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

[GNU bug Tracking System]

Your message dated Wed, 13 Mar 2024 12:36:51 +0000
with message-id <40cecf61-b314-8648-ede4-a86ffdeda226@draigBrady.com>
and subject line Re: bug#69770: [PATCH] build: strengthen 16 bit float support 
checks
has caused the debbugs.gnu.org bug report #69770,
regarding [PATCH] build: strengthen 16 bit float support checks
to be marked as done.

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-- 
69770: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=69770
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> --- Begin Message ---
>
>
>
> Subject: 
>
> [PATCH] build: strengthen 16 bit float support checks
>
>
>
>
> Date: 
>
> Tue, 12 Mar 2024 22:24:34 -0400
>
>
>
>
> 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.
> ---
>  configure.ac | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/configure.ac b/configure.ac
> index 248e30ca2..21bee28d1 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -569,13 +569,14 @@ ac_c_werror_flag=$cu_save_c_werror_flag
>  
>  # Test compiler support for half precision floating point types (for od)
>  AC_MSG_CHECKING([IEEE 16 bit floating point])
> - AC_COMPILE_IFELSE(
> + AC_RUN_IFELSE(
>     [AC_LANG_SOURCE([[
>       int
>       main (void)
>       {
> -        _Float16 hf;
> +        volatile _Float16 hf = 1;
>          float f = hf;  /* Ensure compiler can promote to float.  */
> +        return !(f == 1.0f);
>       }
>    ]])
>    ],[
> @@ -589,13 +590,14 @@ if test $ieee_16_bit_supported = yes; then
>  fi
>  
>  AC_MSG_CHECKING([Brain 16 bit floating point])
> - AC_COMPILE_IFELSE(
> + AC_RUN_IFELSE(
>     [AC_LANG_SOURCE([[
>       int
>       main (void)
>       {
> -        __bf16 hf;
> +        volatile __bf16 hf = 1;
>          float f = hf;  /* Ensure compiler can promote to float.  */
> +        return !(f == 1.0f);
>       }
>    ]])
>    ],[
> -- 
> 2.44.0
>
>
>
>
> --- End Message ---
> --- Begin Message ---
>
>
>
> Subject: 
>
> Re: bug#69770: [PATCH] build: strengthen 16 bit float support checks
>
>
>
>
> Date: 
>
> Wed, 13 Mar 2024 12:36:51 +0000
>
>
>
>
> User-agent: 
>
> Mozilla Thunderbird
>
>
>
>
> 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
>
>
> --- End Message ---