Re: [PATCH] Reduce floating-point difficulties in timevar.cc

[Richard Biener <rguenther@suse.de>]

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 <rguenther@suse.de>
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