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Re: lower subreg optimization

On 04/07/2010 10:48 PM, roy rosen wrote:
I saw in arm/ 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 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.


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