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[LV] Don't mark ptrs as safe to speculate if fed by UB/poison op. #143204

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Merged
merged 3 commits into from
Jun 20, 2025
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[LV] Don't mark ptrs as safe to speculate if fed by UB/poison op. #143204

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3e2a4c4
[LV] Don't mark ptrs as safe to load unconditionally if fed by UB/poi…
fhahn Jun 6, 2025
5f1a523
Merge remote-tracking branch 'origin/main' into lv-check-for-ub-poiso…
fhahn Jun 20, 2025
9724538
!fixup update test after merge, update name.
fhahn Jun 20, 2025
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41 changes: 41 additions & 0 deletions llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
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Expand Up @@ -1491,10 +1491,51 @@ bool LoopVectorizationLegality::canVectorizeWithIfConvert() {
SmallVector<const SCEVPredicate *, 4> Predicates;
for (Instruction &I : *BB) {
LoadInst *LI = dyn_cast<LoadInst>(&I);

// Make sure we can execute all computations feeding into Ptr in the loop
// w/o triggering UB and that none of the out-of-loop operands are poison.
// We do not need to check if operations inside the loop can produce
// poison due to flags (e.g. due to an inbounds GEP going out of bounds),
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Does include things like nsw, nuw for sub/add operations? I'm just thinking of a scalar loop that has something like:

  %index = phi i64 ...
  %index_plus_offset = add nuw nsw i64 %index, 12345678
  %gep = getelementptr inbounds i64, ptr %ptr, i64 %index_plus_offset

where the %index_plus_offset could be poison-generating. Are you saying that after vectorisation any such flags on adds or subs that feed into the GEP cannot survive?

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Yes it includes all relevant flags, including nuw, nsw, inbounds. The flags are only dropped for any op that computes a pointer for loads that are executed conditionally in the original loop but executed unconditionally in the vector loop.

// because flags will be dropped when executing them unconditionally.
// TODO: Results could be improved by considering poison-propagation
// properties of visited ops.
Comment on lines +1500 to +1501
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Post-commit nit: another TODO is to potentially save compile time by computing all SpeculatablePointers defined in the loop together, instead of analyzing the expression of each pointer independently, as some pointers may be used to define others?

auto CanSpeculatePointerOp = [this](Value *Ptr) {
SmallVector<Value *> Worklist = {Ptr};
SmallPtrSet<Value *, 4> Visited;
while (!Worklist.empty()) {
Value *CurrV = Worklist.pop_back_val();
if (!Visited.insert(CurrV).second)
continue;

auto *CurrI = dyn_cast<Instruction>(CurrV);
if (!CurrI || !TheLoop->contains(CurrI)) {
// If operands from outside the loop may be poison then Ptr may also
// be poison.
if (!isGuaranteedNotToBePoison(CurrV, AC,
TheLoop->getLoopPredecessor()
->getTerminator()
->getIterator()))
return false;
continue;
}

// A loaded value may be poison, independent of any flags.
if (isa<LoadInst>(CurrI) && !isGuaranteedNotToBePoison(CurrV, AC))
return false;

