[Bug fortran/29549] matmul slow for complex matrices

["tkoenig at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #6 from tkoenig at gcc dot gnu dot org  2008-02-10 22:47 -------
(In reply to comment #5)
> The big culprit seems to be -fcx-limited-range. The other flags enabled by
> -ffast-math help very little.

C has some strange rules for complex types, which are mandated by the
C standard and aren't much use for other languages.

This is controlled by the variable flag_complex_method.  For C, this
is either 2 (meaning full C rules) or 0, which implies limited range
for complex division.  Complex multiplication can be expanded into
a libcall for flag_complex_method == 2 under circumstances I don't
understand (line 981, tree-complex.c).

Fortran usually has 1, which means sane rules for complex division
and multiplication.

Unfortunately, our matmul routines are written in C, so we
get what we don't need in Fortran - full C rules and possibly
a call to a library routine.

Solutions?  We could introduce an option to set flag_complex_method to
1 in C.  We could also set -fcx-limited-range for our matmul
routines, which should be safe as they don't use complex division
(at least they should not :-)

CC:ing rth as he wrote the code in question.


-- 

tkoenig at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rth at gcc dot gnu dot org,
                   |                            |tkoenig at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29549