[PATCH] D80691: Proposed fix for PR46114

[Qirun Zhang via Phabricator via llvm-commits]

helloqirun created this revision.
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  call void @llvm.dbg.value(metadata i32 0, metadata !22, metadata !DIExpression()), !dbg !31
  call void @llvm.dbg.value(metadata i32 %0, metadata !22, metadata !DIExpression()), !dbg !31

`llvm::RemoveRedundantDbgInstrs` will do a backward scan  to remove the first `dbg.value(.. i32 0)`.

The root case is in EarlyCSE.

Before EarlyCSE, we have

  call void @llvm.dbg.value(metadata i32 %0, metadata !21, metadata !DIExpression()), !dbg !31
  call void @llvm.dbg.value(metadata i32 0, metadata !22, metadata !DIExpression()), !dbg !31
  %1 = load i32, i32* @a, align 4, !dbg !32, !tbaa !27
  call void @llvm.dbg.value(metadata i32 %1, metadata !22, metadata !DIExpression()), !dbg !31

When removing `%1 = load i32, i32* @a, align 4, !dbg !32, !tbaa !27`, we should salvage the debug info early, before replacing all uses with `%0`.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D80691

Files:
  llvm/lib/Transforms/Scalar/EarlyCSE.cpp


Index: llvm/lib/Transforms/Scalar/EarlyCSE.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -1136,6 +1136,8 @@
             LLVM_DEBUG(dbgs() << "Skipping due to debug counter\n");
             continue;
           }
+	  //Salvage debug info early to be compatible with removeRedundantDbgInstrsUsingBackwardScan().
+          salvageDebugInfoOrMarkUndef(Inst);
           if (!Inst.use_empty())
             Inst.replaceAllUsesWith(Op);
           salvageKnowledge(&Inst, &AC);


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