Re: [PATCH] compat/bswap.h: detect ARM64 when using MSVC

From: brian m. carlson <sandals@crustytoothpaste.net>
Date: Nov 10, 2020, 23:38 UTC
Message-ID: <20201110233850.GJ6252@camp.crustytoothpaste.net>
In-Reply-To: <nycvar.QRO.7.76.6.2011101418550.18437@tvgsbejvaqbjf.bet>

On 2020-11-10 at 13:58:21, Johannes Schindelin wrote:

> The biggest question now is: are we certain that `_M_ARM64` implies
> little-endian?

For Windows? Yes. I'm almost certain Windows has only supported little-endian architectures, possibly with the exception of gaming consoles.

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> I remember that ARM (the 32-bit variety, that is) has support for
> switching endianness on the fly. Happily, MSVC talks specifically about
> _not_ supporting that:
> https://docs.microsoft.com/en-us/cpp/build/overview-of-arm-abi-conventions
> 
> Likewise, it says the same about ARM64 (mentioning that it would be much
> harder to switch endianness there to begin with):
> https://docs.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions
> 
> So does that make us confident that we can just add that `_M_ARM64` part?
> Yes. Does it make me confident that we can just drop all of the
> architecture-dependent conditions? No, it does not. There _were_ versions
> of MSVC that could compile code for PowerPC, for example, which _is_
> big-endian.

PowerPC can actually be either. Most 64-bit PowerPC machines these days are run as little endian, and Windows has always run it in little-endian mode. Macs ran it in big-endian mode.

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> > As far as I know, Windows has always run on little-endian hardware.
> 
> I think that depends on your point of view... IIRC an early version of
> Windows NT (or was it still VMS Plus?) ran on DEC Alpha, which I seem to
> _vaguely_ remember was big-endian.

Alpha appears to have supported both, but as far as I know, both Windows and Linux used it in little-endian mode.

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> > [0] Wikipedia does not specify the endiannesses supported by the MIPS
> > edition.
> 
> I have another vague memory about MIPS (a wonderful SGI machine I had the
> pleasure of banging my head against, for lack of Python support and Git
> requiring Python back then) being big-endian, too.

Another architecture that supports both endiannesses. Debian supports both, but I believe Windows only supported the little-endian version. I have a small MIPS board that uses the little endian port for Debian.

> Short version: while I managed to convince myself that _currently_ there
> are no big-endian platforms that we can support via MSVC, I would like to
> stay within the boundaries of caution and _not_ drop those `defined(_M_*)`
> parts.

While I'm confident in my statements, you're the relevant subsystem maintainer here, so I'm happy to defer to your judgment. I think Junio can just pick up the earlier patch version and we should be good to go, since that patch seemed to meet everyone's needs.

-- 
brian m. carlson (he/him or they/them)
Houston, Texas, US