[RFC, PATCH] Disable -fprofile-use related optimizations if corresponding .gcda file not found.

[Richard Biener]

On Thu, Oct 8, 2015 at 9:54 AM, Nikolai Bozhenov <n.bozhenov@samsung.com> wrote:
>
> On 10/07/2015 08:07 PM, pinskia@gmail.com wrote:
>>
>> Yes. But I am saying some one could do -fprofile-use -frename-registers
>> and expect rename registers to stay on even if there is no profile.
>
>
> That's true, we shouldn't disable any options that were explicitly requested
> by
> the user.
>
>> Why not figure out how much to improve that instead. Rename register is
>> just doing renaming of register usage and not undoing any scheduling at all
>> (well it might cause some moves to be removed).  I think you really should
>> dig into deeper why it is causing a performance issue.
>
>
> Yep, the better way is to fix all the problems in all the passes. We're
> working on it but it may take a while. Meanwhile, we want to use PGO and we
> need a way to avoid degradations caused by it. We want to fallback to the
> usual compilation process for a source file if we don't like how it is
> compiled with PGO. And what we want to do is to simply remove the
> corresponding gcda file and have the compiler ignore the -fprofile-use
> option
> for the translation unit.
>
> Anyway, I find current GCC's behavior to be confusing. Currently it silently
> ignores missed profile data but triggers some opaque internal options which
> affect further compilation. I think in case of a missed gcda file GCC should
> either ignore -fprofile-use completely or issue a warning/error.

I agree that it is confusing.  You have to watch out for explicit
-fprofile-use -fXXX
and not clear flags set explicitely though.  Didn't look at the patch in detail.

Richard.

> Thanks,
> Nikolai