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[LoopFusion] Assert failure in the issue 80301 #167837

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[LoopFusion] Assert failure in the issue 80301 #167837

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12 changes: 5 additions & 7 deletions llvm/lib/Transforms/Scalar/LoopFuse.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -406,16 +406,14 @@ struct FusionCandidateCompare {

// Do this compare first so if LHS == RHS, function returns false.
if (DT->dominates(RHSEntryBlock, LHSEntryBlock)) {
// RHS dominates LHS
// Verify LHS post-dominates RHS
assert(LHS.PDT->dominates(LHSEntryBlock, RHSEntryBlock));
return false;
// RHS dominates LHS.
// Check if LHS post-dominates RHS.
return !LHS.PDT->dominates(LHSEntryBlock, RHSEntryBlock);
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@amehsan amehsan Nov 13, 2025 •
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I am not sure what are the implication of violating control flow equivalence of the loops in the candidate set. There is a comment in the code that says members of FusionCandidateSet are control flow equivalent. but I haven't looked into it enough to know what happens if we violate the condition. @1997alireza @CongzheUalberta Do you know about this?

your comment says , while post-domination is required for fusing candidates, it is not required for adding something to the candidate set. Does this mean there is another check of post-dominance relationship later on?

I was going to post a comment on this issue and the one extracted from it, but have been very busy with DA stuff. Will try to post a comment soon (But I may need to read some parts of fusion code, if DA stuff leaves enough time for me).

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@sushgokh sushgokh Nov 13, 2025

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your comment says , while post-domination is required for fusing candidates, it is not required for adding something to the candidate set. Does this mean there is another check of post-dominance relationship later on?

'isAdjacent' is doing the same thing I believe ? In fact, 'isAdjacent' is more stricter than saying 2 loops are control flow equivalent.

}

if (DT->dominates(LHSEntryBlock, RHSEntryBlock)) {
// Verify RHS Postdominates LHS
assert(LHS.PDT->dominates(RHSEntryBlock, LHSEntryBlock));
return true;
// Check if RHS postdominates LHS.
return LHS.PDT->dominates(RHSEntryBlock, LHSEntryBlock);
}

// If two FusionCandidates are in the same level of dominator tree,
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139 changes: 139 additions & 0 deletions llvm/test/Transforms/LoopFusion/guarded.ll
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Expand Up @@ -390,3 +390,142 @@ for.2: ; preds = %for.cond13, %for.body6,
exit: ; preds = %for.cond13
ret void
}

define void @foo(ptr noalias %A, ptr noalias %B, i64 %N) {
; CHECK-LABEL: @foo(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[CMP4:%.*]] = icmp slt i64 0, [[N:%.*]]
; CHECK-NEXT: br i1 [[CMP4]], label [[BB3:%.*]], label [[BB14:%.*]]
; CHECK: bb3:
; CHECK-NEXT: br label [[BB5:%.*]]
; CHECK: bb5:
; CHECK-NEXT: [[I_05:%.*]] = phi i64 [ [[INC:%.*]], [[BB5]] ], [ 0, [[BB3]] ]
; CHECK-NEXT: [[I_0510:%.*]] = phi i64 [ [[INC10:%.*]], [[BB5]] ], [ 0, [[BB3]] ]
; CHECK-NEXT: [[SUB:%.*]] = sub nsw i64 [[I_05]], 3
; CHECK-NEXT: [[ADD:%.*]] = add nsw i64 [[I_05]], 3
; CHECK-NEXT: [[MUL:%.*]] = mul nsw i64 [[SUB]], [[ADD]]
; CHECK-NEXT: [[REM:%.*]] = srem i64 [[MUL]], [[I_05]]
; CHECK-NEXT: [[CONV:%.*]] = trunc i64 [[REM]] to i32
; CHECK-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds i32, ptr [[A:%.*]], i64 [[I_05]]
; CHECK-NEXT: store i32 [[CONV]], ptr [[ARRAYIDX]], align 4
; CHECK-NEXT: [[INC]] = add nsw i64 [[I_05]], 1
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i64 [[INC]], [[N]]
; CHECK-NEXT: [[ARRAYIDX10:%.*]] = getelementptr inbounds i32, ptr [[B:%.*]], i64 [[I_0510]]
; CHECK-NEXT: [[LOAD_B:%.*]] = load i32, ptr [[ARRAYIDX10]], align 4
; CHECK-NEXT: [[ARRAYIDX11:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[I_0510]]
; CHECK-NEXT: store i32 0, ptr [[ARRAYIDX11]], align 4
; CHECK-NEXT: [[INC10]] = add nsw i64 [[I_0510]], 1
; CHECK-NEXT: [[CMP10:%.*]] = icmp slt i64 [[INC10]], [[N]]
; CHECK-NEXT: br i1 [[CMP10]], label [[BB5]], label [[BB1010:%.*]]
; CHECK: bb1010:
; CHECK-NEXT: br label [[BB14]]
; CHECK: bb14:
; CHECK-NEXT: br i1 [[CMP4]], label [[BB8:%.*]], label [[BB12:%.*]]
; CHECK: bb8:
; CHECK-NEXT: br label [[BB9:%.*]]
; CHECK: bb9:
; CHECK-NEXT: [[I1_02:%.*]] = phi i64 [ [[INC14:%.*]], [[BB9]] ], [ 0, [[BB8]] ]
; CHECK-NEXT: [[SUB7:%.*]] = sub nsw i64 [[I1_02]], 3
; CHECK-NEXT: [[ADD8:%.*]] = add nsw i64 [[I1_02]], 3
; CHECK-NEXT: [[MUL9:%.*]] = mul nsw i64 [[SUB7]], [[ADD8]]
; CHECK-NEXT: [[REM10:%.*]] = srem i64 [[MUL9]], [[I1_02]]
; CHECK-NEXT: [[CONV11:%.*]] = trunc i64 [[REM10]] to i32
; CHECK-NEXT: [[ARRAYIDX12:%.*]] = getelementptr inbounds [1024 x i32], ptr @B, i64 0, i64 [[I1_02]]
; CHECK-NEXT: store i32 [[CONV11]], ptr [[ARRAYIDX12]], align 4
; CHECK-NEXT: [[INC14]] = add nsw i64 [[I1_02]], 1
; CHECK-NEXT: [[CMP3:%.*]] = icmp slt i64 [[INC14]], [[N]]
; CHECK-NEXT: br i1 [[CMP3]], label [[BB9]], label [[BB15:%.*]]
; CHECK: bb15:
; CHECK-NEXT: br label [[BB80:%.*]]
; CHECK: bb80:
; CHECK-NEXT: br label [[BB90:%.*]]
; CHECK: bb90:
; CHECK-NEXT: [[I1_0210:%.*]] = phi i64 [ [[INC1410:%.*]], [[BB90]] ], [ 0, [[BB80]] ]
; CHECK-NEXT: [[CONV12:%.*]] = trunc i64 [[I1_0210]] to i32
; CHECK-NEXT: [[ARRAYIDX1210:%.*]] = getelementptr inbounds [1024 x i32], ptr @B, i64 0, i64 [[I1_0210]]
; CHECK-NEXT: store i32 [[CONV12]], ptr [[ARRAYIDX1210]], align 4
; CHECK-NEXT: [[INC1410]] = add nsw i64 [[I1_0210]], 1
; CHECK-NEXT: [[CMP310:%.*]] = icmp slt i64 [[INC1410]], [[N]]
; CHECK-NEXT: br i1 [[CMP310]], label [[BB90]], label [[BB150:%.*]]
; CHECK: bb150:
; CHECK-NEXT: br label [[BB12]]
; CHECK: bb12:
; CHECK-NEXT: ret void
;
entry:
%cmp4 = icmp slt i64 0, %N
br i1 %cmp4, label %bb3, label %bb14

