Possible CSE quirk involving SUBREG on the i386

[John Wehle]

> Based on the comments in combine.c, I think at one time MODES_TIEABLE_P
> did have to be symmetric because of the reload issue.  I'm starting to
> think we can relax this now.
> 
> The last issue is register allocation -- I get the impression from reading
> the code in cse, combine & regclass that this change may make register 
> allocation worse in some cases.  So we need to have some guess about the
> benefit vs the cost.

Yep, I got the same impression though sometimes it's hard to guess without
trying it.

> I propose we table this until after egcs-1.1.  Or better yet, instead of
> tabling it, we just install your patch in the mainline sources after egcs-1.1
> branches.

Okay, I'll see about throwing together a patch.

> That gives us plenty of time to study what actually happens instead of
> doing something rather risky at this stage.

Sounds reasonable to me.

-- John
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