Re: Reload patch to improve 386 code

[Joern Rennecke <amylaar@cygnus.co.uk>]

> Then, there are some simplifications that could be done. I don't like the
> inheritance code, find_equiv_reg and all that. IMHO reload shouldn't try to be
> very clever about this sort of thing - the reload_cse_regs pass can be made
> more clever. I've already submitted a patch to Kenner that enables
> reload_cse_regs to generate optional reloads. If we could add some more
> cleverness (e.g. deleting redundant stores into spill slots or eliminating
> register-register copies), quite a bit of code in reload could be deleted.
> I've already made some experiments in this direction which indicate that this
> approach may be feasible.

The problem with this is that you might have additional spills if you
remove reload inherits - when you inherit a reload that needs a secondary
reload, you extend the lifetime of only one reload register, instead of needing
two (or more, when a tertiary reload would be needed too.)

Moreover, if you remove reload inherits from / to memory, you might end
up with code that looks like it could have an alias problem for doing cse on
it, and reload_cse can't do anything.

I actually think that the reload inheriting code should be mae smarter.
I did some patches to do that and got more benefit for the code I tested
than from reload_cse.  However, they never got accepted for the FSF gcc...
I'll re-fit them into egcs when I find some time for that.  Should I
sent them as multiple (conflicting) patches or as a single patch?

The main issues are:

- inheriting reloads from non-reload registers

- emitting more complete REG_ notes when doing reloads, so that they can
  be found for inherits

- being smarter about finding possible inherits

> Another way of simplifying reload would be to try to generate auto-inc
> addressing after reload has run, not before. That would eliminate some more
> special-case code (and it might make other parts of the compiler simpler as
> well).

But then you wouldn't get the heuristics for the earlier passes right.