Re: RCSP and modulo scheduling

[Jim Wilson <wilson at redhat dot com>]

>         Is RCSP suppose to be a complete solution for software pipelining
> or used in conjunction with modulo scheduling?  The GCC IA-64 Wish List
> and GCC IA-64 Summit both seem to imply that RCSP is a fallback when
> modulo scheduling cannot be applied.  Should both be implemented and work
> together for a complete solution?

IA-64 has hardware register rotation which is supported by modulo scheduling
but not RCSP.

RCSP can handle loops containing if statements better than modulo scheduling.
IA-64 has relatively cheap predication support that lets us eliminate if
statements thus allowing modulo scheduling to succeed more often.

There are also a number of other issues to consider, but the main point here
is that the IA-64 architecture was designed assuming the compiler would support
modulo scheduling.  So modulo scheduling is clearly the best choice for an
IA-64 target, but is not necessarily the best choice for other targets.

As for which one is right for gcc, that issue has not been decided yet, and
may not be decided for a long time.

There was a previous thread that discussed some of the issues here.
See for instance...
        http://gcc.gnu.org/ml/gcc/2002-09/msg01263.html

Jim