Re: Is detecting endianness at compile-time unworkable?

[Michael Felt <aixtools@felt.demon.nl>]

[-- Attachment #1: Type: text/plain, Size: 1744 bytes --]



On 7/30/2018 11:39 AM, Ævar Arnfjörð Bjarmason wrote:
> The reason we're in this hole is because we use this
> sha1collisiondetection library to do SHA-1, and the reason we have
> issues with it specifically (not OpenSSL et al) is because its only
> method of detecting endianness is at compile time.
When using gcc (no xlc available for Linux on Power)

POWER6 (Big Endian by definition)
root@x068:[/data/httpd/gcc]gcc -dM -E - < /dev/null | grep -i end
#define __ORDER_LITTLE_ENDIAN__ 1234
#define __BIG_ENDIAN__ 1
#define __FLOAT_WORD_ORDER__ __ORDER_BIG_ENDIAN__
#define __ORDER_PDP_ENDIAN__ 3412
#define _BIG_ENDIAN 1
#define __ORDER_BIG_ENDIAN__ 4321
#define __BYTE_ORDER__ __ORDER_BIG_ENDIAN__

SLES12 on POWER8
suse12test:~ # gcc -dM -E - < /dev/null | grep -i end
#define __ORDER_LITTLE_ENDIAN__ 1234
#define _LITTLE_ENDIAN 1
#define __FLOAT_WORD_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __ORDER_PDP_ENDIAN__ 3412
#define __LITTLE_ENDIAN__ 1
#define __ORDER_BIG_ENDIAN__ 4321
#define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__

*So, for compile time tests, when gcc is the compiler it seems the
following defines are available**
**__BIG_ENDIAN__, _BIG_ENDIAN,  __LITTLE__ENDIAN__, _LITTLE_ENDIAN**
**or something based on the value of __BYTE_ORDER__*

I'll see if I can find something similar for xlc, but will only be able
to test xlc on AIX.

>
> This didn't use to be the case, it was changed in this commit:
> https://github.com/cr-marcstevens/sha1collisiondetection/commit/d597672
>
> Dan Shumow: Since the commit message doesn't say why, can you elaborate
> a bit on why this was done, i.e. is determining this at runtime harmful
> for performance? If not, perhaps it would be best to bring this back, at
> least as an option.


[-- Attachment #2: Type: text/html, Size: 2629 bytes --]