[cfe-dev] r222220 causes real debug-info bloat

Frédéric Riss friss at apple.com
Mon May 4 10:39:55 PDT 2015

> On May 4, 2015, at 9:51 AM, Robinson, Paul <Paul_Robinson at playstation.sony.com> wrote:
>> -----Original Message-----
>> From: Frédéric Riss [mailto:friss at apple.com]
>> Sent: Friday, May 01, 2015 6:34 PM
>> To: Robinson, Paul
>> Cc: cfe-dev at cs.uiuc.edu Developers (cfe-dev at cs.uiuc.edu)
>> Subject: Re: r222220 causes real debug-info bloat
>> Hi!
>>> On May 1, 2015, at 5:29 PM, Robinson, Paul
>> <Paul_Robinson at playstation.sony.com> wrote:
>>> We were doing some size analysis and noticed some ridiculous numbers
>>> related to debug-info size.  Investigation showed that essentially all
>>> of the bloat came from DW_TAG_imported_declaration pointing to
>>> DW_TAG_subprogram and the associated DW_TAG_formal_parameter DIEs.
>>> We tracked this to r222220, which basically caused every 'using' decl
>>> of a function or variable to have a forward declaration emitted to the
>>> DWARF, whether or not that 'using' decl itself was used in the CU.
>>> #include <stdlib.h>
>>> using ::abort
>>> In Clang 3.5, this produces a pretty minimal .debug_info section (just
>>> the DW_TAG_compile_unit).
>>> In Clang 3.6, we see an additional DW_TAG_subprogram for abort() and
>> then
>>> a DW_TAG_imported_declaration pointing to that declaration.
>>> #include <cstdlib>
>>> on Linux, Clang 3.5 wrote a .debug_info of 185 bytes, 3.6 was 1458.
>>> Multiply this by more headers and again by hundreds to thousands
>>> of modules and pretty soon you're talking multiple megabytes.
>>> Getting away from the benchmarks, a real game saw .debug_info increase
>>> by 13% (6 MB).
>>> r222220 basically causes a 'using' declaration of a function or global
>>> variable to conjure up a forward declaration, if we haven't already
>>> seen a declaration or definition.  The commentary talks about how this
>>> will be RAUW'd later on.  But I'm not sure what motivated this in the
>>> first place, and it clearly can have a huge adverse effect.
>> The whole story is that I was working on getting debug info emitted
>> for function argument default values (which I haven’t gotten back to
>> yet BTW), and that my implementation didn’t work if the default value
>> was a call to a forward declared function. Our decl tracking didn’t
>> handle forward declarations at all, and David pointed out that this
>> was why we were also missing some DW_TAG_imported_declaration. I
>> then implemented support for forward declarations and tested it using
>> the the only current user that cared about forward decls, that is the
>> imported_declaration stuff.
>>> I don't mind having a DW_TAG_imported_declaration for something that
>>> actually gets used in the CU, but a 'using' declaration all by itself
>>> should not count as "used" for purposes of emitting debug info.
>> It’s not that the using clause counts as a ‘use’, it’s just a
>> question of source fidelity.
> Source fidelity is not about emitting every declaration you see.
> It's about, *if* you're going emit something, do it in a way that is
> faithful to the source-as-written.

and I’d add “gives means to the debugger to evaluate every source expression
as it is written in the source.” 

>> Your above example isn’t really
>> compelling. By changing it a little bit to:
>> #include <stdlib.h>
>> namespace A {
>> using ::abort;
>> }
>> The goal of the imported_declaration information is to inform
>> the debugger that in this CU, A::abort is the same thing as
>> ::abort. It’s just a matter of describing aliased name to
>> the debugger so that it can correctly evaluate source
>> expressions.
> Consider this:
> void abort();
> namespace A {
> #if USING
>  using ::abort();
> #else
>  void abort();
> #endif
> };
> In the not-USING case, Clang emits nothing but the CU DIE, because
> neither abort() declaration is used.
> In the USING case, we see the imported_declaration and the associated
> subprogram.  In both cases, the set of declared names is the same, and
> there are no *actual* uses of either name.

