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Re: stepanov on sparc; sparc backend scheduling issues


> On Fri, May 25, 2001 at 05:55:29PM -0700, Joe Buck wrote:
> > So, a question: since I configured the compiler on an Ultrasparc, why
> > doesn't it set -mtune=ultrasparc by default?  What are we defaulting to?
> 
> config.guess doesn't know you configured on an ultrasparc.

OK, I understand that, since it isn't in the target triplet (though this
seems inconsistent with what we do for the ia32 ports).

> You can use --with-cpu=ultrasparc, but that affects -mcpu,
> not -mtune, which may not be what you want.
> 
> Default tuning matches default architecture, which is sparc v7.

But I don't understand this.  Why tune by default for an architecture
no one is using?  Certainly we are best off by default using only the
v7 instruction set, but I think we should tune for whatever is the
most common architecture currently in use.  I don't know if that should
be Ultrasparc, but since the new architectures generally have longer
pipelines, I suspect tuning for a newer architecture will also work
reasonably for older architectures though not vice versa (in this case,
it seems to "push" writes and reads of registers further apart without
making the loop longer).





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