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

From: "Pádraig Brady" <P@draigBrady.com>
To: Grisha Levit <grishalevit@gmail.com>, 69770-done@debbugs.gnu.org
Subject: bug#69770: [PATCH] build: strengthen 16 bit float support checks
Date: Wed, 13 Mar 2024 12:36:51 +0000	[thread overview]
Message-ID: <40cecf61-b314-8648-ede4-a86ffdeda226@draigBrady.com> (raw)
In-Reply-To: <20240313022545.13117-1-grishalevit@gmail.com>

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