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

[David Blaikie]

On Mon, Dec 16, 2013 at 8:06 PM, Adrian Prantl <aprantl at apple.com> wrote:

>
> On Dec 16, 2013, at 4:54 PM, Greg Clayton <gclayton at apple.com> wrote:
>
> >
> > On Dec 16, 2013, at 2:55 PM, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >>
> >>
> >>
> >> On Mon, Dec 16, 2013 at 2:44 PM, Adrian Prantl <aprantl at apple.com>
> wrote:
> >> Hi Chandler and David,
> >>
> >> unfortunately it looks more like case 1. This optimization breaks
> several assumptions that tools in our software stack depend on.
> >>
> >> It's a fairly substantial debug info size savings that seems worth
> investigating whether you can keep it enabled at least in
> >>
> >> - For example, it breaks dtrace, which on Darwin relies on being able
> to pull the (complete) CTF info (compact C type format) out of the DWARF in
> the .dSYM for a given module.
> >>
> >> I take it you're already using -fno-limit-debug-info for these
> scenarios, then? (are you using -flimit-debug-info at all?)
> >
> > Yes, this is what will need to happen now that clang has changed.
>
> In case this wasn't clear enough: Currently, this optimization is totally
> orthogonal to limited debug info. Passing -fno-limit-debug-info will not
> make any of these examples work. This thread is about adding an additional
> flag to the frontend that controls this optimization.
>
> I guess we could make also it part of -flimit-debug-info, either way will
> work.


Agreed. Though I'd rather separate it from -flimit-debug-info.
-flimit-debug-info is more aggressive. But, yes, for this particular kernel
scenario, if you do end up needing to disable the vtable-based
optimization, you should already be disabling -flimit-debug-info.

- David
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