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

[Eric Christopher]

>> The accelerator table should help - since it only has definitions and
>> it's on by default for darwin. What's up that's causing a performance
>> degradation?
>
>
> I don't have numbers right now, in theory extra search means slower response
> (maybe insignificant).
>

Probably insignificant here given the debugger is using the
accelerator tables. I'd need to know what other use you're thinking
about to determine whether or not the slowdown could be damaging
otherwise.

> I was asking whether it is worthwhile to guard this change with a flag. If
> -no-limit-debug-info is not the right flag,
> is it reasonable to have a different flag for this? Does gcc has a flag to
> turn this off?
>

Nope.

-eric


> Manman
>
>>
>> -eric
>>
>> >>>
>> >>>
>> >>>>
>> >>>> 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.
>> >
>> >
>> > -flimit-debug-info has some relatively rough heuristics that aren't as
>> > high
>> > confidence as the vtable based debug info emission mechanism, I'm not
>> > sure
>> > we want to group these together. (eg: it seems likely that a user might
>> > have
>> > issues with the minimization caused by -flimit-debug-info and want to
>> > turn
>> > it off, if in doing so they also turn off the vtable based,
>> > higher-confidence optimization, that would be unfortunate)
>> >
>> > - David
>> >
>> >>
>> >>
>> >> Thanks,
>> >> Manman
>> >>
>> >>>
>> >>>
>> >>>
>> >>> - David
>> >>>
>> >>>>
>> >>>>
>> >>>> Thanks,
>> >>>> Manman
>> >>>>
>> >
>> >
>> > _______________________________________________
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>> > cfe-commits at cs.uiuc.edu
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>> >
>
>