subregs

[Kenneth Zadeck <Kenneth dot Zadeck at NaturalBridge dot com>]

This is a reply to jeff's mail about subreg's in 

http://gcc.gnu.org/ml/gcc/2004-04/msg01278.html

what is desirable is depends on where you are during a compilation.

It is desirable to have the front end of the compiler generate each
load and store as a seperate sequence, i.e. not combine them.  The
motivation here is the same one where gcc currently treats the fields
of small structures as separate variables: each of the small
operations more likely to be optimized by the commoning and code
motion algorithms than if they are left as aggragates.

> (sequence [
>   (insn (set (reg X_1) (reg X_0)))
>   (insn (set (subreg (reg X_1)) Y))
> ])


However, it is also quite desirable to combine the loads and stores
back into aggragates (the jeff extension) as a optimization to be
performed very late, (ideally after some preliminary scheduling)
because you really can do a much better job with the low level code
generation if you can glom together the masking and shifting
operations for several loads and stores.  This can be done with a
simple hack to any full redundancy elimination algorithm.

> This SEQUENCE, considered as a whole, is in SSA form.  It has an
> input, X_0, and an output, X_1.  X_1 will be set only by this insn.
> 
> As an extension, it's nice to recognize cases like
> 
> (insn (clobber (reg X)))
> (insn (set (subreg (reg X) 0) Y))
> (insn (set (subreg (reg X) 1) Z))
> 
> which together set all of X, and turn them into
> 
> (sequence [
>   (insn (clobber (reg X_1)))
>   (insn (set (subreg (reg X_1) 0) Y))
>   (insn (set (subreg (reg X_1) 1) Z))
> ])
> 
> to eliminate the dependency on the previous value of X; and it's also
> nice to eliminate unnecessary copies when converting out of SSA form
> (I think this is just an extension of the usual unnecessary copy
> elimination).
>