[PATCH v3 17/19] math: Handle fabsf128 !__USE_EXTERN_INLINES

[Adhemerval Zanella Netto]

On 01/11/25 00:40, H.J. Lu wrote:
> On Sat, Nov 1, 2025 at 4:04 AM Adhemerval Zanella
> <adhemerval.zanella@linaro.org> wrote:
>>
>> clang on x86_64 fails to build s_fabsf128.c with:
>>
>> ../sysdeps/ieee754/float128/../ldbl-128/s_fabsl.c:32:1: error: attribute declaration must precede definition [-Werror,-Wignored-attributes]
>>    32 | libm_alias_ldouble (__fabs, fabs)
>>       | ^
>> ../sysdeps/generic/libm-alias-ldouble.h:63:38: note: expanded from macro 'libm_alias_ldouble'
>>    63 | #define libm_alias_ldouble(from, to) libm_alias_ldouble_r (from, to, )
>>       |                                      ^
>> ../sysdeps/ieee754/float128/float128_private.h:133:43: note: expanded from macro 'libm_alias_ldouble_r'
>>   133 | #define libm_alias_ldouble_r(from, to, r) libm_alias_float128_r (from, to, r)
>>       |                                           ^
>> ../sysdeps/ieee754/float128/s_fabsf128.c:5:3: note: expanded from macro 'libm_alias_float128_r'
>>     5 |   static_weak_alias (from ## f128 ## r, to ## f128 ## r);       \
>>       |   ^
>> ./../include/libc-symbols.h:166:46: note: expanded from macro 'static_weak_alias'
>>   166 | #  define static_weak_alias(name, aliasname) weak_alias (name, aliasname)
>>       |                                              ^
>> ./../include/libc-symbols.h:154:38: note: expanded from macro 'weak_alias'
>>   154 | # define weak_alias(name, aliasname) _weak_alias (name, aliasname)
>>       |                                      ^
>> ./../include/libc-symbols.h:156:52: note: expanded from macro '_weak_alias'
>>   156 |   extern __typeof (name) aliasname __attribute__ ((weak, alias (#name))) \
>>       |                                                    ^
>> ../include/math.h:134:1: note: previous definition is here
>>   134 | fabsf128 (_Float128 x)
> 
> Why does GCC work?

This is one of the known clang limitation wrt inline function and attribute definition,
where it does not allow to 'add' new attribute if a function is already defined:

---
$ cat t.c
typedef __float128 _Float128;

extern _Float128 fabsf128 (_Float128 __x) __attribute__ ((__nothrow__ )) __attribute__ ((__const__)); extern _Float128 __fabsf128 (_Float128 __x) __attribute__ ((__nothrow__ )) __attribute__ ((__const__));

extern inline _Float128
fabsf128 (_Float128 x)
{
  return __builtin_fabsf128 (x);
}

_Float128
__fabsf128 (_Float128 x)
{
  return __builtin_fabsf128 (x);
}
$ gcc -std=gnu11 -fgnu89-inline -O2 -Wall -frounding-math -ftrapping-math -fstack-protector-strong -fno-common -fno-math-errno t.c -c
$ clang -target x86_64-linux-gnu -std=gnu11 -fgnu89-inline -O2 -Wall -frounding-math -ftrapping-math -fstack-protector-strong -fno-common -fno-math-errno t.c -c
t.c:18:55: warning: attribute declaration must precede definition [-Wignored-attributes]
   18 | extern __typeof (__fabsf128) fabsf128 __attribute__ ((weak, alias ("__fabsf128"))) ;;
      |                                                       ^
t.c:8:1: note: previous definition is here
    8 | fabsf128 (_Float128 x)
      | ^
1 warning generated.
---

I think we some code refactoring we can enable the fabs128 optimization on clang,
but now I am focusing and enabling the build.

> 
>> If compiler does not support __USE_EXTERN_INLINES we need to route
>> fabsf128 call to an internal symbol.
>> ---
>>  include/math.h                        | 11 ++++++++---
>>  sysdeps/ieee754/float128/s_fabsf128.c |  3 +++
>>  2 files changed, 11 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/math.h b/include/math.h
>> index 12150e5d64d..d349f237dce 100644
>> --- a/include/math.h
>> +++ b/include/math.h
>> @@ -114,9 +114,11 @@ __issignalingf (float x)
>>
>>  # if __HAVE_DISTINCT_FLOAT128
>>
>> +#  ifdef __USE_EXTERN_INLINES
>> +
>>  /* __builtin_isinf_sign is broken in GCC < 7 for float128.  */
>> -#  if ! __GNUC_PREREQ (7, 0)
>> -#   include <ieee754_float128.h>
>> +#   if ! __GNUC_PREREQ (7, 0)
>> +#    include <ieee754_float128.h>
>>  extern inline int
>>  __isinff128 (_Float128 x)
>>  {
>> @@ -126,13 +128,16 @@ __isinff128 (_Float128 x)
>>    lx |= -lx;
>>    return ~(lx >> 63) & (hx >> 62);
>>  }
>> -#  endif
>> +#   endif
>>
>>  extern inline _Float128
>>  fabsf128 (_Float128 x)
>>  {
>>    return __builtin_fabsf128 (x);
>>  }
>> +#  else
>> +libm_hidden_proto (fabsf128)
>> +#  endif
>>  # endif
>>
>>
>> diff --git a/sysdeps/ieee754/float128/s_fabsf128.c b/sysdeps/ieee754/float128/s_fabsf128.c
>> index 2c1f277653e..9d64f150402 100644
>> --- a/sysdeps/ieee754/float128/s_fabsf128.c
>> +++ b/sysdeps/ieee754/float128/s_fabsf128.c
>> @@ -6,3 +6,6 @@
>>    libm_alias_float128_other_r (from, to, r)
>>  #endif
>>  #include "../ldbl-128/s_fabsl.c"
>> +#ifndef __USE_EXTERN_INLINES
>> +libm_hidden_def (fabsf128)
>> +#endif
>> --
>> 2.43.0
>>
> 
>