[Bug c/103348] New: Bad code generated for fabs(long double) under aarch64

[Bug c/103348] New: Bad code generated for fabs(long double) under aarch64

[jacob at jacob dot remcomp.fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103348

            Bug ID: 103348
           Summary: Bad code generated for fabs(long double) under aarch64
           Product: gcc
           Version: 6.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jacob at jacob dot remcomp.fr
  Target Milestone: ---

Machine aarch64 linux
when compiling:
long double ld;

ld = fabs(ld);

The compiler generates:
    ldr q0, [x29, 16] 
    bl  __trunctfdf2
    fabs    d0, d0
    bl  __extenddftf2
    str q0, [x29, 16] 
You truncate de 128 bit long double to 64, then extend it to 128 bits! You
loose ALL PRECISION!

Incredible

[Bug c/103348] Bad code generated for fabs(long double) under aarch64

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103348

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-11-21

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Full testcase?
Is this c or c++?
Also if this is c, you should be using fabsl for long double.

[Bug c/103348] Bad code generated for fabs(long double) under aarch64

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103348

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
#include <math.h>


long double 
f(long double ld) 
{
ld = fabsl(ld);
return ld;
};

Works correctly.

[Bug c/103348] Bad code generated for fabs(long double) under aarch64

[jacob at jacob dot remcomp.fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103348

--- Comment #3 from jacob navia <jacob at jacob dot remcomp.fr> ---
1) The complete program is as follows:
#include <math.h>

int main(void)
{
        long double ld = -2.3L;
        ld = fabs(ld);
}

Compiler flags
gcc -S -c -std=c99 tafbs1.c

As per standard c99 fabs is a generic function.
After your remark, gcc generates correct code when I explicitly include
<tgmath.h>

This is surprising



> Le 21 nov. 2021 à 14:25, pinskia at gcc dot gnu.org <gcc-bugzilla@gcc.gnu.org> a écrit :
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103348
> 
> Andrew Pinski <pinskia at gcc dot gnu.org> changed:
> 
>           What    |Removed                     |Added
> ----------------------------------------------------------------------------
>             Status|UNCONFIRMED                 |WAITING
>     Ever confirmed|0                           |1
>   Last reconfirmed|                            |2021-11-21
> 
> --- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
> Full testcase?
> Is this c or c++?
> Also if this is c, you should be using fabsl for long double.
> 
> -- 
> You are receiving this mail because:
> You reported the bug.

[Bug c/103348] Bad code generated for fabs(long double) under aarch64

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103348

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to jacob navia from comment #3)

> As per standard c99 fabs is a generic function.

NO, you need to include tgmath.h for generic math functions.