From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: Junio C Hamano <gitster@pobox.com>
Cc: Daniel Gurney <dgurney99@gmail.com>,
"brian m. carlson" <sandals@crustytoothpaste.net>,
Jeff King <peff@peff.net>,
git@vger.kernel.org
Subject: Re: [PATCH v2] compat/bswap.h: simplify MSVC endianness detection
Date: Tue, 10 Nov 2020 23:37:33 +0100 (CET) [thread overview]
Message-ID: <nycvar.QRO.7.76.6.2011102333530.18437@tvgsbejvaqbjf.bet> (raw)
In-Reply-To: <xmqqtutx6rwc.fsf@gitster.c.googlers.com>
Hi Junio,
On Tue, 10 Nov 2020, Junio C Hamano wrote:
> Junio C Hamano <gitster@pobox.com> writes:
>
> > Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:
> >
> >> As a maintainer, I am less concerned about the "result today" than I am
> >> about keeping things easy and effortless to maintain. One of your patches
> >> accomplishes that. The other one made it into `next`:
> >> https://github.com/git/git/commit/91a67b86f77
> >
> > I do not think reverting it and requeuing
> >
> > https://lore.kernel.org/git/20201107221916.1428757-1-dgurney99@gmail.com/
> >
> > would help future folks why we ignore _MSC_VER as any sign usable to
> > detect endianness, so I'd prefer to see a patch *on top* of 1af265f0
> > (compat/bswap.h: simplify MSVC endianness detection, 2020-11-08),
> > which is 91a67b86f77^2, that explains why we prefer to list archs
> > explicitly in its log message, which would be the primary value of
> > that commit.
> >
> > Something along this line, perhaps?
> >
> > -- >8 --
> >
> > Subject: compat/bswap.h: do not assume MSVC is little-endian only
> >
> > Earlier, with 1af265f0 (compat/bswap.h: simplify MSVC endianness
> > detection, 2020-11-08), we tried to simplify endianness detection
> > used in compat/bswap.h by assuming that any version Git compiled by
> > MSVC (detected by _MSC_VER preprocessor macro) is meant to run on
> > little endian boxes, as the versions of old MSVC that support m68k
> > and MIPS do not support some C99 features used in the codebase
> > anyway.
> >
> > While it might hold true that modern versions of Windows are all
> > little-endian, MSVC is and/or can be ported to build for big-endian
> > boxes, so tying _MSC_VER with endianness is a bit too restrictive.
> >
> > Let's go back to the old way to use _MSC_VER to learn what
> > preprocessor macros compiler uses to tell us which arch we are
> > building for, and list these arches that are little-endian
> > explicitly.
> >
> > ... signed-off-by from you and helped-by from others ...
> > Signed-off-by: Junio C Hamano <gitster@pobox.com>
> > ---
> > diffstat
> > patch
>
> Daniel's patch adds _M_ARM64 to the list, but do we need to do
> anything further to tell the endian on such a bi-endian arch, or
> does MSVC only support little-endian for that architecture?
Yes, MSVC only supports little-endian for that architecture. From
https://docs.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions?view=msvc-160#endianness:
Endianness
As with the ARM32 version of Windows, on ARM64 Windows executes in
little-endian mode. Switching endianness is difficult to achieve
without kernel mode support in AArch64, so it's easier to enforce.
> Just double-checking as the "confusion" that started this thread
> came from an assumption that MSVC == Windows == big-endian, and you
> told us MSVC != Windows. Now the patch assumes ARM64-on-MSVC is
> little-endian only and we want to make sure that assumption is true.
I did not say that MSVC != Windows, but Visual Studio != Windows. But I
did say that I do not want to assume MSVC == Windows for all eternity.
> And perhaps it is worth documenting in the log, perhaps
>
> ... that are little-endian explicitly. Note that ARM64 is
> bi-endian in nature but we treat it little-endian as MSVC
> does not treat the arch as bi-endian.
>
> or something like that at the end (I do not know what MSVC actually
> does---just illustrating the level of details I expect in the
> explanation).
Absolutely. If nobody beats me to the punch, I hope to get around to
prepare the patch that you suggested.
Thanks,
Dscho
next prev parent reply other threads:[~2020-11-10 22:37 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-07 23:47 [PATCH] compat/bswap.h: Simplify MSVC endianness detection Daniel Gurney
2020-11-08 0:12 ` brian m. carlson
2020-11-08 9:57 ` [PATCH v2] compat/bswap.h: simplify " Daniel Gurney
2020-11-10 0:31 ` Jeff King
2020-11-10 2:36 ` brian m. carlson
2020-11-10 6:00 ` Junio C Hamano
2020-11-10 14:04 ` Johannes Schindelin
2020-11-10 15:35 ` Daniel Gurney
2020-11-10 15:47 ` Johannes Schindelin
2020-11-10 17:20 ` Junio C Hamano
2020-11-10 17:30 ` Junio C Hamano
2020-11-10 22:37 ` Johannes Schindelin [this message]
2020-11-10 14:21 ` Jeff King
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