Re: [PATCH 21/28] powerpc: Refactor powerpc32 lround/lroundf/llround/llroundf

["Gabriel F. T. Gomes" <gabriel at inconstante dot eti dot br>]

On Fri, Mar 29 2019, Adhemerval Zanella wrote:
> 
> This patches consolidates all the powerpc llround{f} implementations on
> the generic sysdeps/powerpc/powerpc32/fpu/s_llround{f}.  The only missing
> optimization is the power6x one which I could not make GCC generates
> mftgpr for 32 bits output.

Similar to the llrint case, no harm done as such optimization wasn't
available, anyway.

> +  /* The barrier prevents compiler from optimizing it to llround when
> +     compiled with -fno-math-errno */
> +  math_opt_barrier (x);

I don't actually understand what this accomplishes, and I don't see any
difference in the code generated with and without this barrier (all my
builds have -fno-math-errno passed to the compiler).  Could you help me
understand this?

This is the code I get (with or without the barrier):

  00079dc0 <__llround_power6>:
    79dc0:       fc 20 0b 10     frin    f1,f1
    79dc4:       94 21 ff f0     stwu    r1,-16(r1)
    79dc8:       fc 00 0e 5e     fctidz  f0,f1
    79dcc:       d8 01 00 08     stfd    f0,8(r1)
    79dd0:       80 61 00 08     lwz     r3,8(r1)
    79dd4:       80 81 00 0c     lwz     r4,12(r1)
    79dd8:       38 21 00 10     addi    r1,r1,16
    79ddc:       4e 80 00 20     blr

  00079de0 <__llround_power5plus>:
    79de0:       94 21 ff f0     stwu    r1,-16(r1)
    79de4:       fc 20 0b 10     frin    f1,f1
    79de8:       fc 00 0e 5e     fctidz  f0,f1
    79dec:       d8 01 00 08     stfd    f0,8(r1)
    79df0:       60 00 00 00     nop
    79df4:       60 00 00 00     nop
    79df8:       60 00 00 00     nop
    79dfc:       80 61 00 08     lwz     r3,8(r1)
    79e00:       80 81 00 0c     lwz     r4,12(r1)
    79e04:       38 21 00 10     addi    r1,r1,16
    79e08:       4e 80 00 20     blr

> --- a/sysdeps/powerpc/powerpc32/fpu/s_llround.c
> +++ b/sysdeps/powerpc/powerpc32/fpu/s_llround.c
>
> [...]
>
> +    {
> +       /* IEEE 1003.1 lround function.  IEEE specifies "round to the nearest
> +	  integer value, rounding halfway cases away from zero, regardless of
> +	  the current rounding mode."  However PowerPC Architecture defines
> +	  "round to Nearest" as "Choose the best approximation. In case of a
> +	  tie, choose the one that is even (least significant bit o).".
> +	  So we can't use the PowerPC "round to Nearest" mode. Instead we set
> +	  "round toward Zero" mode and round by adding +-0.5 before rounding
> +	  to the integer value.
> +  
   ~~
Two white-spaces here.

> --- a/sysdeps/powerpc/powerpc32/fpu/s_lround.S
> +++ /dev/null
>
> [...]
>
> -	fcmpu	cr5, fp1, fp9	/* if x >= 0x7fffffff.8p0  */
> -	fcmpu	cr1, fp1, fp8	/* if x <= -0x80000000.8p0  */

OK.  These have also been converted to c code in lround.c...

> --- /dev/null
> +++ b/sysdeps/powerpc/powerpc32/fpu/s_lround.c
>
> [...]
>
> +  if (x >= 0x7fffffff.8p0 || x <= -0x80000000.8p0)
> +    x = (x < 0.0) ? -0x1p+52 : 0x1p+52;

... Here.