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[SimplifyCFG] Drop debug loc in SpeculativelyExecuteBB

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Summary:
According to HowToUpdateDebugInfo.rst:

```
Preserving the debug locations of speculated instructions can make
it seem like a condition is true when it's not (or vice versa), which
leads to a confusing single-stepping experience
```

This patch follows the recommendation to drop debug locations on
speculated instructions.

Reviewers: aprantl, davide

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82420
This commit is contained in:
Vedant Kumar 2020-06-23 15:53:09 -07:00
parent 4b2eead6d5
commit 8bce4cf299
2 changed files with 46 additions and 2 deletions
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9
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -2141,9 +2141,14 @@ bool SimplifyCFGOpt::SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
}
// Metadata can be dependent on the condition we are hoisting above.
// Conservatively strip all metadata on the instruction.
for (auto &I : *ThenBB)
// Conservatively strip all metadata on the instruction. Drop the debug loc
// to avoid making it appear as if the condition is a constant, which would
// be misleading while debugging.
for (auto &I : *ThenBB) {
if (!SpeculatedStoreValue || &I != SpeculatedStore)
I.setDebugLoc(DebugLoc());
I.dropUnknownNonDebugMetadata();
}
// Hoist the instructions.
BB->getInstList().splice(BI->getIterator(), ThenBB->getInstList(),

39
test/Transforms/SimplifyCFG/drop-debug-loc-when-speculating.ll Normal file
View File

@ -0,0 +1,39 @@
; RUN: opt -S -o - %s -simplifycfg | FileCheck %s
declare i1 @make_condition()
; CHECK-LABEL: @test1(
; CHECK: and i1 [[COND:%.*]], [[COND]]{{$}}
; CHECK: select i1 [[COND]]
define void @test1() !dbg !6 {
start:
%cond = call i1 @make_condition(), !dbg !8
br i1 %cond, label %then, label %else, !dbg !9
then: ; preds = %start
%and = and i1 %cond, %cond, !dbg !10
br label %else, !dbg !11
else: ; preds = %then, %start
%phi = phi i1 [ %cond, %start ], [ %and, %then ], !dbg !12
ret void, !dbg !13
}
!llvm.dbg.cu = !{!0}
!llvm.debugify = !{!3, !4}
!llvm.module.flags = !{!5}
!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
!1 = !DIFile(filename: "test.ll", directory: "/")
!2 = !{}
!3 = !{i32 6}
!4 = !{i32 0}
!5 = !{i32 2, !"Debug Info Version", i32 3}
!6 = distinct !DISubprogram(name: "test1", linkageName: "test1", scope: null, file: !1, line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
!7 = !DISubroutineType(types: !2)
!8 = !DILocation(line: 1, column: 1, scope: !6)
!9 = !DILocation(line: 2, column: 1, scope: !6)
!10 = !DILocation(line: 3, column: 2, scope: !6)
!11 = !DILocation(line: 4, column: 2, scope: !6)
!12 = !DILocation(line: 5, column: 3, scope: !6)
!13 = !DILocation(line: 6, column: 3, scope: !6)