RFC: allowing fwprop to propagate subregs

[Paolo Bonzini]

On 01/11/2012 05:55 PM, Ulrich Weigand wrote:
> In either case, it is always odd to have RTX in the insn stream
> that isn't "stable" under common simplication ...   Do you have
> any suggestions on how to fix this?  If we add the fwprop patch,
> should we then disable apply_distributive_law for SUBREGs?

This strange canonicalization is also the cause of PR39726, where I also 
wanted to move subregs inside arithmetic operations.  See the attached 
patch (which is not enough to fix the PR; this is just the combine part).

Unfortunately, doing this also caused x86_64 regressions, I think in 
zero_extract patterns.

Paolo
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