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From: Gaelan Steele <gbs3@st-andrews.ac.uk>
To: David Brown <david@westcontrol.com>,
	Jonathan Wakely <jwakely.gcc@gmail.com>
Cc: "gcc-help@gcc.gnu.org" <gcc-help@gcc.gnu.org>
Subject: Re: Why does this unrolled function write to the stack?
Date: Wed, 8 Feb 2023 18:52:58 +0000	[thread overview]
Message-ID: <VE1PR06MB705480C397197BE7CA931012F1D89@VE1PR06MB7054.eurprd06.prod.outlook.com> (raw)
In-Reply-To: <69488f61-fac0-0c1d-1034-d8d84779f8e2@westcontrol.com>

Thanks for the help investigating, both of you!

> It seems to be a regression between gcc 10 and gcc 11 (discovered by
> changing the compiler on godbolt.org).  With gcc 11 onwards, the
> compiler seems to be using the stack to combine two 4-byte elements at a
> time into a single 8-byte element.  It's easy to see the effect by
> changing the loop size to 2.

I gave this a bisect, and it looks like the commit that caused it is this one:

commit 33c0f246f799b7403171e97f31276a8feddd05c9
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Oct 30 11:26:18 2020 +0100

    tree-optimization/97626 - handle SCCs properly in SLP stmt analysis

The details of how this ties back to the behavior we're seeing go far
above my head, unfortunately.

I'll go ahead and file a bug on the tracker.

Thanks again,
Gaelan
PS Apologies for the horrendous wrapping in my previous email! I'm
using webmail because my normal machine is in repair, and list etiquette
totally slipped my mind.

      reply	other threads:[~2023-02-08 18:53 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-02-08 13:29 Gaelan Steele
2023-02-08 13:49 ` Jonathan Wakely
2023-02-08 13:53   ` Jonathan Wakely
2023-02-08 15:32     ` David Brown
2023-02-08 18:52       ` Gaelan Steele [this message]

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