Re: lower subreg optimization

[Jim Wilson <wilson at codesourcery dot com>]

On 04/07/2010 10:48 PM, roy rosen wrote:
> I saw in arm/neon.md that they have a similar problem:
> ...
> Their solution is also not complete.
> What is the proper way to handle such a case and how do I let gcc know
> that this is a simple move instruction so that gcc would be able to
> optimize it out?

The only simple solution at the moment is the one that the ARM port is 
using.  You avoid emitting the move when you got the lucky reg-alloc 
result, and you emit the move when you aren't lucky.

As the neon.md comment suggests, and as Ian Taylor mentioned in his 
response, a possible solution is to modify the lower-subreg.c pass 
somehow so that it no longer splits subregs of vector modes, possibly 
controlled by a hook.

We might be able to modify the register allocator to look for this 
pattern, to increase the chances of getting the good reg-alloc result, 
but the lower-subreg.c change is probably better.

Another solution might be to add a pass (or modify an existing one like 
regmove.c) to try to put things back together again, but this is 
probably also not as good as the lower-subreg.c change.

Jim