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[SCEV] Do not attempt to collect loop guards for loops without predecessor. #123662

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merged 2 commits into from
Jan 22, 2025
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[SCEV] Do not attempt to collect loop guards for loops without predecessor. #123662

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ed95df9
[SCEV] Do not attempt to collect loop guards for loops without predec…
juliannagele Jan 20, 2025
c950590
fixup! [SCEV] Do not attempt to collect loop guards for loops without…
juliannagele Jan 22, 2025
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2 changes: 2 additions & 0 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -15328,6 +15328,8 @@ ScalarEvolution::LoopGuards::collect(const Loop *L, ScalarEvolution &SE) {
BasicBlock *Header = L->getHeader();
BasicBlock *Pred = L->getLoopPredecessor();
LoopGuards Guards(SE);
if (!Pred)
return Guards;
SmallPtrSet<const BasicBlock *, 8> VisitedBlocks;
collectFromBlock(SE, Guards, Header, Pred, VisitedBlocks);
return Guards;
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28 changes: 28 additions & 0 deletions ...st/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
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Expand Up @@ -336,3 +336,31 @@ exit:
ret void

}

; Checks correct traversal for loops without a unique predecessor
; outside the loop.
define void @pr122913() {
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Can the crash also be reproduced without one of the predecessors being unreachable?

Would be good to add a test case where bb is reachable if possible

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@danilaml danilaml Jan 22, 2025 •
edited
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define void @pr122913() {
entry:
  br i1 1, label %bb, label %header

bb:
  br i1 1, label %exit, label %header

header:
  %0 = phi i32 [ %1, %body ], [ 0, %bb ], [ 0, %entry ]
  br label %body

body:
  %1 = add i32 %0, 1
  %2 = icmp ult i32 %1, 0
  br i1 %2, label %header, label %exit

exit:
  ret void
}

Seems to crash in the same way: https://godbolt.org/z/8rdhM11vh

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Thanks! -- updated the test

; CHECK-LABEL: pr122913
; CHECK-NEXT: Determining loop execution counts for: @pr122913
; CHECK-NEXT: Loop %header: backedge-taken count is i1 false
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i1 false
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is i1 false
; CHECK-NEXT: Loop %header: Trip multiple is 1
entry:
br i1 1, label %bb, label %header

bb:
br i1 1, label %exit, label %header

header:
%0 = phi i32 [ %1, %body ], [ 0, %bb ], [ 0, %entry ]
br label %body

body:
%1 = add i32 %0, 1
%2 = icmp ult i32 %1, 0
br i1 %2, label %header, label %exit

exit:
ret void
}
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