// For other ops, assume poison can only be introduced via flags,
// which can be dropped.
if (!isa<PHINode>(CurrI) && !isSafeToSpeculativelyExecute(CurrI))
return false;
append_range(Worklist, CurrI->operands());
}
return true;
};
// Pass the Predicates pointer to isDereferenceableAndAlignedInLoop so
// that it will consider loops that need guarding by SCEV checks. The
// vectoriser will generate these checks if we decide to vectorise.
if (LI && !LI->getType()->isVectorTy() && !mustSuppressSpeculation(*LI) &&
CanSpeculatePointerOp(LI->getPointerOperand()) &&
isDereferenceableAndAlignedInLoop(LI, TheLoop, SE, *DT, AC,
&Predicates))
SafePointers.insert(LI->getPointerOperand());
Expand Down
124 changes: 82 additions & 42 deletions llvm/test/Transforms/LoopVectorize/X86/drop-poison-generating-flags.ll
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Expand Up @@ -666,34 +666,54 @@ define void @pr70590_recipe_without_underlying_instr(i64 %n, ptr noalias %dst) {
; CHECK-NEXT: [[BROADCAST_SPLAT:%.*]] = shufflevector <4 x i64> [[BROADCAST_SPLATINSERT]], <4 x i64> poison, <4 x i32> zeroinitializer
; CHECK-NEXT: br label %[[VECTOR_BODY:.*]]
; CHECK: [[VECTOR_BODY]]:
; CHECK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[PRED_SREM_CONTINUE6:.*]] ]
; CHECK-NEXT: [[VEC_IND:%.*]] = phi <4 x i64> [ <i64 0, i64 1, i64 2, i64 3>, %[[VECTOR_PH]] ], [ [[VEC_IND_NEXT:%.*]], %[[PRED_SREM_CONTINUE6]] ]
; CHECK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[PRED_LOAD_CONTINUE6:.*]] ]
; CHECK-NEXT: [[VEC_IND:%.*]] = phi <4 x i64> [ <i64 0, i64 1, i64 2, i64 3>, %[[VECTOR_PH]] ], [ [[VEC_IND_NEXT:%.*]], %[[PRED_LOAD_CONTINUE6]] ]
; CHECK-NEXT: [[TMP0:%.*]] = icmp eq <4 x i64> [[VEC_IND]], [[BROADCAST_SPLAT]]
; CHECK-NEXT: [[TMP1:%.*]] = xor <4 x i1> [[TMP0]], splat (i1 true)
; CHECK-NEXT: [[TMP2:%.*]] = extractelement <4 x i1> [[TMP1]], i32 0
; CHECK-NEXT: br i1 [[TMP2]], label %[[PRED_SREM_IF:.*]], label %[[PRED_SREM_CONTINUE:.*]]
; CHECK: [[PRED_SREM_IF]]:
; CHECK-NEXT: br label %[[PRED_SREM_CONTINUE]]
; CHECK: [[PRED_SREM_CONTINUE]]:
; CHECK-NEXT: br i1 [[TMP2]], label %[[PRED_LOAD_IF:.*]], label %[[PRED_LOAD_CONTINUE:.*]]
; CHECK: [[PRED_LOAD_IF]]:
; CHECK-NEXT: [[TMP3:%.*]] = add i64 [[INDEX]], 0
; CHECK-NEXT: [[TMP4:%.*]] = add i64 [[TMP3]], poison
; CHECK-NEXT: [[TMP23:%.*]] = getelementptr [5 x i8], ptr @c, i64 0, i64 [[TMP4]]
; CHECK-NEXT: [[TMP6:%.*]] = load i8, ptr [[TMP23]], align 1
; CHECK-NEXT: [[TMP24:%.*]] = insertelement <4 x i8> poison, i8 [[TMP6]], i32 0
; CHECK-NEXT: br label %[[PRED_LOAD_CONTINUE]]
; CHECK: [[PRED_LOAD_CONTINUE]]:
; CHECK-NEXT: [[TMP8:%.*]] = phi <4 x i8> [ poison, %[[VECTOR_BODY]] ], [ [[TMP24]], %[[PRED_LOAD_IF]] ]
; CHECK-NEXT: [[TMP5:%.*]] = extractelement <4 x i1> [[TMP1]], i32 1
