Re: Large slowdown with gfortran vs f77 (x7)

[Tobias Burnus <burnus at net-b dot de>]

On 09/04/2009 05:04 PM, FX wrote:
>   -- it's unarguably a glibc issue: if exp() is fast and expf() is
> slow, why doesn't glibc implement expf() by calling exp()? (yes, there
> can be other issues like rounding or so, but they can also be dealt
> with separately)

If I recall correctly, it is mostly an x86-64 problem. AMD has some math
patches for GLIBC which speed things up a lot. I think those are used in
openSUSE/SLES but not in Fedora. On the other hand, the AMD patches have
a problem with signaling NaN, which is being fixed [1,4].

Some older timings (from PR 34128) on openSUSE (!)  -- for "sin" but
there is the same problem as for exp:

              g77     gfortran
-m32  real(4) 0.408s  0.421s
-m64  real(4) 1.040s  0.589s ! sinf on x86-64: 40% faster!
-m32  real(8) 0.411s  0.408s
-m64  real(8) 0.976s  0.968s ! sin on x86-64


As this is a math-library problem, one cannot do much from the
GCC/gfortran side. You could consider using the AMD Math Core Library
[2] which implements fast versions of the trigonometric functions and
exp [3]. Those functions are not fully IEEE compliant but it might not
be needed in your case [3,4]. (See AMCL manual [3] for the details.)
Intel's MKL should have something similar if you are on Intel hardware
and have by chance the library.

Switching to SUSE or applying the patches oneself is another
possibility. (I do not know why the patches are not included in the
upstream version of glibc. There must be some (somewhat) well-founded
reason.)

> -- a similar bug was already reported a year and a half ago, and no
> activity was recorded on that front 
> (http://sources.redhat.com/bugzilla/show_bug.cgi?id=5997);

Well, it is assigned to someone @suse and as written it is not an issue
on openSUSE. It might be also related to the AMD patches and the reason
why they are not included in GLIBC.

Tobias
(who uses openSUSE [11.1/Factory] at home, Fedora [version 6 (!)]  at work)


[1] For sNaN, see http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39314 and
https://bugzilla.novell.com/show_bug.cgi?id=487576
[2] ACML (free as in free beer): http://www.amd.com/acml
[3]
http://developer.amd.com/cpu/Libraries/acml/onlinehelp/Documents/Simple.html
; the functions are prefixed by "fast" but if you include the library
before the math library ("-lm") the fast version is used instead of the
libm version; "-lm" is automatically appended (internally) at the end of
the command line when using "gfortran" thus simply adding "-lacml_mv"
(or was it -lacml ?) to the command line should be sufficient.
[4] Note, the GLIBC patches of AMD are supposed to be fully IEEE
complient while the fastexp etc. of ACML are not (esp. regarding
denormal numbers and signaling NaN.)