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

[David Blaikie]

On Fri, 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).
>

Did you happen to go back further to my original implementation of
DW_TAG_imported_module too? I imagine it'd have a similar issue, though
perhaps there are fewer cases of a large number of using directives
compared to the large number of using declarations in your real world
example.

In any case, I believe there were a few issues when I originally
implemented both imported_module and imported_declaration:

We don't keep track of whether these imported entities are used via the
imported name or not, so we can't keep track of that. So I conservatively
emitted them all - I wouldn't be entirely averse to only emitting those
that are referenced, but it'd be a Clang change to keep track of how the
names were referenced (or even just keep track of the fact that the
original entity was referenced, then emit any using declarations/directives
that pertain to it - and keep track of those so we could do that when
needed).

Emitting these only where the entity is defined isn't going to work - as
Frederick showed, users may have using declarations/directives for foreign
entities.

Long story short: There's some improvements that could be made here, but
they're a fair bit of work to keep track of more info in Clang, so far as I
know/recall.

- David



>
> 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.
>
> 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.
>
> 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?
>
> 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.
>
> Thanks,
> --paulr
>
>
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