Re: [PATCH 1/3] sha1dc: update from my PR #36

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 27, 2017, 15:22 UTC
Message-ID: <xmqqmv8t317c.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <20170627121718.12078-2-avarab@gmail.com>

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

> Update sha1dc from my PR #36[1] which'll hopefully be integrated by
> upstream soon.

Please be careful about the title of the patch. "log --oneline" does not even let you tell your readers who calls this as "my" change, and readers would be clueless what PR #36 is. Something like

    sha1dc: correct endian detection for Solaris

may give us more relevant information in the oneline output.

Show 11 quoted lines

> @@ -23,6 +23,13 @@
>  #include "sha1.h"
>  #include "ubc_check.h"
>  
> +#if (defined(__amd64__) || defined(__amd64) || defined(__x86_64__) || defined(__x86_64) || \
> +     defined(i386) || defined(__i386) || defined(__i386__) || defined(__i486__)  || \
> +     defined(__i586__) || defined(__i686__) || defined(_M_IX86) || defined(__X86__) || \
> +     defined(_X86_) || defined(__THW_INTEL__) || defined(__I86__) || defined(__INTEL__) || \
> +     defined(__386) || defined(_M_X64) || defined(_M_AMD64))
> +#define SHA1DC_ON_INTEL_LIKE_PROCESSOR
> +#endif

It is good that you made this orthogonal to the rest.

Show 12 quoted lines

> +#else /* Not under GCC-alike */
>  
> +#if defined(__BYTE_ORDER) && defined(__BIG_ENDIAN)
> +/*
> + * Should detect Big Endian under glibc.git since 14245eb70e ("entered
> + * into RCS", 1992-11-25). Defined in <endian.h> which will have been
> + * brought in by standard headers. See glibc.git and
> + * https://sourceforge.net/p/predef/wiki/Endianness/
> + */
> +#if __BYTE_ORDER == __BIG_ENDIAN
>  #define SHA1DC_BIGENDIAN
>  #endif

Note that this part of the file considers it a valid way for a platform to define a constant BIG_ENDIAN that can be compared to BYTE_ORDER to determine the endianness, implying that such a scheme would also define LITTLE_ENDIAN and a port of such a platform to a little endian box will still _define_ the constant BIG_ENDIAN; it aill have BYTE_ORDER defined to the same value as LITTLE_ENDIAN, though.

Show 9 quoted lines

> +#if (defined(__ARMEB__) || defined(__THUMBEB__) || defined(__AARCH64EB__) || \
>       defined(__MIPSEB__) || defined(__MIPSEB) || defined(_MIPSEB) || \
>       defined(__sparc))
> +/*
> + * Should define Big Endian for a whitelist of known processors. See
> + * https://sourceforge.net/p/predef/wiki/Endianness/ and
> + * http://www.oracle.com/technetwork/server-storage/solaris/portingtosolaris-138514.html
> + */
>  #define SHA1DC_BIGENDIAN

These look sensible.

Show 17 quoted lines

> +#else /* Not under GCC-alike or glibc or <processor whitelist> */
> +
> +#if defined(SHA1DC_ON_INTEL_LIKE_PROCESSOR)
> +/*
> + * As a last resort before we fall back on _BIG_ENDIAN or whatever
> + * else we're not 100% sure about below, we blacklist specific
> + * processors here. We could add more, see
> + * e.g. https://wiki.debian.org/ArchitectureSpecificsMemo
> + */
> +#else /* Not under GCC-alike or glibc or <processor whitelist>  or <processor blacklist> */
> +
> +#ifdef _BIG_ENDIAN
> +/*
> + * Solaris / illumos defines either _LITTLE_ENDIAN or _BIG_ENDIAN in
> + * <sys/isa_defs.h>.
> + */
> +#define SHA1DC_BIGENDIAN

This makes readers of this patch wonder why we assume platforms won't define _LITTLE_ENDIAN and _BIG_ENDIAN at the same time, just like we saw in the section with __BIG_ENDIAN above.

Thanks, but this is starting to feel like watching a whack-a-mole played while blindfolded. At some point, somebody upstream should declare that enough is enough and introduce the "SHA1DC_FORCE_ENDIAN" macro.