[DebugInfo][NewGVN] Debug value loss caused by the missing salvage

[Apochens]

NewGVN deletes trivially dead instructions without salvaging the corresponding debug values (markInstructionForDeletion(&I)).

llvm-project/llvm/lib/Transforms/Scalar/NewGVN.cpp

Lines 3044 to 3049 in 145b6cd

	if (isInstructionTriviallyDead(&I, TLI)) {
	InstrDFS[&I] = 0;
	LLVM_DEBUG(dbgs() << "Skipping trivially dead instruction " << I << "\n");
	markInstructionForDeletion(&I);
	continue;
	}

Here is an example: https://godbolt.org/z/G4PWEv6nr. In this example, the icmp instruction is deleted without its debug value being slavaged.

Before the deletion:

bb1:
  %tmp = load i8, ptr null, align 1, !dbg !25
    #dbg_value(i8 %tmp, !9, !DIExpression(), !25)
  %tmp2 = icmp eq i8 %tmp, 8, !dbg !26
    #dbg_value(i1 %tmp2, !11, !DIExpression(), !26)
  br label %bb3, !dbg !27

After the deletion:

bb1:
  %tmp = load i8, ptr null, align 1, !dbg !25
    #dbg_value(i8 %tmp, !9, !DIExpression(), !25)
    #dbg_value(i1 poison, !11, !DIExpression(), !26)
  br label %bb3, !dbg !27

[llvmbot]

@llvm/issue-subscribers-debuginfo

Author: Shan Huang (Apochens)

NewGVN deletes trivially dead instructions without salvaging the corresponding debug values (`markInstructionForDeletion(&I)`).

llvm-project/llvm/lib/Transforms/Scalar/NewGVN.cpp

Lines 3044 to 3049 in 145b6cd

	if (isInstructionTriviallyDead(&I, TLI)) {
	InstrDFS[&I] = 0;
	LLVM_DEBUG(dbgs() << "Skipping trivially dead instruction " << I << "\n");
	markInstructionForDeletion(&I);
	continue;
	}

Here is an example: https://godbolt.org/z/G4PWEv6nr. In this example, the icmp instruction is deleted without its debug value being slavaged.

Before the deletion:

bb1:
  %tmp = load i8, ptr null, align 1, !dbg !25
    #dbg_value(i8 %tmp, !9, !DIExpression(), !25)
  %tmp2 = icmp eq i8 %tmp, 8, !dbg !26
    #dbg_value(i1 %tmp2, !11, !DIExpression(), !26)
  br label %bb3, !dbg !27

After the deletion:

bb1:
  %tmp = load i8, ptr null, align 1, !dbg !25
    #dbg_value(i8 %tmp, !9, !DIExpression(), !25)
    #dbg_value(i1 poison, !11, !DIExpression(), !26)
  br label %bb3, !dbg !27