Fails to detect endianness on 64-bit ARM

[musicinmybrain]

On 64-bit ARM (aarch64), which can technically be little- or big-endian, since c233f40 the project cannot compile because the endianness is not successfully detected from predefined preprocessor macros.

I’ve tested this on Linux, with recent (11.x) GCC, on little-endian aarch64.

In file included from /builddir/build/BUILD/c4core-0.1.3/src/c4/config.hpp:34,
                 from /builddir/build/BUILD/c4core-0.1.3/src/c4/error.hpp:8,
                 from /builddir/build/BUILD/c4core-0.1.3/src/c4/error.cpp:1:
/builddir/build/BUILD/c4core-0.1.3/src/c4/cpu.hpp:76:9: error: #error "unknown endianness"
   76 | #       error "unknown endianness"
      |         ^~~~~

Perhaps it would help, across all architectures, to look for __BYTE_ORDER__ and/or __FLOAT_WORD_ORDER__, which may be defined to __ORDER_BIG_ENDIAN__, __ORDER_LITTLE_ENDIAN__, or (if you are cursed) __ORDER_PDP_ENDIAN__. These are GCC-specific, but they allow you to tell which mode is in use on bi-endian architectures.

Similarly, some toolchains (clang, I think) define __BIG_ENDIAN__ or __LITTLE_ENDIAN__. When present, they are conclusive.

Finally, according to https://sourceforge.net/p/predef/wiki/Endianness/, some environments apparently define __AARCH64EB__ or __AARCH64EL__ on aarch64. These are not present on gcc/Linux.

[biojppm]

Thanks for reporting.

The byte order check for aarch64 (and all other ARMs) is this:

   #ifdef __ARMEL__
       #define C4_BYTE_ORDER _C4EL
   #elif defined(__ARMEB__)
       #define C4_BYTE_ORDER _C4EB
   #else
       #error "unknown endianness"
   #endif

There are no tests for this architecture (or s390x) in the current pipeline, and although I intend to add them unfortunately I cannot work on that ATM.

So can you confirm this (or something similar) fixes your problem?

   #if defined(__ARMEL__) || defined(__LITTLE_ENDIAN__) || defined(__AARCH64EL__) \
       || (defined(__BYTE_ORDER__) && (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__))
       #define C4_BYTE_ORDER _C4EL
   #elif defined(__ARMEB__) || defined(__BIG_ENDIAN__) || defined(__AARCH64EB__) \
       || (defined(__BYTE_ORDER__) && (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__))
       #define C4_BYTE_ORDER _C4EB
   #elif defined(__BYTE_ORDER__) && (__BYTE_ORDER__ == __ORDER_PDP_ENDIAN__)
       #define C4_BYTE_ORDER _C4EM
   #else
       #error "unknown endianness"
   #endif

[biojppm]

I added s390x tests to the CI (as well as aarch64 and ppc64le), and they succeed with this change. So the PR will close this.

[musicinmybrain]

Thanks! I didn’t have a chance to test your change yet, but this is appreciated.