linux-kernel - Re: [PATCH] word-at-a-time: use the same return type for has

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Message-Id: <847747a8-b773-4b7b-8c14-b8ef4ba7c022@app.fastmail.com>
Date:   Wed, 02 Aug 2023 07:59:48 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>
Cc:     "Nathan Chancellor" <nathan@...nel.org>,
        "Tom Rix" <trix@...hat.com>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        "Heiko Carstens" <hca@...ux.ibm.com>,
        "Vasily Gorbik" <gor@...ux.ibm.com>,
        "Alexander Gordeev" <agordeev@...ux.ibm.com>,
        "Christian Borntraeger" <borntraeger@...ux.ibm.com>,
        "Sven Schnelle" <svens@...ux.ibm.com>, linux-s390@...r.kernel.org,
        "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>
Subject: Re: [PATCH] word-at-a-time: use the same return type for has_zero regardless
 of endianness

On Wed, Aug 2, 2023, at 03:07, Linus Torvalds wrote:
> On Tue, 1 Aug 2023 at 15:22, <ndesaulniers@...gle.com> wrote:

> Who ends up being affected by this? Powerpc does its own
> word-at-a-time thing because the big-endian case is nasty and you can
> do better with special instructions that they have.

powerpc needs the same patch though.

> Who else is even BE any more? Some old 32-bit arm setup?
>
> I think the patch is fine, but I guess I'd like to know that people
> who are affected actually don't see any code generation changes (or
> possibly see improvements from not turning it into a bool until later)

s390 is the main one here, maintainers added to Cc.

Atheros and Realtek MIPS are older but still shipping in low-end
network equipment, and there are still users on old embedded
big-endian mips (broadcom, cavium, intel/lantiq and arm
(intel ixp4xx) hardware. All modern Arm hardware (v6/v7/v8/v9)
can do both big-endian and little-endian, but actual user numbers
for big-endian are close to zero -- I have not heard from anyone
actually using it in years.

And then there was a lot of the old-school workstation/server
hardware (m68k, mips, parisc and sparc, not alpha/ia64) that is
mostly big-endian and still maintained to some degree.

      Arnd