GVN-PRE sucks -- well, for SPECCFP2000 mgrid anyway

[Mark Mitchell]

Daniel Berlin wrote:

> Yeah.
> I'll see if i can come up with some simple heuristics that help here, 
> without hurting anything else too much.

I wonder if it's better to restrain PRE, or let it go ahead, and then 
have the register allocator move computations back into the loop.

I've copied Kenny here to see if he happens to know of any magic bullets 
for this kind of thing.

(Kenny, the issue is that PRE is pulling so much stuff out of loops that 
we get register pressure, and hence bad code.)

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Mark Mitchell
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