Re: [PATCH v2 2/4] iee754: provide gcc builtins based generic fma functions

[Stefan Liebler <stli@linux.ibm.com>]

On 6/2/20 7:13 PM, Vineet Gupta wrote:
> On 6/2/20 5:51 AM, Stefan Liebler via Libc-alpha wrote:
>>  #endif /* math-use-builtins.h */
>> Please also update the current architecture specific math-use-builtins.h
>> file: sysdeps/s390/fpu/math-use-builtins.h
>> Otherwise it will break build on s390x.
> 
> Done.
> 
>>> diff --git a/sysdeps/ieee754/dbl-64/s_fma.c b/sysdeps/ieee754/dbl-64/s_fma.c
>>> index 876df6e78bdc..9dc5b132b9ee 100644
>>> --- a/sysdeps/ieee754/dbl-64/s_fma.c
>>> +++ b/sysdeps/ieee754/dbl-64/s_fma.c
>>> @@ -25,6 +25,7 @@
>>>  #include <fenv_private.h>
>>>  #include <libm-alias-double.h>
>>>  #include <tininess.h>
>>> +#include <math-use-builtins.h>
>>>
>>>  /* This implementation uses rounding to odd to avoid problems with
>>>     double rounding.  See a paper by Boldo and Melquiond:
>>> @@ -33,6 +34,10 @@
>>>  double
>>>  __fma (double x, double y, double z)
>>>  {
>>> +#if USE_FMA_BUILTIN
>>> +  return __builtin_fma (x, y, z);
>>
>> Architectures which have support for ldbl-128 will use the file
>> sysdeps/ieee754/ldbl-128/s_fma.c instead of
>> sysdeps/ieee754/dbl-64/s_fma.c. Should this file also be adjusted in
>> order to use the builtin if USE_FMA_BUILTIN is set to one?
> 
> Right.
> 
> I used commit f82996f815 "Use GCC builtins for round functions if desired" as
> starting point for my change. And seems it was not an ideal reference :-) as round
> has far fewer instances than fma. Indeed fma is present in ldbl-128 and dbl-64 so
> needs updating in both.
> 
> But just to be sure s390 is currently not using the newly introduced builtins so
> I'll keep them as follows.
> 
> #define USE_SQRT_BUILTIN 0
> #define USE_SQRTF_BUILTIN 0
> 
> #define USE_FMA_BUILTIN 0
> #define USE_FMAF_BUILTIN 0
> #define USE_FMAL_BUILTIN 0
> #define USE_FMAF128_BUILTIN 0
> 
Yes. Those should be disabled for now in order to not break anything. As
soon as you've committed your patches, I'll have a look if I can
activate some of them and remove s390 specific implementations. But I
first have to check with the gcc guys, if the builtins are always
expanded correctly in various gcc versions.

Thanks.
Stefan
> -Vineet
> 


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