r188739 - Revert "Revert "Revert "Revert "DebugInfo: Omit debug info for dynamic classes in TUs that do not have the vtable for that class""""

[Manman Ren]

On Fri, Oct 11, 2013 at 12:57 PM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
>
> On Fri, Oct 11, 2013 at 12:51 PM, Manman Ren <manman.ren at gmail.com> wrote:
>
>> Hi, David
>>
>> I just noticed this has great size reduction on xalan (a SPEC benchmark),
>> thanks for the work.
>> The number of MDNodes in a lto debug build decreased from 29M to 12M.
>>
>
> Great - thanks for the stats.
>
>
>>  This may degrade the debugger's speed since we are now emitting
>> declarations instead of definitions, and the debugger will have to search
>> for the definition.
>>
>
> Perhaps. I suppose moreso on Apple platforms where debug info isn't
> linked, so searching multiple object files' debug info could be more
> costly. But do the accelerator tables handle this by making it easy to find
> the object file with the definition?
>
>
>> One example is building XercesWrapperNavigator.cpp with -O0 -g, we are
>> now emitting a forward declaration for XercesDocumentWrapper.
>> XercesWrapperNavigator has a member (m_d) pointing to
>> XercesDocumentWrapper, and we call m_d->func() inside
>> XercesWrapperNavigator.cpp.
>>
>
> Do you have any evidence that this adversely affects debugger performance?
>
For our internal tool chains, we now have to perform a string search to
find the definition. In theory, the extra search should make it slower.
Yes, the accelerator table can help the search.


>
>
>> Is it reasonable for this commit to be enabled only for limit-debug-info,
>> when no-limit-debug-info is on, we don't see effects of this patch?
>>
>
> GCC does this by default, at least on Linux - I haven't tested on MacOS. I
> would guess it does so there too (but as you point out, there are different
> tradeoffs there, so I might be wrong).
>
>  I'd like to understand the performance tradeoff to justify such a large
> regression in debug info size (it's worth about ~20% on Clang/LLVM based on
> my experiments).
>

Our default is -limit-debug-info, so this will be on by default. I was
asking whether we should guard this so when people use
"-fno-limit-debug-info", they will get the definition.

Thanks,
Manman


>
>
> - David
>
>
>>
>> Thanks,
>> Manman
>>
>>
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