[llvm-dev] llvm-dwarfdump stats for inlined functions

[David Blaikie via llvm-dev]

On Wed, Apr 28, 2021 at 8:03 AM via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>
> Pedantically, DW_AT_abstract_origin is not required on a DW_TAG_inlined_subroutine; the DWARF spec says a concrete instance “may” omit attributes not specific to that instance, in which case there will be an abstract origin that will have those attributes.  It then goes on to point out that declaration coordinates (DW_AT_decl_[file,line,column]) are for the declaration, not the inlining site, which makes them good candidates for hoisting to the abstract origin.

Sure enough. Potentially one could argue that abstract_origin is
necessary to describe the scope where the original entity was written
(that can't be/isn't described in attributes) - though using
DW_AT_specification to refer to a declaration in the source scope
would probably suffice (even if that wasn't exactly representative of
the source code - perhaps the declaration was in one place, the
definition in another, and the inlining in a third place - there
wouldn't be any way to describe that second place without
abstract_origin (but there's probbaly not a lot of value in describing
that second place anyway).

> That is, DW_AT_abstract_origin is a way to optimize for size; it isn’t mandatory.  So, I think llvm-dwarfdump is correct to allow it.  I might prefer that “inlined functions without abstract origin” be what’s called out separately, rather than the “with” case, as the “without” cases represent a likely opportunity for size reduction.
>
>
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> I don’t *think* we ever emit an inlined_subroutine without an abstract origin?  If we want to make it an assertion on the emission side, that would be fine.  In fact, now that we omit DW_TAG_formal_parameter where the parameter has been optimized away, it’s probably a good idea to have an assertion like that.

I don't believe we have - and the architecture isn't really designed
for that as an option. If there's some good places for extra
assertions to ensure it (no doubt we've had bugs where we fail to emit
the abstract origin or stitch it up to the inlined or concrete
definition) that's OK - but I probably wouldn't go hunting for it or
personally wouldn't go tracking it as a statistic (at least for LLVM's
output, any time those two stats differ would be a bug - so I'd
probably track it more with a light warning (not hard error) in the
debug info verifier mode, perhaps - if we have warnings/if it is
important)




> --paulr
>
>
>
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Djordje Todorovic via llvm-dev
> Sent: Wednesday, April 28, 2021 7:12 AM
> To: llvm-dev <llvm-dev at lists.llvm.org>
> Cc: asowda at cisco.com
> Subject: [llvm-dev] llvm-dwarfdump stats for inlined functions
>
>
>
> Hi,
>
>
>
> As discussed on the https://reviews.llvm.org/D101025, we have noticed that there are two different stat categories for inlined functions when using `llvm-dwarfdump –statistics`:
>
> # inlined functions
>
> # inlined functions with abstract origin
>
> and it was introduced after the D58849. We were wondering if there is a particular motivation of doing so.
>
>
>
> Having an inlined_subroutine DIE with no abstract_origin attribute does not have so many benefits, so it might be better if we have an assertion/error (e.g., in the AsmPrinter) when we face such situation (or just to avoid dumping it into the final DWARF).
>
>
>
> Any thoughts?
>
>
>
> Best regards,
>
> Djordje
>
>
>
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