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From: Paul Zimmermann <>
To: Alexander Monakov <>
Subject: Re: slowdown with -std=gnu18 with respect to -std=c99
Date: Tue, 03 May 2022 13:45:00 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <> (message from Alexander Monakov on Tue, 3 May 2022 12:09:32 +0300 (MSK))

thank you very much Alexander.

> Date: Tue, 3 May 2022 12:09:32 +0300 (MSK)
> From: Alexander Monakov <>
> cc:,,
> On Tue, 3 May 2022, Paul Zimmermann via Gcc-help wrote:
> > Does anyone have a clue?
> I can reproduce a difference, but in my case it's simply because in -std=gnuXX
> mode (as opposed to -std=cXX) GCC enables FMA contraction, enabling the last few
> steps in the benchmarked function to use fma instead of separate mul/add
> instructions.

but then you should get better (i.e. smaller) timings with -std=gnuXX than
with -std=cXX, instead of worse timings as we get?

> (regarding __builtin_expect, it also makes a small difference in my case,
> it seems GCC generates some redundant code without it, but the difference is
> 10x smaller than what presence/absence of FMA gives)
> I think you might be able to figure it out on your end if you run both variants
> under 'perf stat', note how cycle count and instruction counts change, and then
> look at disassembly to see what changed. You can use 'perf record' and 'perf
> report' to easily see the hot code path; if you do that, I'd recommend to run
> it with the same sampling period in both cases, e.g. like this:
>     perf record -e instructions:P -c 500000 ./perf ...

thank you, we'll investigate that.

Best regards,

  reply	other threads:[~2022-05-03 11:45 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-03  8:28 Paul Zimmermann
2022-05-03  9:09 ` Alexander Monakov
2022-05-03 11:45   ` Paul Zimmermann [this message]
2022-05-03 12:12     ` Alexander Monakov
2022-05-05  8:57   ` Stéphane Glondu
2022-05-05 14:31     ` Stéphane Glondu
2022-05-05 14:41       ` Marc Glisse
2022-05-05 14:56         ` Alexander Monakov
2022-05-06  7:46           ` Paul Zimmermann
2022-05-06  9:27             ` Alexander Monakov
2022-05-07  6:11               ` Paul Zimmermann
2022-05-11 13:26               ` Alexander Monakov
2022-05-05 17:50         ` Paul Zimmermann

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