From: David Laight <david.laight.linux@gmail.com>
To: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>,
Ilya Leoshkevich <iii@linux.ibm.com>,
Sven Schnelle <svens@linux.ibm.com>,
Vasily Gorbik <gor@linux.ibm.com>,
Christian Borntraeger <borntraeger@linux.ibm.com>,
linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org
Subject: Re: [PATCH 2/4] s390/percpu: Drop unused output register from arch_this_cpu_add()
Date: Wed, 14 Oct 2026 13:27:36 +0100 [thread overview]
Message-ID: <20261014132736.10a32cfe@pumpkin> (raw)
In-Reply-To: <4907dbfd-3c5c-483b-be25-b0df83af2d20-agordeev@linux.ibm.com>
On Mon, 12 Oct 2026 15:24:20 +0200
Alexander Gordeev <agordeev@linux.ibm.com> wrote:
> On Mon, Oct 12, 2026 at 01:42:47PM +0200, Heiko Carstens wrote:
> > The laa/laag instructions return the old memory content, but
> > arch_this_cpu_add() does not need it. Instead of allocating a dedicated
> > output register old__ just to discard the returned value, change the
> > val__ constraint from "d" to "+&d" so it is used as input/output
> > register.
> >
> > This reduces register pressure.
>
> Do you know whether the compiler is may be smart enough to select the
> same register for old and val himself?
Given gcc tends to allocate a new register for x++ I doubt it
(and then has to do a reg-reg move to put it back).
David
next prev parent reply other threads:[~2026-10-14 12:27 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-12 11:42 [PATCH 0/4] s390/percpu: More small improvements Heiko Carstens
2026-10-12 11:42 ` [PATCH 1/4] s390/percpu: Use asm_inline for arch_this_cpu_add() Heiko Carstens
2026-10-12 11:46 ` sashiko-bot
2026-10-12 12:41 ` Alexander Gordeev
2026-10-12 11:42 ` [PATCH 2/4] s390/percpu: Drop unused output register from arch_this_cpu_add() Heiko Carstens
2026-10-12 11:51 ` sashiko-bot
2026-10-12 13:24 ` Alexander Gordeev
2026-10-12 14:16 ` Heiko Carstens
2026-10-14 12:27 ` David Laight [this message]
2026-10-12 14:34 ` Alexander Gordeev
2026-10-12 11:42 ` [PATCH 3/4] s390/percpu: Drop unused output register from arch_this_cpu_to_op() Heiko Carstens
2026-10-12 11:54 ` sashiko-bot
2026-10-12 12:18 ` Heiko Carstens
2026-10-12 14:35 ` Alexander Gordeev
2026-10-12 11:42 ` [PATCH 4/4] s390/percpu: Implement this_cpu_try_cmpxchg() Heiko Carstens
2026-10-12 11:51 ` sashiko-bot
2026-10-12 15:11 ` Alexander Gordeev
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