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From: torek@elf.ee.lbl.gov (Chris Torek)
Newsgroups: comp.arch
Subject: Re: register save
Message-ID: <10945@dog.ee.lbl.gov>
Date: 14 Mar 91 22:18:58 GMT
References: <1991Mar11.192116.1974@dgbt.doc.ca> <912@spim.mips.COM> <3255@crdos1.crd.ge.COM> <10896@dog.ee.lbl.gov> <1991Mar13.200326.29116@zoo.toronto.edu>
Reply-To: torek@elf.ee.lbl.gov (Chris Torek)
Organization: Lawrence Berkeley Laboratory, Berkeley
Lines: 30
X-Local-Date: Thu, 14 Mar 91 14:18:58 PST

In article <1991Mar13.200326.29116@zoo.toronto.edu>
henry@zoo.toronto.edu (Henry Spencer) writes:
>AHEM.  Chris, the Am29k has *both*.  You can use 128 of those registers
>as a register-window bank if you want, with the added bonus that the
>size of the windows is completely up to you and can be different for
>each function.

True.  I was originally going to use SPARC and MIPS in my examples, but
I figured people would object to comparing a 127-register machine
(Sun's SPARC chips, 8 windows of which 1 is reserved for traps, 16
registers per window, plus 8 more overlapping into the trap window
[this makes the trap handlers somewhat painful], plus 8 `global'
registers of which one is wired zero) to a 31-register machine (MIPS
R[23]000, 32 registers, one wired zero).  So when I said:

>>... and which thus fit perfectly in the windows,
>>will get either large gains, or (if your register allocation is good
>>enough) none at all, on machines with register windows (SPARC) vs
>>machines with `just a lot of registers' (AM29000).

perhaps I should have made it `...with register windows (SPARC, AM29000)
vs machines with ``just a lot of registers'' (AM29000)'.

(Acutally, the SPARC has 8 `user-usable' global registers, the last one
being the %y register, but it is a special case.  Still, it might make
a good holding register for leaf functions.  Now I am tempted to hack
gcc to do this. :-) )
-- 
In-Real-Life: Chris Torek, Lawrence Berkeley Lab EE div (+1 415 486 5427)
Berkeley, CA		Domain:	torek@ee.lbl.gov