[llvm] [DebugInfo][LoopStrengthReduce] Fix missing debug location updates (PR #97519)
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Tue Jul 2 21:38:24 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Shan Huang (Apochens)
<details>
<summary>Changes</summary>
Fix #<!-- -->97510 .
Note that, for the new phi instruction `NewPH`, which replaces the old phi `PH` and the cast `ShadowUse`, I choose to propagate the debug location of `PH` to it, because the cast is eliminated according to the optimization semantics.
BTW, after patching the debug location update of `NewPH`, I found that in the optimized IR, another phi instruction `%lsr.iv` has the same debug location as `%IV.S.` (`NewPH`). However, `%lsr.iv` should inherent the debug location of `%iv` in the unoptimized program, _i.e._, "line: 3" instead of "line: 2". The same thing happens to `%lsr.iv.next`, which has the same debug location as `%exitcond`. Yet, `%lsr.iv.next` should inherent the debug location of `%iv.next`, _i.e._, "line: 7" instead of "line: 8". The program before and after LSR are shown below.
**Before optimization**:
```LLVM IR
define i32 @<!-- -->foobar6() !dbg !5 {
entry:
br label %loop, !dbg !8
loop: ; preds = %loop, %entry
%accum = phi i32 [ -3220, %entry ], [ %accum.next, %loop ], !dbg !9
%iv = phi i32 [ 12, %entry ], [ %iv.next, %loop ], !dbg !10
%tmp1 = sitofp i32 %accum to double, !dbg !11
tail call void @<!-- -->foo(double %tmp1), !dbg !12
%accum.next = add nsw i32 %accum, 9597741, !dbg !13
%iv.next = add nuw nsw i32 %iv, 1, !dbg !14
%exitcond = icmp ugt i32 %iv, 235, !dbg !15
br i1 %exitcond, label %exit, label %loop, !dbg !16
exit: ; preds = %loop
ret i32 %accum.next, !dbg !17
}
...
!9 = !DILocation(line: 2, column: 1, scope: !5)
!10 = !DILocation(line: 3, column: 1, scope: !5)
...
!14 = !DILocation(line: 7, column: 1, scope: !5)
!15 = !DILocation(line: 8, column: 1, scope: !5)
...
```
**After optimization**:
```LLVM IR
define i32 @<!-- -->foobar6() !dbg !5 {
entry:
br label %loop, !dbg !8
loop: ; preds = %loop, %entry
->%lsr.iv = phi i32 [ %lsr.iv.next, %loop ], [ 11, %entry ], !dbg !9
%IV.S. = phi double [ -3.220000e+03, %entry ], [ %IV.S.next., %loop ], !dbg !9
tail call void @<!-- -->foo(double %IV.S.) #<!-- -->0, !dbg !10
%IV.S.next. = fadd double %IV.S., 0x41624E65A0000000, !dbg !11
->%lsr.iv.next = add nuw nsw i32 %lsr.iv, 1, !dbg !12
%exitcond = icmp ugt i32 %lsr.iv.next, 235, !dbg !12
br i1 %exitcond, label %exit, label %loop, !dbg !13
exit: ; preds = %loop
ret i32 -2135478791, !dbg !14
}
...
!9 = !DILocation(line: 2, column: 1, scope: !5)
...
!12 = !DILocation(line: 8, column: 1, scope: !5)
...
```
I guess it's caused by inserting new instructions with IRBuilder, in which the next instructions are specified as the insertion points. (Yet, I haven't found the corresponding code.🧐)
---
Full diff: https://github.com/llvm/llvm-project/pull/97519.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp (+2)
- (added) llvm/test/Transforms/LoopStrengthReduce/X86/preserving-debugloc-phi-binop.ll (+59)
``````````diff
diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
index 1c4a0d92dcde9..8ebead8b8fbd7 100644
--- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -2404,6 +2404,7 @@ void LSRInstance::OptimizeShadowIV() {
/* Add new PHINode. */
PHINode *NewPH = PHINode::Create(DestTy, 2, "IV.S.", PH->getIterator());
+ NewPH->setDebugLoc(PH->getDebugLoc());
/* create new increment. '++d' in above example. */
Constant *CFP = ConstantFP::get(DestTy, C->getZExtValue());
@@ -2411,6 +2412,7 @@ void LSRInstance::OptimizeShadowIV() {
Incr->getOpcode() == Instruction::Add ? Instruction::FAdd
: Instruction::FSub,
NewPH, CFP, "IV.S.next.", Incr->getIterator());
+ NewIncr->setDebugLoc(Incr->getDebugLoc());
NewPH->addIncoming(NewInit, PH->getIncomingBlock(Entry));
NewPH->addIncoming(NewIncr, PH->getIncomingBlock(Latch));
diff --git a/llvm/test/Transforms/LoopStrengthReduce/X86/preserving-debugloc-phi-binop.ll b/llvm/test/Transforms/LoopStrengthReduce/X86/preserving-debugloc-phi-binop.ll
new file mode 100644
index 0000000000000..1aecd34082a2a
--- /dev/null
+++ b/llvm/test/Transforms/LoopStrengthReduce/X86/preserving-debugloc-phi-binop.ll
@@ -0,0 +1,59 @@
+; RUN: opt -S -passes=loop-reduce -mtriple=x86_64-unknown-unknown < %s | FileCheck %s
+
+; Check that LoopStrengthReduce's OptimizeShadowIV() propagates the debug
+; locations of the old phi (`%accum`) and binop (`%accum.next`) instruction
+; to the new phi and binop instruction, respectively.
+
+target datalayout = "n8:16:32:64"
+
+define i32 @foobar6() !dbg !5 {
+; CHECK-LABEL: define i32 @foobar6(
+; CHECK: loop:
+; CHECK: [[IV_S_:%.*]] = phi double [ -3.220000e+03, %[[ENTRY:.*]] ], [ [[IV_S_NEXT_:%.*]], %loop ], !dbg [[DBG9:![0-9]+]]
+; CHECK: [[IV_S_NEXT_]] = fadd double [[IV_S_]], 0x41624E65A0000000, !dbg [[DBG11:![0-9]+]]
+; CHECK: exit:
+;
+entry:
+ br label %loop, !dbg !8
+
+loop: ; preds = %loop, %entry
+ %accum = phi i32 [ -3220, %entry ], [ %accum.next, %loop ], !dbg !9
+ %iv = phi i32 [ 12, %entry ], [ %iv.next, %loop ], !dbg !10
+ %tmp1 = sitofp i32 %accum to double, !dbg !11
+ tail call void @foo(double %tmp1), !dbg !12
+ %accum.next = add nsw i32 %accum, 9597741, !dbg !13
+ %iv.next = add nuw nsw i32 %iv, 1, !dbg !14
+ %exitcond = icmp ugt i32 %iv, 235, !dbg !15
+ br i1 %exitcond, label %exit, label %loop, !dbg !16
+
+exit: ; preds = %loop
+ ret i32 %accum.next, !dbg !17
+}
+
+declare void @foo(double)
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!2, !3}
+!llvm.module.flags = !{!4}
+
+; CHECK: [[DBG9]] = !DILocation(line: 2,
+; CHECK: [[DBG11]] = !DILocation(line: 6,
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "main.ll", directory: "/")
+!2 = !{i32 10}
+!3 = !{i32 0}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "foobar6", linkageName: "foobar6", 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)
``````````
</details>
https://github.com/llvm/llvm-project/pull/97519
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