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

[Sourabh Singh Tomar via llvm-dev]

> 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.


          > Correct, you should not be mixing dbg.declare and other
instrinsics for the same variable

How about patching up llvm for the same, currently the IR showed above is
valid and can be processed by llvm for target code generation.
Should we move ahead invalidate this behavior as in "declare and value
intrinsic can't be specified for same local variable". ?

So that no FE should generate this sort of thing in first place. clang
doesn't do that so this change should not affect clang.

Thanks,
Sourabh.

On Tue, Mar 31, 2020 at 2:24 AM Adrian Prantl <aprantl at apple.com> wrote:

>
>
> On Mar 30, 2020, at 4:13 AM, Jeremy Morse via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> 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.
>
>
> Correct, you should not be mixing dbg.declare and other instrinsics for
> the same variable.
>
> See also https://llvm.org/docs/SourceLevelDebugging.html#llvm-dbg-declare
>
> -- adrian
>
>
> 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
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>
>
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