Re: Register Allocation

[Joern RENNECKE <joern dot rennecke at st dot com>]

In http://gcc.gnu.org/ml/gcc/2005-11/msg01163.html, Ian Lance Taylor wrote:

> Either way, register elimination can cause addresses which were valid
> to become invalid, typically because valid offsets from the frame
> pointer become invalid offsets from the stack pointer.  So that needs
> to be cleaned up somewhere.

This is not just about some requiring some cleanup somewhere.  Register
elimination and stack slot allocation determine the exact addresses that
are used, which in turn determine what reload inheritance is possible for
address reloads that are for stack slots which are close together on the
stack.  Getting this right is essential to avoid performance degradation on
some platforms.  These targets typically use LEGITIMIZE_RELOAD_ADDRESS to
split out-of-range addresses into a normal form with a base address load
and a memory access using this base with a small offset.

On the other hand, the hard register spills appear to offer a new
opportunity: we have talked about shrink-wrapping code in the past, but
have never implemented this in gcc.
I think that register saves/restores can be considered
special cases of hard register spills.  In order to do this efficiently,
there would have to be some interface with the target to exploit insn
sequences that can save/restore multiple registers more efficiently in
bulk, .e.g load/store multiple, or auto-increment use on targets that
are otherwise ACCUMULATE_OUTGOING_ARGS.  On the other hand, these
techniques can also help when we need to spill multiple hard registers
around a tight loop.