bug#69770: [PATCH] build: strengthen 16 bit float support checks

[Pádraig Brady]

On 13/03/2024 02:24, Grisha Levit wrote:
> Recent clang provides __bf16 on aarch64 but it is broken.
>
> If built with -O0, the conversion is wrong:
>
>      $ printf '\x3F\x80' | od --end=big -An -tfB | tr -d ' '
>      1.875
>
> If built with -O1 or higher, compilation fails:
>
>      fatal error: error in backend: Cannot select: 0xb400007a58d29780: f32 = 
> fp_extend 0xb400007a58d31720
>        0xb400007a58d31720: bf16,ch = CopyFromReg 0xb400007b78c53720, 
> Register:bf16 %13
>          0xb400007a58d29470: bf16 = Register %13
>      In function: print_bfloat
>
> The latter issue does not cause the existing configure test to fail
> because the promotion is optimized out.
>
> * configure.ac: Ensure 16 bit float promotion code does not get
> optimized out, and produces an expected result.

Looks good!

I'll follow up with another patch to all these involved checks to be cached / 
bypassed,
by wrapping them in AC_CACHE_VAL().

Marking this as done.

thanks!
Pádraig