; CHECK-NEXT: br i1 [[TMP5]], label %[[PRED_SREM_IF1:.*]], label %[[PRED_SREM_CONTINUE2:.*]]
; CHECK: [[PRED_SREM_IF1]]:
; CHECK-NEXT: br label %[[PRED_SREM_CONTINUE2]]
; CHECK: [[PRED_SREM_CONTINUE2]]:
; CHECK-NEXT: br i1 [[TMP5]], label %[[PRED_LOAD_IF1:.*]], label %[[PRED_LOAD_CONTINUE2:.*]]
; CHECK: [[PRED_LOAD_IF1]]:
; CHECK-NEXT: [[TMP10:%.*]] = add i64 [[INDEX]], 1
; CHECK-NEXT: [[TMP11:%.*]] = add i64 [[TMP10]], poison
; CHECK-NEXT: [[TMP25:%.*]] = getelementptr [5 x i8], ptr @c, i64 0, i64 [[TMP11]]
; CHECK-NEXT: [[TMP26:%.*]] = load i8, ptr [[TMP25]], align 1
; CHECK-NEXT: [[TMP14:%.*]] = insertelement <4 x i8> [[TMP8]], i8 [[TMP26]], i32 1
; CHECK-NEXT: br label %[[PRED_LOAD_CONTINUE2]]
; CHECK: [[PRED_LOAD_CONTINUE2]]:
; CHECK-NEXT: [[TMP29:%.*]] = phi <4 x i8> [ [[TMP8]], %[[PRED_LOAD_CONTINUE]] ], [ [[TMP14]], %[[PRED_LOAD_IF1]] ]
; CHECK-NEXT: [[TMP7:%.*]] = extractelement <4 x i1> [[TMP1]], i32 2
; CHECK-NEXT: br i1 [[TMP7]], label %[[PRED_SREM_IF3:.*]], label %[[PRED_SREM_CONTINUE4:.*]]
; CHECK: [[PRED_SREM_IF3]]:
; CHECK-NEXT: br label %[[PRED_SREM_CONTINUE4]]
; CHECK: [[PRED_SREM_CONTINUE4]]:
; CHECK-NEXT: br i1 [[TMP7]], label %[[PRED_LOAD_IF3:.*]], label %[[PRED_LOAD_CONTINUE4:.*]]
; CHECK: [[PRED_LOAD_IF3]]:
; CHECK-NEXT: [[TMP17:%.*]] = add i64 [[INDEX]], 2
; CHECK-NEXT: [[TMP18:%.*]] = add i64 [[TMP17]], poison
; CHECK-NEXT: [[TMP19:%.*]] = getelementptr [5 x i8], ptr @c, i64 0, i64 [[TMP18]]
; CHECK-NEXT: [[TMP20:%.*]] = load i8, ptr [[TMP19]], align 1
; CHECK-NEXT: [[TMP21:%.*]] = insertelement <4 x i8> [[TMP29]], i8 [[TMP20]], i32 2
; CHECK-NEXT: br label %[[PRED_LOAD_CONTINUE4]]
; CHECK: [[PRED_LOAD_CONTINUE4]]:
; CHECK-NEXT: [[TMP22:%.*]] = phi <4 x i8> [ [[TMP29]], %[[PRED_LOAD_CONTINUE2]] ], [ [[TMP21]], %[[PRED_LOAD_IF3]] ]
; CHECK-NEXT: [[TMP9:%.*]] = extractelement <4 x i1> [[TMP1]], i32 3
; CHECK-NEXT: br i1 [[TMP9]], label %[[PRED_SREM_IF5:.*]], label %[[PRED_SREM_CONTINUE6]]
; CHECK: [[PRED_SREM_IF5]]:
; CHECK-NEXT: br label %[[PRED_SREM_CONTINUE6]]
; CHECK: [[PRED_SREM_CONTINUE6]]:
; CHECK-NEXT: [[TMP12:%.*]] = add i64 [[INDEX]], poison
; CHECK-NEXT: br i1 [[TMP9]], label %[[PRED_LOAD_IF5:.*]], label %[[PRED_LOAD_CONTINUE6]]
; CHECK: [[PRED_LOAD_IF5]]:
; CHECK-NEXT: [[TMP30:%.*]] = add i64 [[INDEX]], 3
; CHECK-NEXT: [[TMP12:%.*]] = add i64 [[TMP30]], poison
; CHECK-NEXT: [[TMP13:%.*]] = getelementptr [5 x i8], ptr @c, i64 0, i64 [[TMP12]]
; CHECK-NEXT: [[TMP14:%.*]] = getelementptr i8, ptr [[TMP13]], i32 0
; CHECK-NEXT: [[WIDE_LOAD:%.*]] = load <4 x i8>, ptr [[TMP14]], align 1
; CHECK-NEXT: [[TMP27:%.*]] = load i8, ptr [[TMP13]], align 1
; CHECK-NEXT: [[TMP28:%.*]] = insertelement <4 x i8> [[TMP22]], i8 [[TMP27]], i32 3
; CHECK-NEXT: br label %[[PRED_LOAD_CONTINUE6]]
; CHECK: [[PRED_LOAD_CONTINUE6]]:
