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From: Alexander Monakov <>
To: Paul Zimmermann <>
Subject: Re: slowdown with -std=gnu18 with respect to -std=c99
Date: Wed, 11 May 2022 16:26:05 +0300 (MSK)	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Fri, 6 May 2022, Alexander Monakov wrote:

> The primary issue here is false dependency on vcvtss2sd instruction. In the
> snippet shown in Stéphane's email, the slower variant begins with
>     vcvtss2sd   -0x4(%rsp),%xmm1,%xmm1
> The cvtss2sd instruction is specified to take the upper bits of SSE register
> unmodified, so here it merges high bits of xmm1 with results of float->double
> conversion (in low bits) into new xmm1. Unless the CPU can track dependencies
> separately for vector register components, it has to delay this instruction
> until the previous computation that modified xmm1 has completed (AMD Zen2 is
> an example of a microarchitecture that apparently can).

For future reference, my statement in parenthesis was a bit inaccurate: Zen 2
avoids the false dependency provided that xmm1 carries all-zeroes in high bits
after being idiomatically zeroed (i.e. via pxor). Thanks to Andreas Abel for
pointing out there's a limitation.

(nevertheless, the "blessed" state seemingly survives context switches, so
it's quite useful, including this testcase)


  parent reply	other threads:[~2022-05-11 13:26 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
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 [this message]
2022-05-05 17:50         ` Paul Zimmermann

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