RS6000 call pattern clobbers

[Richard Kenner]

> The patterns clobber and use the rs6000 link register as a match_scratch
> with constraint of the link register class:
> 
>    (clobber (match_scratch:SI 0 "=l"))
> 
> instead of clobbering the link register hard register directly in the
> early insn generation.  This style dates to the original rs6000 port.  A
> naked use that starts as a pseudo causes problems for dataflow.
> 
> Do you remember why you wrote the call patterns this way?  Was
> there a problem with reload and clobbers of hard registers in a register
> class containing a single register or some other historical quirk?

I think the former.  I no longer remember the details, but if you had
a clobber of a hard reg, there were a number of things that such a hard
reg couldn't be used for (this is where the details are murky) and in order
to avoid that problem a match_scratch was used to delay the explicit hard
register usage as long as possible.