Re: Predicated execution

[Michael Meissner <meissner at cygnus dot com>]

On Fri, Jan 14, 2000 at 02:24:18PM -0800, Richard Henderson wrote:
> > Hmm, how would the @pred work?
> 
> That's some stuff I'm currently working on in genrecog and genattrtab. 
> 
> The idea is that @foo magically generates a predicate for genrecog to
> use that is true iff get_attr_foo is true.  This makes it easy to 
> mark which instructions are or are not predicable.

Note in general, I don't think you can use attributes until after reload, since
the attribute can be different based on the register class, etc.  For a machine
that provides predicated execution in all instructions (presumably like the
IA64) it might not matter.  However, there are machines out there that only a
subset of the instructions provided can be predicated, and you might not know
what you have until register allocation has been done.

> > Or would there be some other way of describing whether an insn can
> > be safely predicated?
> 
> You can use any method you can code up into a boolean function.
> Using attributes is almost certainly going to be easier though.
> 
> > I'm assuming that it 
> > wouldn't be necessary to have a pattern like this for every insn in the 
> > description that can be predicated.
> 
> No.
> 
> 
> 
> r~

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Michael Meissner, Cygnus Solutions
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