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]

>>> 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.
>
>
> It'd be really nice to quantify this.
>
>>
>> Yes, the accelerator table can help the search.
>>

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?

-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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