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Re: [PATCH,rs6000] disable mfcr pattern for TARGET_ISEL


On Wed, Jul 21, 2010 at 11:51 AM, Nathan Froyd <froydnj@codesourcery.com> wrote:

>> Do processors with TARGET_ISEL also implement MFCRF? ?mfcrf does not
>> have the same expense as mfcr. ?There may be other reasons to use ISEL
>> more aggressively, but I am surprised that this would be a problem on
>> embedded processors with ISEL. ?Is this for old processors with ISEL
>> or did new processors not enable TARGET_MFCRF in GCC?
>
> The motivation was for some newer embedded processors. ?Older processors
> with ISEL do not implement MFCRF; they can still benefit from using ISEL
> instead of MFCR. ?Some newer embedded processors do implement MFCRF, but
> MFCRF on those implementations carries the same synchronization
> penalties as MFCR, so it's beneficial to use ISEL there as well.

Someone implemented mfcrf as inefficiently as mfcr?  Sigh.

> I see now that the pattern cleverly uses 'Q' so that it works for
> TARGET_MFCRF as well. ?Will !TARGET_ISEL || TARGET_MFCRF DTRT?

I'm not sure what you are trying to capture with the new final
constraint, unless you now are saying that the pattern *is* a good
idea for TARGET_ISEL processors with TARGET_MFCRF, which sort of
contradicts your statement above.

Thanks, David


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