bb3: ; preds = %entry
br label %bb5

bb5: ; preds = %bb3, %bb5
%i.05 = phi i64 [ %inc, %bb5 ], [ 0, %bb3 ]
%sub = sub nsw i64 %i.05, 3
%add = add nsw i64 %i.05, 3
%mul = mul nsw i64 %sub, %add
%rem = srem i64 %mul, %i.05
%conv = trunc i64 %rem to i32
%arrayidx = getelementptr inbounds i32, ptr %A, i64 %i.05
store i32 %conv, ptr %arrayidx, align 4
%inc = add nsw i64 %i.05, 1
%cmp = icmp slt i64 %inc, %N
br i1 %cmp, label %bb5, label %bb103

bb103: ; preds = %bb103
br label %bb105

bb105: ; preds = %bb103, %bb105
%i.0510 = phi i64 [ %inc10, %bb105 ], [ 0, %bb103 ]
%arrayidx10 = getelementptr inbounds i32, ptr %B, i64 %i.0510
%load.b = load i32, ptr %arrayidx10, align 4
%arrayidx11 = getelementptr inbounds i32, ptr %A, i64 %i.0510
store i32 0, ptr %arrayidx11, align 4
%inc10 = add nsw i64 %i.0510, 1
%cmp10 = icmp slt i64 %inc10, %N
br i1 %cmp10, label %bb105, label %bb1010

bb1010: ; preds = %bb105
br label %bb14

bb14: ; preds = %bb1010, %entry
br i1 %cmp4, label %bb8, label %bb12

bb8: ; preds = %bb14
br label %bb9

bb9: ; preds = %bb8, %bb9
%i1.02 = phi i64 [ %inc14, %bb9 ], [ 0, %bb8 ]
%sub7 = sub nsw i64 %i1.02, 3
%add8 = add nsw i64 %i1.02, 3
%mul9 = mul nsw i64 %sub7, %add8
%rem10 = srem i64 %mul9, %i1.02
%conv11 = trunc i64 %rem10 to i32
%arrayidx12 = getelementptr inbounds [1024 x i32], ptr @B, i64 0, i64 %i1.02
store i32 %conv11, ptr %arrayidx12, align 4
%inc14 = add nsw i64 %i1.02, 1
%cmp3 = icmp slt i64 %inc14, %N
br i1 %cmp3, label %bb9, label %bb15

bb15: ; preds = %bb9
br label %bb80

bb80: ; preds = %bb15
br label %bb90

bb90: ; preds = %bb80, %bb90
%i1.0210 = phi i64 [ %inc1410, %bb90 ], [ 0, %bb80 ]
%conv12 = trunc i64 %i1.0210 to i32
%arrayidx1210 = getelementptr inbounds [1024 x i32], ptr @B, i64 0, i64 %i1.0210
store i32 %conv12, ptr %arrayidx1210, align 4
%inc1410 = add nsw i64 %i1.0210, 1
%cmp310 = icmp slt i64 %inc1410, %N
br i1 %cmp310, label %bb90, label %bb150

bb150: ; preds = %bb90
br label %bb12


bb12: ; preds = %bb15, %bb14
ret void
}
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