[PATCH] Sparc exp(), expf() performance improvement

[Patrick McGehearty]

On 8/2/2017 6:19 PM, David Miller wrote:
> From: Patrick McGehearty <patrick.mcgehearty@oracle.com>
> Date: Wed, 2 Aug 2017 14:53:19 -0500
>
>> I concur that it would be beneficial to use -mcpu=niagara4 for
>> selected libm functions, but do not believe it should be default on
>> without more extensive testing (and likely more compiler tuning).
> This is precisely what has been suggested, have an niagara4 IFUNC
> vector for math functions that benefit greatly from the fp<-->int
> register move instructions.

I'm new to linux glibc development and have been using existing examples 
to suggest
how I should do things like use ifunc. If there is a different approach 
that is recommended,
I'm willing to use it but will need some hints/instruction on what is.

My current approach of using
sparc_libm_ifunc (exp, hwcap & HWCAP_SPARC_CRYPTO ?
                   __exp_niagara4 : __ieee754_exp);
in sysdeps/sparc/sparc64/fpu/multiarch/e_exp-niagara4.c matches the 
approach used in
sysdeps/sparc/sparc64/fpu/multiarch/s_ceil.c
sysdeps/sparc/sparc64/fpu/multiarch/s_ceilf.c
sysdeps/sparc/sparc64/fpu/multiarch/s_floor.c
sysdeps/sparc/sparc64/fpu/multiarch/s_floorf.c
sysdeps/sparc/sparc64/fpu/multiarch/s_fma.c
sysdeps/sparc/sparc64/fpu/multiarch/s_fmaf.c
sysdeps/sparc/sparc64/fpu/multiarch/s_trunc.c
sysdeps/sparc/sparc64/fpu/multiarch/s_truncf.c

I'll note that simply using -mcpu=niagara4 on the existing ieee754 exp() 
code only
provides a 5% performance gain. Due to other assignments, I will not 
have time/resources
to redo other libm functions at this time.

- patrick