Skip to content

[DebugInfo][LoopLoadElim] Fix missing debug location updates #91839

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 3 commits into from
May 17, 2024
Merged

[DebugInfo][LoopLoadElim] Fix missing debug location updates #91839

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
23d0f3c
fix missing debug location updates
Apochens May 11, 2024
ab97b2d
nit
Apochens May 13, 2024
b3f4235
refine the test
Apochens May 16, 2024
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
12 changes: 11 additions & 1 deletion llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -448,6 +448,10 @@ class LoadEliminationForLoop {
new LoadInst(Cand.Load->getType(), InitialPtr, "load_initial",
/* isVolatile */ false, Cand.Load->getAlign(),
PH->getTerminator()->getIterator());
// We don't give any debug location to Initial, because it is inserted
// into the loop's preheader. A debug location inside the loop will cause
// a misleading stepping when debugging. The test update-debugloc-store
// -forwarded.ll checks this.

PHINode *PHI = PHINode::Create(Initial->getType(), 2, "store_forwarded");
PHI->insertBefore(L->getHeader()->begin());
Expand All @@ -462,14 +466,20 @@ class LoadEliminationForLoop {
"The type sizes should match!");

Value *StoreValue = Cand.Store->getValueOperand();
if (LoadType != StoreType)
if (LoadType != StoreType) {
StoreValue = CastInst::CreateBitOrPointerCast(StoreValue, LoadType,
"store_forward_cast",
Cand.Store->getIterator());
// Because it casts the old `load` value and is used by the new `phi`
// which replaces the old `load`, we give the `load`'s debug location
// to it.
cast<Instruction>(StoreValue)->setDebugLoc(Cand.Load->getDebugLoc());
}

PHI->addIncoming(StoreValue, L->getLoopLatch());

Cand.Load->replaceAllUsesWith(PHI);
PHI->setDebugLoc(Cand.Load->getDebugLoc());
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

This is an interesting case. When promoting store/loads I think we usually give phis a debugloc from the store (or if it's from stores, plural, it's a merged location). At least that's what we do in mem2reg. I think in this case using the load's DebugLoc makes intuitive sense - we're just eliminating the load here after all, but I'd be interested to hear what others think about this case too (cc @SLTozer , @adrian-prantl , @dwblaikie, @jryans - there's a comment at the top of the function that explains the transformation) and what you make of this @Apochens?

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Using the load's location here does make intuitive sense to me as well. I'm less sure what we've done for such cases historically though, so I'm interested to hear what others think as well.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I think in this case using the load's DebugLoc makes intuitive sense - we're just eliminating the load here after all

That's what I agree with.

When promoting store/loads I think we usually give phis a debugloc from the store (or if it's from stores, plural, it's a merged location).

I'd like to know why give the store'd debug location to an instruction that replaces a load instruction. Are there some conventions or something?

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Hmm, should we ping other guys again? 🤔

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Probably best to wait a few more days first, as people may be on holiday or have a long queue to work through.

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Looking at the git history, it doesn't look like there's a particular reason for using the store instead of the load - the main point was to ensure that the scope was correct. I think using the load makes more sense, so current state SGTM.

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Seems like enough of us have appeared and all concluded the same thing here, so I think we can leave this part as it is.

}

/// Top-level driver for each loop: find store->load forwarding
Expand Down
85 changes: 85 additions & 0 deletions llvm/test/Transforms/LoopLoadElim/update-debugloc-store-forwarded.ll
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,85 @@
; RUN: opt -passes=loop-load-elim -S < %s | FileCheck %s

; LoopLoadElimination's propagateStoredValueToLoadUsers() replaces the
; `load` (`%a`) with a hoisted initial `load` and a `phi` that forwards
; the stored value.
; This test checks that the debug location is propagated to the new `phi`
; from the original `load` it replaces in block `%for.body` and the debug
; location drop of the hoisted `load` in block `%entry`.
; Moreover, this test also checks the debug location update of the new
; `bitcast` created when the `load` type is mismatched with the `store` type:
; store i32 ...
; %a = load float, ...
; Because the `bitcast` casts the old `load` value, it has the same debug
; location as the old `load` (ie., the same as the new `phi`).

; If the store and the load use different types, but have the same
; size then we should still be able to forward the value.
;
; for (unsigned i = 0; i < 100; i++) {
; A[i+1] = B[i] + 2;
; C[i] = ((float*)A)[i] * 2;
; }

define void @f(ptr noalias %A, ptr noalias %B, ptr noalias %C, i64 %N) !dbg !5 {
; CHECK-LABEL: define void @f(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[LOAD_INITIAL:%.*]] = load float, ptr {{.*}}, align 4{{$}}
; CHECK: for.body:
; CHECK-NEXT: [[STORE_FORWARDED:%.*]] = phi float [ [[LOAD_INITIAL]], %entry ], [ [[STORE_FORWARD_CAST:%.*]], %for.body ], !dbg [[DBG9:![0-9]+]]
; CHECK: [[STORE_FORWARD_CAST]] = bitcast i32 {{.*}} to float, !dbg [[DBG9]]
; CHECK: [[DBG9]] = !DILocation(line: 11,
;
entry:
br label %for.body, !dbg !8

for.body: ; preds = %for.body, %entry
%indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ], !dbg !9
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1, !dbg !10
%Aidx_next = getelementptr inbounds i32, ptr %A, i64 %indvars.iv.next, !dbg !11
%Bidx = getelementptr inbounds i32, ptr %B, i64 %indvars.iv, !dbg !12
%Cidx = getelementptr inbounds i32, ptr %C, i64 %indvars.iv, !dbg !13
%Aidx = getelementptr inbounds i32, ptr %A, i64 %indvars.iv, !dbg !14
%b = load i32, ptr %Bidx, align 4, !dbg !15
%a_p1 = add i32 %b, 2, !dbg !16
store i32 %a_p1, ptr %Aidx_next, align 4, !dbg !17
%a = load float, ptr %Aidx, align 4, !dbg !18
%c = fmul float %a, 2.000000e+00, !dbg !19
%c.int = fptosi float %c to i32, !dbg !20
store i32 %c.int, ptr %Cidx, align 4, !dbg !21
%exitcond = icmp eq i64 %indvars.iv.next, %N, !dbg !22
br i1 %exitcond, label %for.end, label %for.body, !dbg !23

for.end: ; preds = %for.body
ret void, !dbg !24
}

!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: "type-mismatch.ll", directory: "/")
!2 = !{i32 17}
!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 = !{}
!8 = !DILocation(line: 1, column: 1, scope: !5)
!9 = !DILocation(line: 2, column: 1, scope: !5)
!10 = !DILocation(line: 3, column: 1, scope: !5)
!11 = !DILocation(line: 4, column: 1, scope: !5)
!12 = !DILocation(line: 5, column: 1, scope: !5)
!13 = !DILocation(line: 6, column: 1, scope: !5)
!14 = !DILocation(line: 7, column: 1, scope: !5)
!15 = !DILocation(line: 8, column: 1, scope: !5)
!16 = !DILocation(line: 9, column: 1, scope: !5)
!17 = !DILocation(line: 10, column: 1, scope: !5)
!18 = !DILocation(line: 11, column: 1, scope: !5)
!19 = !DILocation(line: 12, column: 1, scope: !5)
!20 = !DILocation(line: 13, column: 1, scope: !5)
!21 = !DILocation(line: 14, column: 1, scope: !5)
!22 = !DILocation(line: 15, column: 1, scope: !5)
!23 = !DILocation(line: 16, column: 1, scope: !5)
!24 = !DILocation(line: 17, column: 1, scope: !5)
Loading