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

Tue Aug 18 11:52:14 PDT 2015

On Tue, Aug 18, 2015 at 11:34 AM, Robinson, Paul <
Paul_Robinson at playstation.sony.com> wrote:

> | I don't understand why "without requiring* additional smarts on the
> debugger side" is a goal.
>
> It is explicitly a goal of DWARF to be language-neutral,
>

This statement seems to be problematically ambiguous. The DWARF features
are language neutral, certainly - so they can be used to describe a variety
of language features. There are suggested ways that certain language
features can be described in DWARF, but those are not required.

This does not imply that the DWARF produced by a debugger can be usefully
interpreted to perform certain actions (such as name lookup) in a language
neutral way.

DWARF is language neutral in the sense that it provides a bunch of tools
and says "use these however you want (here are some suggestions) to
communicate things between producers and consumers".

> and it is contrary to that goal to require a debugger to know a C++ rule
> about anonymous namespaces in order to conjure up a correct list of named
> entities.  I'd go so far as to say it's a bug in the producer.
>

Again, the COBOL example ith regards to nested name lookup seems to provide
the perfect counterexample to this idea.

>
>
> | We require lots of additional smarts on the debugger side to deal with
> ABI knowledge
>
> AFAICT all of those smarts are to deal with debugger features intended to
> imitate things a *compiler* would do, like support dynamically calling a
> function in the process-under-debug.
>

"like performing name lookup" would seem like another thing that could be
described in the same way as you've done here.

> Nobody has shown why a debugger needs ABI knowledge in order to interpret
> the DWARF correctly, in its intended mode of providing a language-neutral
> mapping of source to object.
>

DWARF doesn't remove the language-ness from the mapping. We don't use it to
encode all the language rules/notions into some common abstract
representation. We use it to describe the source, generally.

> Certainly if your fancy-pants debugger wants to pretend to be a compiler,
> JITting ABI-compatible code and such, it needs to know a bunch more stuff
> (like the ABI), but that's not a justification for emitting DWARF that
> *can't* be interpreted correctly without specific C++ rule knowledge.
>

I don't see the distinction you're drawing. "can't be interpreted
correctly" depends on what actions are being undertaken. If all I want to
do is print out a rough summary of what the source looked like, using the
debug info - I could totally do that with an anonymous namespace without
any imported module directive.

Your debugger happens to be want to do name lookup in a particular way (it
could do so by treating an anonymous namespace as not adding any extra
qualification - that wouldn't be a language specific rule? (not any more
than the "look in outer scopes" rule that consumers appear to use by
default)) as one of its tasks.

Mine happens to want to be able to cal functions.

It's not like one of these is fundamental and the other is not. These are
just two different scenarios that DWARF consumers may wish to support. (two
of many)

>
>
> | (SFINAE being an obvious example of happy fun times),
>
> SFINAE (AFAIK) applies to template selection, and since you don't have a
> template declaration in the DWARF, that's irrelevant here.
>

Yep, fair point. Thought about that more & realized that's the case - only
the templates instantiated with the SFINAE Test passing would ever get
generated & then be considered in the overload set.

I imagine there are toher complex lookup cases that there's insufficient
DWARF to get right, though - none immediately spring to mind.

For now, my fundamental question is: How is this goal (adding imported
modules for anonymous namespaces) consistent with the lack of imported
modules for outer namespaces in nested ones? These two scenarios seem quite
the same to me.

> --paulr
>
>
>
> *From:* Eric Christopher [mailto:echristo at gmail.com]
> *Sent:* Tuesday, August 18, 2015 11:06 AM
> *To:* David Blaikie
> *Cc:* Robinson, Paul;
> reviews+D7895+public+7827be49c0b04087 at reviews.llvm.org; Romanova, Katya;
> 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 11:03 AM David Blaikie <dblaikie at gmail.com> wrote:
>
> On Tue, Aug 18, 2015 at 10:59 AM, Eric Christopher <echristo at gmail.com>
> wrote:
>
>
>
> On Tue, Aug 18, 2015 at 10:55 AM David Blaikie <dblaikie at gmail.com> wrote:
>
> 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.
>
>
>
> That said, it seems to be a bit of a double standard here. We require lots
> of additional smarts on the debugger side to deal with ABI knowledge, and
> in fact have numerous odd workarounds in DWARF to deal with parameter
> passing that would be less of an issue if we had generalized ABI support
> (as an example). How is this case any different?
>
>
>
> The conclusion with the ABI stuff was that it would be impossible for a
> client to model it correctly - there were cases Richard demonstrated that
> there was no reasonable extra DWARF we could produce in the type that would
> allow the debugger to make the right choice about the calling convention to
> use when that type was passed (I believe there were particular template
> cases?). Also, the argument was made that these rules change over time -
> and the debugger wouldn't know which set of rules to use without being told
> which ones the compiler had used when it compiled this file (OK, so if we
> had enough type information, we could've just put a "ABI rev number" on the
> CU).
>
> For both of those reasons, it is necessary that we actually just put a tag
> on the type specifying how it should be passed.
>
>
>
> Or specify ABI per function of course.
>
>
>
> These cases don't apply here - C++ has fun/weird/interesting name lookup
> rules (and, admittedly, I'd wager we still don't provide enough DWARF to
> make the name lookup work every time (SFINAE being an obvious example of
> happy fun times), but generally we rely on consumers to do best effort
> rather than by trying to describe it all explicitly in DWARF - the latter
> seems pretty much impractical)
>
>
>
> Oh I agree. :)
>
>
>
> -eric
>
>
>
> - Dave
>
>
>
>
>
> -eric
>
>
>
> 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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