[llvm-dev] Question WRT llvm.dbg.value

[Jeremy Morse via llvm-dev]

Hi Sourabh,

On Mon, Mar 30, 2020 at 8:09 AM Sourabh Singh Tomar via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Under what circumstances should a frontend choose to emit(at -O0(No optimization)) llvm.dbg.value for a local variable.
>
> I saw some debuginfo code in flang(older one), sort of it choose to emit *llvm.dbg.value* for *every load operation* happening on a *local variable*. And as noted below in IR snippet it has already emitted *llvm.dbg.declare* for the local variable.

[...]

>  call void @llvm.dbg.declare(metadata i32* %foo, metadata !9, metadata !DIExpression()), !dbg !11
> %0 = load i32, i32* %foo, align 4, !dbg !13
>   call void @llvm.dbg.value(metadata i32 %0, metadata !9, metadata !DIExpression()), !dbg !11

My understanding is that this isn't correct: dbg.declare specifies the
memory address of a variable for the whole lifetime of the function,
whereas dbg.value (and dbg.addr) specify the value/address until the
next debug intrinsic. Mixing these two kinds of intrinsics creates
ambiguity over where the variable location is at different positions
in the code.

If dbg.value intrinsics are to be used and the variable can be located
in memory too, then the producer needs to specify where the location
switches from a value to an address (and vice versa) with dbg.value /
dbg.addr. Awkwardly,I think there are some issues with dbg.addr at -O0
that Brian ran into here [0, 1], which might need addressing.

[0] http://lists.llvm.org/pipermail/llvm-dev/2020-February/139500.html
[1] http://lists.llvm.org/pipermail/llvm-dev/2020-February/139511.html

--
Thanks,
Jeremy