lower subreg optimization

[Ian Lance Taylor]

Nathan Froyd <froydnj@codesourcery.com> writes:

> Compiling anything that uses doubles on powerpc e500v2 produces awful
> code due in part to lower-subregs (the register allocator doesn't help,
> either, but that's a different story).  Code that looks like:
>
>   rY:DI = r<ARG>:DI
>   rX:DI = rY:DI
>   (subreg:DF rZ:DI 0) = rX:DI
>
> <ARG> is a hard register for argument passing; the code looks equally
> awful inside of a function, too.  The above gets lowered to:
>
> 1:  r<Y>:SI = r<ARG>:SI
> 2:  r<Y+1>:SI = r<ARG+1>:SI
> 3:  (subreg:SI rX:DI 0) = r<Y>:SI
> 4:  (subreg:SI rX:DI 4) = r<Y+1>:SI
> 5:  (subreg:DF rZ:DI 0) = rX:DI
>
> which usually results in two stores and a load against the stack, rather
> than a single-instruction dealing entirely in registers.  I realize
> e500v2 is not exactly a mainstream target, but perhaps a target hook is
> appropriate here?  I suppose checking costs might achieve the same
> thing.

I doubt that a target hook is required to avoid this.  Perhaps
simple_move_operand should reject a mode changing subreg when the two
modes are !MODE_TIEABLE_P.

This code is sort of weird, though; why the conversion from DImode to
DFmode?

Ian