Re: [PATCH v2 2/6] bswap.h: Add support for __BYTE_ORDER__

From: brian m. carlson <sandals@crustytoothpaste.net>
Date: Jun 12, 2025, 00:24 UTC
Message-ID: <aEoeUCcXYByjU5HR@fruit.crustytoothpaste.net>
In-Reply-To: <20250611221444.1567638-3-sebastian@breakpoint.cc>

On 2025-06-11 at 22:14:38, Sebastian Andrzej Siewior wrote:

Show 27 quoted lines

> The __BYTE_ORDER__ define is provided by gcc (since ~v4.6), clang
> (since ~v3.2) and icc (since ~16.0.3). It is not provided by msvc as of
> v19.43 / 17.13.6.
> The __BYTE_ORDER and BYTE_ORDER macros are libc specific and are not
> available on all supported platforms such as mingw.
> 
> Add support for the __BYTE_ORDER__ macro as a fallback.
> 
> Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
> ---
>  compat/bswap.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/compat/bswap.h b/compat/bswap.h
> index b34054f2bd728..0a457542dd76a 100644
> --- a/compat/bswap.h
> +++ b/compat/bswap.h
> @@ -116,6 +116,12 @@ static inline uint64_t git_bswap64(uint64_t x)
>  # define GIT_LITTLE_ENDIAN LITTLE_ENDIAN
>  # define GIT_BIG_ENDIAN BIG_ENDIAN
>  
> +#elif defined(__BYTE_ORDER__) && defined(__ORDER_LITTLE_ENDIAN__) && defined(__ORDER_BIG_ENDIAN__)
> +
> +# define GIT_BYTE_ORDER __BYTE_ORDER__
> +# define GIT_LITTLE_ENDIAN __ORDER_LITTLE_ENDIAN__
> +# define GIT_BIG_ENDIAN __ORDER_BIG_ENDIAN__
> +

One additional option you can add if you want (it's completely optional) is that if `__STDC_VERSION__` is 202311L or larger, then you can `#include <stdbit.h>`, which has `__STDC_ENDIAN_LITTLE__`, `__STDC_ENDIAN_BIG__`, and `__STDC_ENDIAN_NATIVE__`.

That will work on a modern GCC or clang with an appropriate compiler flag. I don't know about MSVC, but it might be an option for people who want to use more esoteric compilers which support standards but aren't very compatible with GCC and clang.

-- 
brian m. carlson (they/them)
Toronto, Ontario, CA