Re: [PATCH 01/17] S390: Use load-fp-integer instruction for nearbyint functions.

[Stefan Liebler <stli at linux dot ibm dot com>]

On 11/4/19 7:22 PM, Adhemerval Zanella wrote:
> On 04/11/2019 12:27, Stefan Liebler wrote:
> > If compiled with z196 zarch support, the load-fp-integer instruction
> > is used to implement nearbyint, nearbyintf, nearbyintl.
> > Otherwise the common-code implementation is used.
>
> > +
> > +double
> > +__nearbyint (double x)
> > +{
> > +  double y;
> > +  /* The z196 zarch "load fp integer" (fidbra) instruction is rounding
> > +     x to the nearest integer according to current rounding mode (M3-field: 0)
> > +     where inexact exceptions are suppressed (M4-field: 4).  */
> > +  __asm__ ("fidbra %0,0,%1,4" : "=f" (y) : "f" (x));
> > +  return y;
> > +}
> > +libm_alias_double (__nearbyint, nearbyint)
>
> At least with recent gcc __builtin_nearbyint generates the expected fidbra
> instruction for -march=z196.  I wonder if we could start to simplify some
> math symbols implementation where new architectures/extensions provide
> direct implementation by a direct mapping implemented by compiler builtins.
>
> I would expect to:
>
>    1. Move all sysdeps/ieee754/dbl-64/wordsize-64 to sysdeps/ieee754/dbl-64/
>       since I hardly doubt these micro-optimizations really pay off with
>       recent architectures and compiler version.
>
>    2. Add internal macros __USE_<SYMBOL>_BUILTIN and use as:
>
>       * sysdeps/ieee754/dbl-64/s_nearbyint.c
>       
>       [...]
>       double
>       __nearbyint (double x)
>       {
>       #if __USE_NEARBYINT_BUILTIN
>         return __builtin_nearbyint (x);
>       #else
>         /* Use generic implementation.  */
>       #endif
>       }
>
>    3. Define the __USE_<SYMBOL>_BUILTIN for each architecture.
>
> It would allow to simplify some architectures, aarch64 for instance.

Currently the long double builtins are generating an extra not needed 
stack frame compared to the inline assembly. But this needs to be fixed 
in gcc.

E.g. if build for s390 (31bit), where the fidbra & co instructions are 
not available, the builtins generate a call to libc which would end in 
an infinite loop.  I will make some tests on s390 starting with the 
current minimum gcc 6.2 to be sure that the instructions are used.  I 
have never build glibc with other compilers like clang.  Is there a 
special need to check this behavior?

In general I can start with those functions where the builtins can be 
used on s390, but I won't move all wordsize-64 functions and adjust them 
to use the builtins with this patch series.
This means for now, I start with using builtins for nearbyint, rint, 
floor, ceil, trunc, round and copysign.

Afterwards the same can be done for the remaining functions.

I will create an own header file, e.g. 
sysdeps/generic/math-use-builtins.h in the same way as 
fix-fp-int-compare-invalid.h.
The generic version contains all USE_XYZ_BUILTIN macros defined to 0
and each architecture can provide its own file with other settings.
For each functions XYZ there will be three macros, e.g. 
USE_NEARBYINT_BUILTIN, USE_NEARBYINTF_BUILTIN, USE_NEARBYINTL_BUILTIN.
How about this?

Thanks,
Stefan