Re: [PATCH] sha1dc: fix for compiling on AIX using IBM XLC compiler

[Jonathan Nieder <jrnieder@gmail.com>]

(cc-ing Marc Stevens for real this time. Sorry for the noise)
Ævar Arnfjörð Bjarmason wrote:
> On Wed, May 09 2018, jens persson wrote:

>> Hello, first patch. I'm having trouble compiling on AIX using IBMs
>> compiler, leading to
>> unusable binaries. The following patch solved the problem for 2.17.0.
>> The patch below is cut&pasted via gmail to allow for firewalls, but
>> exists in an unmolested form on github:
>> https://github.com/MrShark/git/commit/44bfcaca6637e24548ec06f46fb6035a846b14af
>>
>> Best regards
>> /jp

Thanks for the patch.

>> Building on AIX using XLC every checkout gives an error:
>> fatal: pack is corrupted (SHA1 mismatch)
>> fatal: index-pack failed
>>
>> Back tracking it was introduced in 2.13.2, most likely in [1]
>>
>> Add a #ifdef guard based on macros defined at [2] and [3].
>>
>> Should perhaps __xlc__ should should be changed to or combined with _AIX
>> based on the behavour of GCC on AIX or XL C on Linux.
>>
>> 1. https://github.com/git/git/commit/6b851e536b05e0c8c61f77b9e4c3e7cedea39ff8
>> 2. https://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.3/com.ibm.xlc1313.aix.doc/compiler_ref/macros_platform.html
>> 3. https://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.3/com.ibm.xlc1313.aix.doc/compiler_ref/xlmacros.html
>>
>> Signed-off-by: jens persson <jens@persson.cx>
>> ---
>>  sha1dc/sha1.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/sha1dc/sha1.c b/sha1dc/sha1.c
>> index 25eded139..68a8a0180 100644
>> --- a/sha1dc/sha1.c
>> +++ b/sha1dc/sha1.c
>> @@ -84,7 +84,7 @@
>>  /* Not under GCC-alike or glibc or *BSD or newlib */
>>  #elif (defined(__ARMEB__) || defined(__THUMBEB__) || defined(__AARCH64EB__) || \
>>         defined(__MIPSEB__) || defined(__MIPSEB) || defined(_MIPSEB) || \
>> -       defined(__sparc))
>> +       defined(__sparc) || (defined(__powerpc) && defined(__xlc__)))

I wonder if there's a simpler way to detect this.

__powerpc seems orthogonal to the goal, since there are little-endian
powerpc machines.

It appears that XLC defines _BIG_ENDIAN on big-endian machines.  I
wonder if we should do

 #elif defined(_BYTE_ORDER) && defined(_BIG_ENDIAN) && defined(_LITTLE_ENDIAN)
  ... as today ...
 #elif !defined(_BYTE_ORDER) && defined(_BIG_ENDIAN) && !defined(_LITTLE_ENDIAN)
 # define SHA1DC_BIGENDIAN
 #elif !defined(_BYTE_ORDER) && !defined(_BIG_ENDIAN) && defined(_LITTLE_ENDIAN)
  /* little endian. */
 #else
  ...

It also seems to me that Git should enable the #error in the
fallthrough case.  The test suite would catch this kind of problem but
it does not seem that everyone runs the test suite, so a compiler
error is more robust.  Is there a #define we can set to enable that?

>>  /*
>>   * Should define Big Endian for a whitelist of known processors. See
>>   * https://sourceforge.net/p/predef/wiki/Endianness/ and
>
> This patch looks sane to me, but we don't manage this software but
> instead try to pull it as-is from upstream at
> https://github.com/cr-marcstevens/sha1collisiondetection
>
> Which we could apply this, it would be much better if you could submit a
> pull request with this to upstream, and then we can update our copy as I
> did in e.g. 9936c1b52a ("sha1dc: update from upstream", 2017-07-01).

Thanks,
Jonathan