Gcc builtin review: isinf, insnan ...

[Julian Taylor]

On 27.05.2015 16:57, Wilco Dijkstra wrote:
> Ondøej Bílka wrote:
>> I raised this issue before but didn't wrote patch so I should do it now.
>> I would be silent about glibc as it shares same flaw as gcc.
>>
>> Main problem that these functions try to be branchless. Which causes
>> performance regression for most applications versus branched code.
> 
> Being branchless is one issue indeed but the main issue they are never
> inlined on any target as GLIBC headers explicitly disable inlining by GCC.
> 


This is a huge problem for almost all numerical applications that don't
care about signaling nans, so basically all.
I had to fix too many applications having poor performance due to that
(e.g.
http://yarikoptic.github.io/numpy-vbench/vb_vb_ufunc.html#numpy-isnan-a-10types)

I would love when glibc would somehow allow programs to use the builtin
via the isnan function in an easy way, so not via compiler specific
compiler flags like -fno-signaling-nan or preprocessor compiler version
checks.
Maybe alway direct to the builtin/fastest possible non signaling
implementation when some GIVE_ME_FAST_BOOL_ISNAN is defined before
including <math.h>?

Also note that gcc has updated their documentation which might allow to
now use __builtin_isinf
(https://sourceware.org/bugzilla/show_bug.cgi?id=15367)