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

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 02/12/2019 11:56, Stefan Liebler wrote:
> 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.
>>
> This patch is superseded by the patch-series which is always using wordsize-64 version and allows to use the GCC builtins in common-code implementation:
> "[PATCH 00/13] Use GCC builtins for some math functions if desired."
> https://www.sourceware.org/ml/libc-alpha/2019-12/msg00029.html
> 
> Bye,
> Stefan
> 

Thanks, I will review these set.