[DebugInfo][NewGVN] Deleting instructions without salvaging their debug values

[Apochens]

In NewGVN, NewGVN::eliminateInstructions deletes instructions by first marking them for deletion. In the following code, the instruction is deleted after being marked without having its debug value salvaged.

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

Line 4079 in 6c8c836

markInstructionForDeletion(DefI);

Here is an example: https://godbolt.org/z/K4EanM9Ka. In this example, %add2 is deleted and its debug value becomes poison.

; Before NewGVN
define void @binop(i32 %x, i32 %y) !dbg !5 {
  %add1 = add i32 %x, %y, !dbg !12
    #dbg_value(i32 %add1, !9, !DIExpression(), !12)
  %add2 = add i32 %y, %x, !dbg !13
    #dbg_value(i32 %add2, !11, !DIExpression(), !13)
  call void @use(i32 %add1, i32 %add2), !dbg !14
  ret void, !dbg !15
}

; After NewGVN
define void @binop(i32 %x, i32 %y) !dbg !5 {
  %add1 = add i32 %x, %y, !dbg !12
    #dbg_value(i32 %add1, !9, !DIExpression(), !12)
    #dbg_value(i32 poison, !11, !DIExpression(), !13)
  call void @use(i32 %add1, i32 %add1), !dbg !14
  ret void, !dbg !15
}

[llvmbot]

@llvm/issue-subscribers-debuginfo

Author: Shan Huang (Apochens)

In NewGVN, `NewGVN::eliminateInstructions` deletes instructions by first marking them for deletion. In the following code, the instruction is deleted after being marked without having its debug value salvaged.

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

Line 4079 in 6c8c836

markInstructionForDeletion(DefI);

Here is an example: https://godbolt.org/z/K4EanM9Ka. In this example, %add2 is deleted and its debug value becomes poison.

; Before NewGVN
define void @<!-- -->binop(i32 %x, i32 %y) !dbg !5 {
  %add1 = add i32 %x, %y, !dbg !12
    #dbg_value(i32 %add1, !9, !DIExpression(), !12)
  %add2 = add i32 %y, %x, !dbg !13
    #dbg_value(i32 %add2, !11, !DIExpression(), !13)
  call void @<!-- -->use(i32 %add1, i32 %add2), !dbg !14
  ret void, !dbg !15
}

; After NewGVN
define void @<!-- -->binop(i32 %x, i32 %y) !dbg !5 {
  %add1 = add i32 %x, %y, !dbg !12
    #dbg_value(i32 %add1, !9, !DIExpression(), !12)
    #dbg_value(i32 poison, !11, !DIExpression(), !13)
  call void @<!-- -->use(i32 %add1, i32 %add1), !dbg !14
  ret void, !dbg !15
}