patchcompat/bswap.h: detect ARM64 when using MSVC

Signed-off-by: Daniel Gurney <dgurney99@gmail.com>
---
 compat/bswap.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On 2020-11-07 at 22:19:16, Daniel Gurney wrote:

I think this is fine as it is, but I have a question here: why, if we're using MSVC, is that not sufficient to enable this? In other words, why can't this line simply be this:

  #elif defined(_MSC_VER)

As far as I know, Windows has always run on little-endian hardware. It looks like MSVC did run on the M68000 series and MIPS[0] at some point. Are those really versions of MSVC we care about and think Git can practically support, given the fact that we require so many C99 constructs that are not practically available in old versions of MSVC? If not, wouldn't it make sense to simplify?

[0] Wikipedia does not specify the endiannesses supported by the MIPS edition.

That's a fair point, and I doubt there'll be any new developments in this regard. I'll send a new patch with the simplification you suggested.

Hi brian & Daniel,

On Sat, 7 Nov 2020, brian m. carlson wrote:

That is a good question. As far as I can see, this code path has been introduced in 0fcabdeb52b (Use faster byte swapping when compiling with MSVC, 2009-10-19). Then commit looks like this:

    Author: Sebastian Schuberth <sschuberth@gmail.com>
    Date:   Mon Oct 19 18:37:05 2009 +0200
    Subject: Use faster byte swapping when compiling with MSVC

    When compiling with MSVC on x86-compatible, use an intrinsic for
    byte swapping. In contrast to the GCC path, we do not prefer
    inline assembly here as it is not supported for the x64 platform.

    Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>

    diff --git a/compat/bswap.h b/compat/bswap.h
    index 5cc4acbfcce..279e0b48b15 100644
    --- a/compat/bswap.h
    +++ b/compat/bswap.h
    @@ -28,6 +28,16 @@ static inline uint32_t default_swab32(uint32_t val)
     	} \
     	__res; })

    +#elif defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64))
    +
    +#include <stdlib.h>
    +
    +#define bswap32(x) _byteswap_ulong(x)
    +
    +#endif
    +
    +#ifdef bswap32
    +
     #undef ntohl
     #undef htonl
     #define ntohl(x) bswap32(x)

I Cc:ed Sebastian, to confirm my hunch: Looking a bit above that hunk, I see that this merely imitates the way things are done for GCC:

    #if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))

    [...]

Now, let's have a slightly harder look at the patch under consideration: what this does is to re-use the (known to be little-endian) code path of x86 and x86_64 to define the `bswap32()` and `bswap64()` macros also for ARM64.

Further down, the presence of these macros is taken for a sign that we _are_ little-endian and therefore the `ntol()` family of functions isn't a no-op, but has to swap the order, and that's what those macros are then used for.

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

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.

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.

Indeed, and it is more than just the C99 constructs that prevent us from supporting other MSVC versions: we do not support compiling with MSVC out of the box. Nowadays, we rely on CMake to generate the appropriate files to allow us to build with MSVC because our Makefile-based approach very much hardcodes GCC (or compatible) options.

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.

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.

Ciao, Dscho

On Tue, Nov 10, 2020 at 2:58 PM Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:

I believe my intention was not necessarily to imitate the way things are done for GCC, but to independently be on the safe side by checking that this code is only used on x86-style / little-endian architecture.

IMO, strictly speaking, from a semantic point of view this is not about which OS we are running on, but about which compiler is being used. So the question here is: Can MSVC compile for a non-little endian target platform (incl. things like cross-compilation)? And AFAIK the answer is yes, it could in the past / still can nowadays.

Same here, I'd prefer to keep these for explicitness, and for consistency with the GCC check.

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

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

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.

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

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.

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.