Re: [PATCH] gcov: Fix -fprofile-update=atomic

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Dec 16, 2022 at 11:39 AM Sebastian Huber
<sebastian.huber@embedded-brains.de> wrote:
>
> On 16.12.22 10:47, Richard Biener wrote:
> >> No, if you select -fprofile-update=atomic, then the updates shall be
> >> atomic from my point of view. If a fallback is acceptable, then you can
> >> use -fprofile-update=prefer-atomic. Using the fallback in
> >> -fprofile-update=atomic is a bug which prevents the use of gcov for
> >> multi-threaded applications on the lower end targets which do not have
> >> atomic operations in hardware.
> > Ah, OK.  So the fallback there is libatomic calls as well then?  Note
> > not all targets support libatomic, for those the failure mode is likely
> > a link error (which might be fine, but we eventually might want to
> > amend documentation to indicate this failure mode).
>
> It seems these library calls caused issues in the past:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77466
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77378

Hmm, those are testsuite-isms in some way but of course
users would run into the same issue, needing explicit
-latomic (where available).  I suppose target specs could
automatically add that for -fprofile-update=atomic but this
would need to be specified at link time as well then.

> One option could be to emit calls to a new libgcov function:
>
> __gcov_inc_counter(counter) -> updated value
>
> This function could use a __gthread_mutex_t mutex for updates. This ends
> up probably with quite a bad performance.

But that's eventually what libatomic will do as well as fallback.

I don't have a good idea here.

Do you have to explicitely link -latomic on RISCV?

Richard.

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