; CHECK-NEXT: [[WIDE_LOAD:%.*]] = phi <4 x i8> [ [[TMP22]], %[[PRED_LOAD_CONTINUE4]] ], [ [[TMP28]], %[[PRED_LOAD_IF5]] ]
; CHECK-NEXT: [[PREDPHI:%.*]] = select <4 x i1> [[TMP0]], <4 x i8> zeroinitializer, <4 x i8> [[WIDE_LOAD]]
; CHECK-NEXT: [[TMP15:%.*]] = getelementptr i8, ptr [[DST]], i64 [[INDEX]]
; CHECK-NEXT: [[TMP16:%.*]] = getelementptr i8, ptr [[TMP15]], i32 0
Expand Down Expand Up @@ -743,34 +763,54 @@ define void @recipe_without_underlying_instr_lanes_used(i64 %n, ptr noalias %dst
; CHECK-NEXT: [[BROADCAST_SPLAT:%.*]] = shufflevector <4 x i64> [[BROADCAST_SPLATINSERT]], <4 x i64> poison, <4 x i32> zeroinitializer
; CHECK-NEXT: br label %[[VECTOR_BODY:.*]]
; CHECK: [[VECTOR_BODY]]:
; CHECK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[PRED_SREM_CONTINUE6:.*]] ]
; CHECK-NEXT: [[VEC_IND:%.*]] = phi <4 x i64> [ <i64 0, i64 1, i64 2, i64 3>, %[[VECTOR_PH]] ], [ [[VEC_IND_NEXT:%.*]], %[[PRED_SREM_CONTINUE6]] ]
; CHECK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[PRED_LOAD_CONTINUE6:.*]] ]
; CHECK-NEXT: [[VEC_IND:%.*]] = phi <4 x i64> [ <i64 0, i64 1, i64 2, i64 3>, %[[VECTOR_PH]] ], [ [[VEC_IND_NEXT:%.*]], %[[PRED_LOAD_CONTINUE6]] ]
; CHECK-NEXT: [[TMP0:%.*]] = icmp eq <4 x i64> [[VEC_IND]], [[BROADCAST_SPLAT]]
; CHECK-NEXT: [[TMP1:%.*]] = xor <4 x i1> [[TMP0]], splat (i1 true)
; CHECK-NEXT: [[TMP2:%.*]] = extractelement <4 x i1> [[TMP1]], i32 0
; CHECK-NEXT: br i1 [[TMP2]], label %[[PRED_SREM_IF:.*]], label %[[PRED_SREM_CONTINUE:.*]]
; CHECK: [[PRED_SREM_IF]]:
; CHECK-NEXT: br label %[[PRED_SREM_CONTINUE]]
; CHECK: [[PRED_SREM_CONTINUE]]:
; CHECK-NEXT: br i1 [[TMP2]], label %[[PRED_LOAD_IF:.*]], label %[[PRED_LOAD_CONTINUE:.*]]
; CHECK: [[PRED_LOAD_IF]]:
; CHECK-NEXT: [[TMP9:%.*]] = add i64 [[INDEX]], 0
; CHECK-NEXT: [[TMP16:%.*]] = add i64 [[TMP9]], poison
; CHECK-NEXT: [[TMP23:%.*]] = getelementptr [5 x i8], ptr @c, i64 0, i64 [[TMP16]]
; CHECK-NEXT: [[TMP6:%.*]] = load i8, ptr [[TMP23]], align 1
; CHECK-NEXT: [[TMP24:%.*]] = insertelement <4 x i8> poison, i8 [[TMP6]], i32 0
; CHECK-NEXT: br label %[[PRED_LOAD_CONTINUE]]
; CHECK: [[PRED_LOAD_CONTINUE]]:
; CHECK-NEXT: [[TMP25:%.*]] = phi <4 x i8> [ poison, %[[VECTOR_BODY]] ], [ [[TMP24]], %[[PRED_LOAD_IF]] ]
; CHECK-NEXT: [[TMP3:%.*]] = extractelement <4 x i1> [[TMP1]], i32 1
; CHECK-NEXT: br i1 [[TMP3]], label %[[PRED_SREM_IF1:.*]], label %[[PRED_SREM_CONTINUE2:.*]]
; CHECK: [[PRED_SREM_IF1]]:
; CHECK-NEXT: br label %[[PRED_SREM_CONTINUE2]]
; CHECK: [[PRED_SREM_CONTINUE2]]:
; CHECK-NEXT: br i1 [[TMP3]], label %[[PRED_LOAD_IF1:.*]], label %[[PRED_LOAD_CONTINUE2:.*]]
; CHECK: [[PRED_LOAD_IF1]]:
; CHECK-NEXT: [[TMP26:%.*]] = add i64 [[INDEX]], 1
; CHECK-NEXT: [[TMP29:%.*]] = add i64 [[TMP26]], poison
; CHECK-NEXT: [[TMP30:%.*]] = getelementptr [5 x i8], ptr @c, i64 0, i64 [[TMP29]]
