roy rosen <roy.1ro...@gmail.com> writes:
> I am trying to use define_split, but it seems to me that I don't
> understand how it is used.
> It says in the gccint.pdf (which I use as my tutorial (is there
> anything better or more up to date?))
Assuming you built gccint.pdf from the gcc sources that you are using
for development, then, no, there is nothing better.
> that the combiner only uses the
> define_split if it doesn't find any define_insn to match. This is not
> clear to me: If there is no define_insn to match a pattern in the
> combine stage, then how is this pattern there in the first place? The
> only way I can think for it to happen is that such a pattern was
> created by the combiner itself (but it seems unreasonable to me that
> we now want to split what the combiner just combined). Can someone
> please explain this to me?
The combiner works by combining existing insns into new patterns, and
then attempting to recognize the new patterns (that is why it is called
the combiner--becuse it combines patterns). The combiner combines insns
by, essentially, looking for cases like
(set (reg N) (AAA))
(... (reg N) ...)
and turning them into
(... (AAA) ...)
Actually it's a lot more complicated but that's the basic idea.
So the combiner uses splits by constructing large complex patterns and
then seeing if it can split them. If you know that certain code
sequences are likely to fit together in some complex way, then you can
write a define_split to split that complex pattern into a pair of
insns.
That said, this is not a very interesting use of splits these days and
it could probably be removed without doing any noticeable harm. These
days most people just write the complex pattern using define_insn, and
then write a define_split (or a define_insn_and_split) which splits up
the insn just before register allocation or just before instruction
scheduling.
Ian
