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[KeyInstr][LoopUnroll] Remap atoms while unrolling #133489

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May 9, 2025
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[KeyInstr][LoopUnroll] Remap atoms while unrolling #133489

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11 changes: 10 additions & 1 deletion llvm/lib/Transforms/Utils/LoopUnroll.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -752,6 +752,14 @@ llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
}
}

// Remap source location atom instance. Do this now, rather than
// when we remap instructions, because remap is called once we've
// cloned all blocks (all the clones would get the same atom
// number).
Comment on lines +755 to +758
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I get why all the clones need to get distinct atom numbers; what is it here that ensures that's the case, don't we need to reset something in VMap to ensure that's the case? i.e., if we called RemapSourceAtom later what would be different?

(I get the overall picture, but not how it's achieved here.)

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CloneBasicBlock maps the atoms from the cloned block so new ones can be remapped in the duplicated block (the next unroll iteration). It stores that info in VMap. We need to update those instructions now using that info.

remapInstructionsInBlocks (which does also call RemapSourceAtom) which is called later for all cloned is passed LastValueMap. That map just has the "running map" of new cloned instructions, i.e., values are overwritten after each unrolled iteration. If we used that infra to remap source atoms, we'd just remap all atoms x to y, in all the unrolled blocks. But what we want is x -> y1, x->y2, with a unique value in each unrolled iteration.

Does that make any sense? I fear I'm mincing this explanation.

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Ah right -- it's the fact that Something Happens (TM) during CloneBasicBlock that I think I was missing.

if (!VMap.AtomMap.empty())
for (Instruction &I : *New)
RemapSourceAtom(&I, VMap);

// Update our running map of newest clones
LastValueMap[*BB] = New;
for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
Expand Down Expand Up @@ -802,7 +810,8 @@ llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
}
}

// Remap all instructions in the most recent iteration
// Remap all instructions in the most recent iteration.
// Key Instructions: Nothing to do - we've already remapped the atoms.
remapInstructionsInBlocks(NewBlocks, LastValueMap);
for (BasicBlock *NewBlock : NewBlocks)
for (Instruction &I : *NewBlock)
Expand Down
69 changes: 69 additions & 0 deletions llvm/test/DebugInfo/KeyInstructions/Generic/loop-unroll-runtime.ll
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@@ -0,0 +1,69 @@

; RUN: opt %s -S --passes=loop-unroll -unroll-runtime=true -unroll-count=4 -unroll-remainder -o - \
; RUN: | FileCheck %s

;; Check atoms are remapped for runtime unrolling.

; CHECK: for.body.epil:
; CHECK-NEXT: store i64 %indvars.iv.unr, ptr %p, align 4, !dbg [[G2R1:!.*]]

; CHECK: for.body.epil.1:
; CHECK-NEXT: store i64 %indvars.iv.next.epil, ptr %p, align 4, !dbg [[G3R1:!.*]]

; CHECK: for.body.epil.2:
; CHECK-NEXT: store i64 %indvars.iv.next.epil.1, ptr %p, align 4, !dbg [[G4R1:!.*]]

; CHECK: for.body:
; CHECK-NEXT: %indvars.iv = phi i64 [ 0, %for.body.lr.ph.new ], [ %indvars.iv.next.3, %for.body ]
; CHECK-NEXT: %niter = phi i64 [ 0, %for.body.lr.ph.new ], [ %niter.next.3, %for.body ]
; CHECK-NEXT: store i64 %indvars.iv, ptr %p, align 4, !dbg [[G1R1:!.*]]
; CHECK-NEXT: %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
; CHECK-NEXT: store i64 %indvars.iv.next, ptr %p, align 4, !dbg [[G5R1:!.*]]
; CHECK-NEXT: %indvars.iv.next.1 = add nuw nsw i64 %indvars.iv, 2
; CHECK-NEXT: store i64 %indvars.iv.next.1, ptr %p, align 4, !dbg [[G6R1:!.*]]
; CHECK-NEXT: %indvars.iv.next.2 = add nuw nsw i64 %indvars.iv, 3
; CHECK-NEXT: store i64 %indvars.iv.next.2, ptr %p, align 4, !dbg [[G7R1:!.*]]
; CHECK-NEXT: %indvars.iv.next.3 = add nuw nsw i64 %indvars.iv, 4