I’m repeating myself, but this is not about uses, just about describing names.

Then, as a compiler policy, you might want to limit the names you describe
to the ones that are actually used in the program (we have no code to track
the uses and modify the debug info accordingly). You might also want to
emit all the names so that the debugger can evaluate accurately every
expression that could happen in the source code.

I’m not arguing that one of the above is better than the other (the answer can
certainly be different depending on the environment), I mostly want to point
out that this information isn’t as useless as you seem to think.

> Therefore, I argue, this is not about source fidelity but about
> declining to produce declarations not useful to the consumer.

David would need to confirm, but I think that if we revert the change, there
are tests in the GDB test suite that will fail. ‘Not useful’ information should not
be able to break user level tests, should it? 

>>> Can somebody describe how these extra forward declarations fit into
>>> the Grand Scheme of Things in a beneficial way, and can we do something
>>> about unused 'using' declarations?
>> I totally get your point about the size, and according to past
>> conversations, I gather that the use described above isn’t maybe
>> relevant to your debugger (which maybe points to something that
>> can be tuned depending on the target debugger? I’m sorry, but I
>> just came back from a long leave and I’m so much behind on list
>> reading that I have no idea of the status of that idea).
>> IMO, it has nothing to do with the fact that the function/variable
>> is used or not. The using directives create new names and the only
>> way for the debugger(s) to understand these names is to have them
>> described in the debug info.
> By that argument you should emit every name you see in every header,
> whether it is used or not.  That's not what we do, because it's not
> useful to anyone and unnecessarily bloats the debug info.  The case of
> used-only-by-'using' is no different because there's no *actual* use.

I’m sorry, but the fact that all declarations aren’t emitted happen to bother
me from time to time. I’m a heavy user of debugger conditional breakpoints,
and the conditions often involve calling to functions that aren’t defined in my
program, but which were described in the headers (for example libc).

Not having the prototype for these functions available to the debugger requires
me to play casting games so that it gets the calling convention correctly. If all
the declarations were to be emitted I wouldn’t have that issue and my debug
experience would be better.

Do not get me wrong. I’m not arguing for including all the declarations. I’m just
trying to point out the the information isn’t useless as you describe it and that 
there is a balance to find. Including only the names that have been really used
in the program would be a perfectly sensible one, but we do not have the code
that does that tracking!

> I found it instructive to add this to my not-USING example:
> void foo() { ::abort(); A::abort(); }
> which naively I would expect to induce subprogram DIEs for abort() and
> A::abort(), but in fact it doesn't, even with -fstandalone-debug.  That
> seems sub-optimal too.  But, it just further illustrates the discrepancy
> between the 'using' declarations and non-'using' declarations.
> Also that there's a deeper problem here, which might or might not be
> what David Blaikie was getting at.
> The missing DIEs in the non-USING case, along with memories of trying
> to do something else with used/non-used declarations some while ago,
> make me think that even though abort() and A::abort() are (probably)
> being flagged, debug-info generation isn't going back through those
> non-defining declarations to see which ones ought to be emitted after
> all.

As pointed above, such code that goes from the uses to the debug info just
doesn’t exist. The debug info for types and declarations is generated during
AST construction (IIRC) and not touched afterwards.


> It looks like CGDebugInfo::finalize() does a post-pass for types, to
> some extent; maybe that needs to be done for other decls as well?
> --paulr
>>> Given how the patch works, it looks we can just short-circuit the
>>> creation of these forward declarations with no harm done, but I have to
>>> wonder whether we're shooting ourselves in the foot in some situation
>>> that isn't immediately obvious.
>> If the git commit message is still accurate regarding the use of that
>> function, then you’ll just go back to the previous state which you
>> liked better. If the function grows new callers, you might lose
>> more stuff, but IIUC it should mostly be stuff that you don’t care
>> about anyway.
>> Fred
>>> Thanks,
>>> --paulr

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