llvm dev - Mar 2020 - Question WRT llvm.dbg.value

Hello Everyone,

I have general question WRT llvm.dbg.value intrinsic function semantics.

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.

IR snippet of a subprogram from flang -
-----------
 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

*Please note here, that IR snippet is generated by flang(at -O0),  so no
clang/llvm intervention happening till this point. After this llvm takes
over this IR for final target code generation.

Can anybody provide some clarity WRT this.

Thanks a lot!
Sourabh Singh Tomar
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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

Hi!

On Mon, 2020-03-30 at 12:13 +0100, Jeremy Morse via llvm-dev
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.
> 
> 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.
There are also some issues with how SelectionDAG place the resulting DBG_VALUE
instructions for dbg.addr: https://bugs.llvm.org/show_bug.cgi?id=35318

Best regards,
David
> [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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

> 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
<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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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