Recently we found an ICE while compiling a program with auto-fdo
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65972).
The ICE was caused because SSA is not in a valid state when the early inliner
is run. The fix was to update_ssa before running the early inliner
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65972#c4).
However, it remains to be found out which pass caused the SSA to be in that
state, maybe fixing the problem there would be more appropriate.
-Aditya
----------------------------------------
> Date: Sat, 9 May 2015 16:33:02 +0200
> From: hubi...@ucw.cz
> To: hiradi...@msn.com
> CC: de...@google.com; i.palac...@samsung.com; davi...@google.com;
> hubi...@ucw.cz; gcc@gcc.gnu.org; v.bari...@samsung.com; dnovi...@google.com;
> seb...@gmail.com
> Subject: Re: AutoFDO profile toolchain is open-sourced
>
>>> Yes, it will. But it's not well tuned at all. I will start tuning it
>>> if I have free cycles. It would be great if opensource community can
>>> also contribute to this tuning effort.
>>
>> If you could outline portions of code which needs tuning, rewriting, that
>> will help get started in this effort.
>
> Optimization passes in GCC are generally designed to work with any kind of
> edge profile they get.
> There are only few cases where they do care about what profile is around.
>
> At the moment we consider two types of profiles - static (guessed) and FDO.
> For
> static one we shut down use of profile info for some heuristics - for example
> we do not expect loop trip counts to be reliable in the profiles because they
> are not. You can look for code checking profile_status_for_fn.
>
> Auto-FDO does not have special value for profile_status_for_fn and it goes
> with
> same code paths for FDO. Dehao has some patches for Auto-FDO tuning but my
> impression is that he got mostly got around by just makng optimizer bit more
> robust for nonsential profiles that is always good, since even FDO profiles
> can
> get wrong. BTW, Dehao, do you think you can submit these changes for this
> stage1?
>
> I suppose in this case we have yet another kind of profile that is less
> reliable than
> FDO and we need to start by simply benchmarking and looking for cases where
> this profile
> gets worse and handle them one by one :)
>
> Honza