; CHECK-NEXT: [[TMP13:%.*]] = load i8, ptr [[TMP30]], align 1
; CHECK-NEXT: [[TMP14:%.*]] = insertelement <4 x i8> [[TMP25]], i8 [[TMP13]], i32 1
; CHECK-NEXT: br label %[[PRED_LOAD_CONTINUE2]]
; CHECK: [[PRED_LOAD_CONTINUE2]]:
; CHECK-NEXT: [[TMP15:%.*]] = phi <4 x i8> [ [[TMP25]], %[[PRED_LOAD_CONTINUE]] ], [ [[TMP14]], %[[PRED_LOAD_IF1]] ]
; CHECK-NEXT: [[TMP4:%.*]] = extractelement <4 x i1> [[TMP1]], i32 2
; CHECK-NEXT: br i1 [[TMP4]], label %[[PRED_SREM_IF3:.*]], label %[[PRED_SREM_CONTINUE4:.*]]
; CHECK: [[PRED_SREM_IF3]]:
; CHECK-NEXT: br label %[[PRED_SREM_CONTINUE4]]
; CHECK: [[PRED_SREM_CONTINUE4]]:
; CHECK-NEXT: br i1 [[TMP4]], label %[[PRED_LOAD_IF3:.*]], label %[[PRED_LOAD_CONTINUE4:.*]]
; CHECK: [[PRED_LOAD_IF3]]:
; CHECK-NEXT: [[TMP17:%.*]] = add i64 [[INDEX]], 2
; CHECK-NEXT: [[TMP18:%.*]] = add i64 [[TMP17]], poison
; CHECK-NEXT: [[TMP19:%.*]] = getelementptr [5 x i8], ptr @c, i64 0, i64 [[TMP18]]
; CHECK-NEXT: [[TMP20:%.*]] = load i8, ptr [[TMP19]], align 1
; CHECK-NEXT: [[TMP21:%.*]] = insertelement <4 x i8> [[TMP15]], i8 [[TMP20]], i32 2
; CHECK-NEXT: br label %[[PRED_LOAD_CONTINUE4]]
; CHECK: [[PRED_LOAD_CONTINUE4]]:
; CHECK-NEXT: [[TMP22:%.*]] = phi <4 x i8> [ [[TMP15]], %[[PRED_LOAD_CONTINUE2]] ], [ [[TMP21]], %[[PRED_LOAD_IF3]] ]
; CHECK-NEXT: [[TMP5:%.*]] = extractelement <4 x i1> [[TMP1]], i32 3
; CHECK-NEXT: br i1 [[TMP5]], label %[[PRED_SREM_IF5:.*]], label %[[PRED_SREM_CONTINUE6]]
; CHECK: [[PRED_SREM_IF5]]:
; CHECK-NEXT: br label %[[PRED_SREM_CONTINUE6]]
; CHECK: [[PRED_SREM_CONTINUE6]]:
; CHECK-NEXT: [[TMP7:%.*]] = add i64 [[INDEX]], poison
; CHECK-NEXT: br i1 [[TMP5]], label %[[PRED_LOAD_IF5:.*]], label %[[PRED_LOAD_CONTINUE6]]
; CHECK: [[PRED_LOAD_IF5]]:
; CHECK-NEXT: [[TMP31:%.*]] = add i64 [[INDEX]], 3
; CHECK-NEXT: [[TMP7:%.*]] = add i64 [[TMP31]], poison
; CHECK-NEXT: [[TMP8:%.*]] = getelementptr [5 x i8], ptr @c, i64 0, i64 [[TMP7]]
; CHECK-NEXT: [[TMP9:%.*]] = getelementptr i8, ptr [[TMP8]], i32 0
; CHECK-NEXT: [[WIDE_LOAD:%.*]] = load <4 x i8>, ptr [[TMP9]], align 1
; CHECK-NEXT: [[TMP27:%.*]] = load i8, ptr [[TMP8]], align 1
; CHECK-NEXT: [[TMP28:%.*]] = insertelement <4 x i8> [[TMP22]], i8 [[TMP27]], i32 3
; CHECK-NEXT: br label %[[PRED_LOAD_CONTINUE6]]
; CHECK: [[PRED_LOAD_CONTINUE6]]:
; CHECK-NEXT: [[WIDE_LOAD:%.*]] = phi <4 x i8> [ [[TMP22]], %[[PRED_LOAD_CONTINUE4]] ], [ [[TMP28]], %[[PRED_LOAD_IF5]] ]
; CHECK-NEXT: [[PREDPHI:%.*]] = select <4 x i1> [[TMP0]], <4 x i8> zeroinitializer, <4 x i8> [[WIDE_LOAD]]
; CHECK-NEXT: [[PREDPHI7:%.*]] = select <4 x i1> [[TMP0]], <4 x i64> zeroinitializer, <4 x i64> poison
; CHECK-NEXT: [[TMP12:%.*]] = extractelement <4 x i64> [[PREDPHI7]], i32 3
Expand Down
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