Re: PPC64 libmvec sincos/sincosf ABI

[GT <tnggil at protonmail dot com>]

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Thursday, August 8, 2019 4:11 PM, Bill Schmidt wschmidt@linux.ibm.com wrote:

> > 1. I understood sincos to be included in implementation of libmvec on PPC64
> > because x86_64 provides that capability. There is discussion of an initial
> > implementation bug in x86_64 sincos at:
> > https://sourceware.org/bugzilla/show_bug.cgi?id=20024
> >
> >
> > The final comment in the thread declares the bug fixed in GLIBC 2.24.
>
> Interesting.  I believe you -- I'm still learning about this.  It
> appears that, although there is an interface provided in libmvec, there
> isn't GCC code to generate calls to it.  I'd like to be proven wrong
> about that.

Your note below regarding the missing logic for sincos in ix86_veclibabi_svml
is proof that you are correct. Isn't it?

> > Compiled it with gcc version 8.3.1 using the command in the Example:
> > gcc ./tst_sincos.c -O1 -ftree-loop-vectorize -ffast-math -lm -mavx
> > Does not generate vectorized call. There is the following warning issued
> > by gcc:
> > ./tst_sincos.c: In function ‘main’:
> > ./tst_sincos.c:15:5: warning: implicit declaration of function ‘sincos’ [-Wimplicit-function-declaration]
> > sincos (a[i], &b[i], &c[i]);
> > ^~~~~~
> > ./tst_sincos.c:15:5: warning: incompatible implicit declaration of built-in function ‘sincos’
> > ./tst_sincos.c:15:5: note: include ‘<math.h>’ or provide a declaration of ‘sincos’
> > ./tst_sincos.c:2:1:
> > +#include <math.h>
> > ./tst_sincos.c:15:5:
> > sincos (a[i], &b[i], &c[i]);
> > ^~~~~~
> >
> > The same warning is issued even when not requesting vectorization:
> > gcc ./tst_sincos.c -lm
> > All the above was on an x86_64 system. Got the same warning on PPC64 POWER8.
> > What is the issue with tst_sincos.c above? math.h is clearly included.
>
> I agree that this is odd.  I have no explanation for you.

Florian has given the reason for the warnings as being that it is required to build
with _GNU_SOURCE for sincos.

> You aren't going to get the vectorized function calls having removed the
> #pragma omp simd, I think.

GCC flags -O1 and -ftree-loop-vectorize enable vectorization of loops even in the absence
of simd pragmas. Example 2 explicitly states that. The failure to vectorize is specific to
how sincos is being handled. Which goes back to ix86_veclibabi_svml.

Thanks.
Bert.