_RESUME_VAL4]], %[[SCALAR_PH]] ]
; CHECK-NEXT: [[PTR_IV_3:%.*]] = phi ptr [ [[PTR_IV_3_NEXT:%.*]], %[[LOOP_2_LATCH]] ], [ [[BC_RESUME_VAL5]], %[[SCALAR_PH]] ]
; CHECK-NEXT: [[EC_2:%.*]] = icmp eq i32 [[IV]], 1024
; CHECK-NEXT: br i1 [[EC_2]], label %[[EXIT:.*]], label %[[LOOP_2_LATCH]]
; CHECK: [[LOOP_2_LATCH]]:
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15 changes: 6 additions & 9 deletions llvm/test/Transforms/LoopVersioning/invalidate-laa-after-versioning.ll
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Expand Up @@ -56,19 +56,16 @@ define void @test(ptr %arg, i64 %arg1) {
; CHECK-NEXT: [[GEP_5:%.*]] = getelementptr inbounds double, ptr [[LCSSA_PTR_IV_1]], i64 1
; CHECK-NEXT: br label [[INNER_2:%.*]]
; CHECK: inner.2:
; CHECK-NEXT: [[INDVAR:%.*]] = phi i64 [ [[INDVAR_NEXT:%.*]], [[INNER_2]] ], [ 0, [[INNER_1_EXIT]] ]
; CHECK-NEXT: [[PTR_IV_2:%.*]] = phi ptr [ [[GEP_5]], [[INNER_1_EXIT]] ], [ [[PTR_IV_2_NEXT:%.*]], [[INNER_2]] ]
; CHECK-NEXT: [[PTR_IV_2_NEXT]] = getelementptr inbounds double, ptr [[PTR_IV_2]], i64 1
; CHECK-NEXT: [[INDVAR_NEXT]] = add i64 [[INDVAR]], 1
; CHECK-NEXT: br i1 false, label [[INNER_3_LVER_CHECK:%.*]], label [[INNER_2]]
; CHECK: inner.3.lver.check:
; CHECK-NEXT: [[INDVAR_LCSSA:%.*]] = phi i64 [ [[INDVAR]], [[INNER_2]] ]
; CHECK-NEXT: [[LCSSA_PTR_IV_2:%.*]] = phi ptr [ [[PTR_IV_2]], [[INNER_2]] ]
; CHECK-NEXT: [[GEP_6:%.*]] = getelementptr inbounds double, ptr [[PTR_PHI]], i64 1
; CHECK-NEXT: [[GEP_7:%.*]] = getelementptr inbounds double, ptr [[LCSSA_PTR_IV_2]], i64 1
; CHECK-NEXT: [[TMP0:%.*]] = shl i64 [[INDVAR_LCSSA]], 3
; CHECK-NEXT: [[TMP1:%.*]] = add i64 [[TMP0]], 24
; CHECK-NEXT: [[SCEVGEP:%.*]] = getelementptr i8, ptr [[LCSSA_PTR_IV_1]], i64 [[TMP1]]
; CHECK-NEXT: [[TMP0:%.*]] = ptrtoint ptr [[LCSSA_PTR_IV_2]] to i64
; CHECK-NEXT: [[TMP1:%.*]] = add i64 [[TMP0]], 16
; CHECK-NEXT: [[SCEVGEP:%.*]] = inttoptr i64 [[TMP1]] to ptr
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We really shouldn't be emitting ptrtoint+add+inttoptr. Why do we end up with this instead of a GEP? I'd have expected this to not happen as both the phi and the final result are pointers...

; CHECK-NEXT: [[BOUND0:%.*]] = icmp ult ptr [[GEP_7]], [[GEP_1]]
; CHECK-NEXT: [[BOUND1:%.*]] = icmp ult ptr [[PTR_PHI]], [[SCEVGEP]]
; CHECK-NEXT: [[FOUND_CONFLICT:%.*]] = and i1 [[BOUND0]], [[BOUND1]]
Expand All @@ -90,10 +87,10 @@ define void @test(ptr %arg, i64 %arg1) {
; CHECK: inner.3:
; CHECK-NEXT: [[IV_2:%.*]] = phi i64 [ 0, [[INNER_3_PH]] ], [ [[IV_2_NEXT:%.*]], [[INNER_3]] ]
; CHECK-NEXT: [[GEP_8:%.*]] = getelementptr inbounds double, ptr [[GEP_6]], i64 [[IV_2]]
; CHECK-NEXT: store double 0.000000e+00, ptr [[GEP_7]], align 8, !alias.scope !0, !noalias !3
; CHECK-NEXT: store double 0.000000e+00, ptr [[GEP_8]], align 8, !alias.scope !3
; CHECK-NEXT: store double 0.000000e+00, ptr [[GEP_7]], align 8, !alias.scope [[META0:![0-9]+]], !noalias [[META3:![0-9]+]]
; CHECK-NEXT: store double 0.000000e+00, ptr [[GEP_8]], align 8, !alias.scope [[META3]]
; CHECK-NEXT: [[GEP_9:%.*]] = getelementptr double, ptr [[PTR_PHI]], i64 [[IV_2]]
; CHECK-NEXT: [[TMP18:%.*]] = load double, ptr [[GEP_9]], align 8, !alias.scope !3
; CHECK-NEXT: [[TMP18:%.*]] = load double, ptr [[GEP_9]], align 8, !alias.scope [[META3]]
; CHECK-NEXT: [[IV_2_NEXT]] = add nuw nsw i64 [[IV_2]], 1
; CHECK-NEXT: [[C_2:%.*]] = icmp eq i64 [[IV_2]], 1
; CHECK-NEXT: br i1 [[C_2]], label [[OUTER_LATCH_LOOPEXIT3:%.*]], label [[INNER_3]]
Expand Down
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