On Fri, 21 Jul 2023, Matthew Malcomson wrote:
> On some AArch64 bootstrapped builds, we were getting a flaky test
> because the floating point operations in `get_time` were being fused
> with the floating point operations in `timevar_accumulate`.
>
> This meant that the rounding behaviour of our multiplication with
> `ticks_to_msec` was different when used in `timer::start` and when
> performed in `timer::stop`. These extra inaccuracies led to the
> testcase `g++.dg/ext/timevar1.C` being flaky on some hardware.
>
> This change ensures those operations are not fused and hence stops the test
> being flaky on that particular machine. There is no expected change in the
> generated code.
> Bootstrap & regtest on AArch64 passes with no regressions.
I think this is undesriable. With fused you mean we use FMA?
I think you could use -ffp-contract=off for the TU instead.
Note you can't use __attribute__((noinline)) literally since the
host compiler might not support this.
Richard.
> gcc/ChangeLog:
>
> * timevar.cc (get_time): Make this noinline to avoid fusing
> behaviour and associated test flakyness.
>
>
> N.b. I didn't know who to include as reviewer -- guessed Richard Biener as the
> global reviewer that had the most contributions to this file and Richard
> Sandiford since I've asked him for reviews a lot in the past.
>
>
> ############### Attachment also inlined for ease of reply
> ###############
>
>
> diff --git a/gcc/timevar.cc b/gcc/timevar.cc
> index
> d695297aae7f6b2a6de01a37fe86c2a232338df0..5ea4ec259e114f31f611e7105cd102f4c9552d18
> 100644
> --- a/gcc/timevar.cc
> +++ b/gcc/timevar.cc
> @@ -212,6 +212,7 @@ timer::named_items::print (FILE *fp, const
> timevar_time_def *total)
> HAVE_WALL_TIME macros. */
>
> static void
> +__attribute__((noinline))
> get_time (struct timevar_time_def *now)
> {
> now->user = 0;
>
>
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)
