Hi all,

when _Float16 in native precision is supported GCC defines
__FLT_EVAL_METHOD__ to 16 as per ISO/IEC TS 18661-3.

Ex:

$ ./arm-none-eabi-gcc -mthumb -march=armv8.1-m.main+fp  -mfloat-abi=softfp -xc /dev/null -E -dM | grep FLT_EVAL
#define __FLT_EVAL_METHOD__ 16
#define __FLT_EVAL_METHOD_TS_18661_3__ 16

Unfortunately libc/include/math.h understands only values 0, 1 and 2
leading to have newlib non compilable for those configurations.

I think in newlib we should either:

1- Handle __FLT_EVAL_METHOD__ == 16
2- Build in a C mode which does not enable _Float16 (ex -std=c11)
3- Build without the _FloatN extensions -fpermitted-flt-eval-methods=c11

I think 1 should be the favorite approach.

ISO/IEC 9899:201x [1] suggests that for other values of FLT_EVAL_METHOD
other than 0, 1, 2 the definition of float_t and double_t is
implementation-defined.

The suggested patch implements that for arm and Aarch64.

Note, some previous discussion on this topic can be found here [2] as
follow-up to a patch addressing the same issue (only for the
case__FLT_EVAL_METHOD__ == 16 case).

Best Regards

  Andrea

[1] <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf>
[2] <https://sourceware.org/legacy-ml/newlib/2019/msg00597.html>