; CHECK: [[G2R1]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 1)
; CHECK: [[G3R1]] = !DILocation({{.*}}, atomGroup: 3, atomRank: 1)
; CHECK: [[G4R1]] = !DILocation({{.*}}, atomGroup: 4, atomRank: 1)
; CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
; CHECK: [[G5R1]] = !DILocation({{.*}}, atomGroup: 5, atomRank: 1)
; CHECK: [[G6R1]] = !DILocation({{.*}}, atomGroup: 6, atomRank: 1)
; CHECK: [[G7R1]] = !DILocation({{.*}}, atomGroup: 7, atomRank: 1)

define i32 @unroll(ptr %p, i32 %N) local_unnamed_addr !dbg !5 {
entry:
%cmp9 = icmp eq i32 %N, 0
br i1 %cmp9, label %for.cond.cleanup, label %for.body.lr.ph

for.body.lr.ph: ; preds = %entry
%wide.trip.count = zext i32 %N to i64
br label %for.body

for.cond.cleanup: ; preds = %for.body, %entry
%r = phi i32 [ 0, %entry ], [ 1, %for.body ]
ret i32 %r

for.body: ; preds = %for.body, %for.body.lr.ph
%indvars.iv = phi i64 [ 0, %for.body.lr.ph ], [ %indvars.iv.next, %for.body ]
store i64 %indvars.iv, ptr %p, !dbg !8
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%exitcond = icmp eq i64 %indvars.iv.next, %wide.trip.count
br i1 %exitcond, label %for.cond.cleanup, label %for.body
}

!llvm.dbg.cu = !{!0}
!llvm.debugify = !{!2, !3}
!llvm.module.flags = !{!4}

!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
!1 = !DIFile(filename: "test.ll", directory: "/")
!2 = !{i32 17}
!3 = !{i32 0}
!4 = !{i32 2, !"Debug Info Version", i32 3}
!5 = distinct !DISubprogram(name: "unroll", linkageName: "unroll", scope: null, file: !1, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
!6 = !DISubroutineType(types: !7)
!7 = !{}
!8 = !DILocation(line: 1, column: 1, scope: !5, atomGroup: 1, atomRank: 1)
48 changes: 48 additions & 0 deletions llvm/test/DebugInfo/KeyInstructions/Generic/loop-unroll.ll
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@@ -0,0 +1,48 @@
; RUN: opt %s --passes=loop-unroll -S -o - -S | FileCheck %s

; CHECK: store i32 %a, ptr %p, align 4, !dbg [[G1R1:!.*]]
; CHECK: %v.1 = add i32 1, %a, !dbg [[G2R2:!.*]]
; CHECK: store i32 %v.1, ptr %p, align 4, !dbg [[G2R1:!.*]]
; CHECK: %v.2 = add i32 2, %a, !dbg [[G3R2:!.*]]
; CHECK: store i32 %v.2, ptr %p, align 4, !dbg [[G3R1:!.*]]

; CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
; CHECK: [[G2R2]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 2)
; CHECK: [[G2R1]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 1)
; CHECK: [[G3R2]] = !DILocation({{.*}}, atomGroup: 3, atomRank: 2)
; CHECK: [[G3R1]] = !DILocation({{.*}}, atomGroup: 3, atomRank: 1)

define void @f(ptr %p, i32 %a) local_unnamed_addr #0 !dbg !5 {
entry:
br label %for.body

for.cond.cleanup: ; preds = %for.body
ret void

for.body: ; preds = %for.body, %entry
%i = phi i32 [ 0, %entry ], [ %inc, %for.body ]
%v = add i32 %i, %a, !dbg !11
store i32 %v, ptr %p, align 4, !dbg !12
%inc = add i32 %i, 1
%cmp = icmp slt i32 %inc, 3
br i1 %cmp, label %for.body, label %for.cond.cleanup
}

declare i32 @bar(...) local_unnamed_addr

attributes #0 = { nounwind ssp uwtable }

!llvm.dbg.cu = !{!0}
!llvm.debugify = !{!2, !3}
!llvm.module.flags = !{!4}

!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
!1 = !DIFile(filename: "/home/och/scratch/test.ll", directory: "/")
!2 = !{i32 8}
!3 = !{i32 0}
!4 = !{i32 2, !"Debug Info Version", i32 3}
!5 = distinct !DISubprogram(name: "f", linkageName: "f", scope: null, file: !1, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
!6 = !DISubroutineType(types: !7)
!7 = !{}
!11 = !DILocation(line: 4, column: 1, scope: !5, atomGroup: 1, atomRank: 2)
!12 = !DILocation(line: 5, column: 1, scope: !5, atomGroup: 1, atomRank: 1)