[PATCH] Anonymous namespaces are missing import DW_TAG_imported_module.

[David Blaikie]

On Tue, Mar 10, 2015 at 3:25 PM, Romanova, Katya <
Katya_Romanova at playstation.sony.com> wrote:

>  Hi David,
>
>
>
> I wanted to follow up on this bug. I still haven’t heard a definite “yes”
> or “no” from the other developers.
>
>
>
> Here are my thought on why adding DW_TAG_imported_module is
> beneficial:
>
>
>
> (1) We will be more consistent with debug info generated by other
> compilers (e.g. GCC and Intel's ICC).
>

Sure enough - and I'd usually argue from consistency if it was a large
issue in a wide existing install base (I've made that argument for doing
some far weirder things than this to Clang/LLVM's debug info output in the
name of compatibility). But it sounds like in this case you're not shipping
Clang off to a wide variety of machines with existing PS Debuggers that you
don't control/ship, so it just comes down to "fix in LLVM or fix in PS
Debugger"


>  (2) It resolves ambiguity created by lack of specification. LLDB and GDB
> are not the only debuggers in the world. Since it's a grey area in the
> DWARF spec, other debuggers might not automatically import the anonymous
> namespaces.
>

I don't quite see the ambiguity - we've provided the DWARF that describes
the source. It's up to the debugger to use the language semantics to use
that DWARF appropriately. I'd simply consider it (an understandable) bug in
those debuggers & honestly a simple bug/oversight in existing compilers
most likely (because they implemented anonymous namespaces just as the C++
spec says "equivalent to namespace <unmentionable> { } using
<unmentionable>" - because they didn't have a more source-accurate
representation like Clang has) that debuggers just got used to being given,
they haven't had to deal with something more source accurate.


>   About your concerns:
>
> (1) Import is implicit. It's marked as "artificial", so it shouldn't be
> confusing for the debuggers.
>
> (2) The impact of the size increase is minimal, 6 bytes per anonymous
> namespace.
>
> (3) The added complexity is very small. The patch is tiny and clear.
>

It's also in LLVM, building more (certainly this isn't the first instance
by any stretch, but it's certainly not an ideal state of affairs)
language-specific understanding into LLVM, which makes it harder for LLVM
to support other source languages (what happens when some source language
really doesn't want this imported module TAG emitted because it changes the
meaning?).


>   I think the benefits clearly outweigh the drawbacks.
>
>
>
> We are talking about adding just one tag and a couple of attributes in
> some rare cases (like anonymous array).
>

Hmm - what's this about anonymous arrays? (what are anonymous arrays?)


> From what I remember, there are several existing attributes that are not
> exactly universally used, e.g. the DW_AT_APPLE_* attributes that are
> generated for all platforms, I'm not sure if anybody outside Apple has any
> use for it. And I haven't seen anybody raising any concerns related to size
> increases stemming from these attributes. . And we're just asking to add 6
> bytes per anonymous namespace.
>
>
>
> Sony’s debugger is just asking for a tiny bit of information from the
> compiler. It's not much really, but without this help. Fixing this problem
> in the debugger would be much harder. It might help other debuggers as well.
>

Well, now that we've added the IsPS4 handling no DwarfDebug anyway, how's
about we just put it behind that flag? I still don't much like it, but at
least the workaround will be limited to where it's needed, so we'll know
when to remove it, etc.

- David


>
>
> Katya.
>
>
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *Sent:* Tuesday, March 03, 2015 9:44 AM
> *To:* reviews+D7895+public+7827be49c0b04087 at reviews.llvm.org
> *Cc:* Romanova, Katya; Eric Christopher; Frédéric Riss; Duncan P. N. Exon
> Smith; llvm-commits at cs.uiuc.edu
> *Subject:* Re: [PATCH] Anonymous namespaces are missing import
> DW_TAG_imported_module.
>
>
>
>
>
>
>
> On Fri, Feb 27, 2015 at 5:28 PM, Paul Robinson <
> Paul_Robinson at playstation.sony.com> wrote:
>
> It's an arguable gray area in the compiler/debugger contract.  How much
> does the compiler _require_ the debugger to know?  (Which is different
> from how much the debugger knows for the purpose of improving the user's
> debugging experience.)
>
>
> I don't quite see the grey here - I thought the line of reasoning you've
> provided in the past was fairly simple: source fidelity. The source doesn't
> have a using directive in it, so I don't know why we'd expect to see one in
> the debug info.
>
>
> This change reduces that requirement, with very low size cost (which we
> are pretty sensitive to).  A debugger that auto-imports anonymous
> namespaces will see the import and recognize that it has already imported
>  that scope, so the cost there is also very low.
>
> Not seeing a whole lot of down-side here.
>
>
> Mostly just principles & a desire not to add cruft in source or output.
>
> Not sure if other people have any opinion on this, I'm happy to defer.
>
>
>
>
> http://reviews.llvm.org/D7895
>
> EMAIL PREFERENCES
>   http://reviews.llvm.org/settings/panel/emailpreferences/
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150310/be950d81/attachment.html>