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

Mon May 4 12:46:04 PDT 2015

From Fred:
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.”

Agreed.  But a 'using' declaration is not an expression.  If the declared name is not used in any source expression, it can hardly be needed to evaluate a source expression as written in the source.

I admit calling this stuff "useless" is a bit of hyperbole.  Clang is being inconsistent.  If a normal unqualified function declaration isn't emitted, and a normal function declaration inside a namespace isn't emitted, I see no argument justifying emitting a function that happens to be declared with a 'using' declaration. A 'using' versus a normal declaration is typically an implementation detail, not something inherently significant that can change the meaning of a program.

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.

But it looks to me like these "missing" imported_declarations aren't any more missing than the equivalent non-'using' declarations.  Is there a principled justification for the inconsistency?  I'm not hearing one.  What I'm hearing is the Clang can't keep track of what's actually needed in the source, so we arbitrarily emit some things and not others when we're not sure.  That's really unsatisfactory.
Now, if the forward declaration bit is something that was part of a larger project that is half-implemented, and the imported_declaration bit was a momentary convenience that you intended to take out later, well that's fine so long as you now intend to finish the job.

I am sympathetic to the desire to have the full power of libc available in conditional breakpoints, and so forth.  If that's your preferred mode then perhaps a more –gfull-to-bursting mode might be the ticket.  If it didn't cost multi-megabytes to get there, and the time it takes to write all that out, and link it together, then I'd have no problem with always emitting everything.  As it is, we seem to have at least two modes of not-emitting-everything, and the balance point of things-most-likely-to-be-useful doesn't really seem to warrant including "anything that the standard-library implementor decided to implement with a 'using' declaration."

(Hmmm… could we suppress this stuff just for standard-header declarations?  That's where our immediate pain-point seems to be.)
--paulr

From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Monday, May 04, 2015 10:56 AM
To: Frédéric Riss
Cc: Robinson, Paul; cfe-dev at cs.uiuc.edu Developers (cfe-dev at cs.uiuc.edu)
Subject: Re: [cfe-dev] r222220 causes real debug-info bloat

On Mon, May 4, 2015 at 10:39 AM, Frédéric Riss <friss at apple.com<mailto:friss at apple.com>> wrote:

On May 4, 2015, at 9:51 AM, Robinson, Paul <Paul_Robinson at playstation.sony.com<mailto: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<mailto:cfe-dev at cs.uiuc.edu> Developers (cfe-dev at cs.uiuc.edu<mailto: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<mailto: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.

I don't recall precisely - but yes, I think we un-XFAIL'd some tests after you made this change. Could check.

‘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.

Fred

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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