[PATCH] D7895: Anonymous namespaces are missing import DW_TAG_imported_module.

[David Blaikie via llvm-commits]

On Tue, Aug 18, 2015 at 10:43 AM, Robinson, Paul <
Paul_Robinson at playstation.sony.com> wrote:

> The as-if suggests that an explicit artificial import is an entirely
> appropriate, natural and good way to handle it, because it models the
> language behavior exactly without *requiring* additional smarts on the
> debugger side.
>

I agree it is a way to handle it - but I don't understand why "without
requiring* additional smarts on the debugger side" is a goal.

natural/good - I just don't agree with. The stated goal in many cases has
been that the DWARF model the source as written. This is a case where
that's easy and obvious to do.

* it's not requiring - it's just a degraded experience (as you say, quality
of implementation - as with many places where, if the debugger doesn't
understand C++, it gets a less-than-ideal user experience)


> Not all as-if statements have to be modeled directly in the DWARF, because
> they aren't relevant to DWARF or DWARF has other ways of achieving the same
> effect.  For example, [namespace.udir]/2 says a using-directive means "the
> names appear * as if *they were declared in the nearest enclosing
> namespace…" but there's no reason to generate piles of declarations in the
> outer namespace because, we have DW_TAG_imported_module which means exactly
> that.
>
> I don't see any good reason for artificial namespaces to be treated
> specially just because the namespace is anonymous and the 'using' directive
> isn't explicit in the source.
>

>From my perspective it seems like you're asking the producer to special
case anonymous namespaces so the debugger doesn't have to. I'd simply argue
the opposite - I'd like to not special case them in the producer, instead
producing source-accurate debug info as much as possible, and leave it to
the debugger to implement appropriate behavior.

The Cobol example seems perfect: We rely on consumers modeling languages
correctly to import outer namespaces into inner ones in languages that
support that. In the languages that don't, we rely on the consumer to know
that and model it differently. If a consumer does not do this, the user
gets a degraded experience (their cobol name lookup is wrong (or perhaps
their non-cobol name lookup is wrong, if the debugger decided to only model
the cobol side and assume all other languages work that way too)).

This seems directly analogous to anonymous namespaces - we describe the
source and let the consumer model it appropriately.


> --paulr
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *Sent:* Tuesday, August 18, 2015 10:27 AM
> *To:* Robinson, Paul
> *Cc:* reviews+D7895+public+7827be49c0b04087 at reviews.llvm.org; Romanova,
> Katya; Eric Christopher; Frédéric Riss; Duncan P. N. Exon Smith;
> llvm-commits
> *Subject:* Re: [PATCH] D7895: Anonymous namespaces are missing import
> DW_TAG_imported_module.
>
>
>
>
>
>
>
> On Tue, Aug 18, 2015 at 10:23 AM, Robinson, Paul <
> Paul_Robinson at playstation.sony.com> wrote:
>
> | suggests that DW_TAG_imported_module could be used for C++ using
> directives (of which there are none in the source code in question).
>
>
>
> [namespace.unnamed]/1 says there is an implicit (as-if) using directive.
>
>
>
> Lots of things are as-if, doesn't mean we model them in the DWARF that
> way, it's just an easy way of defining the behavior in terms of other stuff
> that's already defined in the standard. Generally we try to model things
> like the actual source where possible, it seems/usually sounds like/etc.
>
>
>
> --paulr
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *Sent:* Tuesday, August 18, 2015 10:15 AM
>
>
> *To:* Robinson, Paul
> *Cc:* reviews+D7895+public+7827be49c0b04087 at reviews.llvm.org; Romanova,
> Katya; Eric Christopher; Frédéric Riss; Duncan P. N. Exon Smith;
> llvm-commits
> *Subject:* Re: [PATCH] D7895: Anonymous namespaces are missing import
> DW_TAG_imported_module.
>
>
>
> All I'm saying is that I think this is a fine thing to have as a
> debugger-specific tuning for your debugger.
>
> DWARF-the-standard is language neutral, yes, so it provides features that
> can be used to describe many different languages. It describes
> (non-normatively) possible uses of DWARF features for language features,
> and suggests that DW_TAG_imported_module could be used for C++ using
> directives (of which there are none in the source code in question). It
> doesn't suggest anywhere that it should, let alone must, be used to
> explicitly describe name lookup rules (or we'd need those using directives
> in nested scopes for all outer scopes, for example).
>
>
>
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