Path: utzoo!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!mcsun!tuvie!mike
From: mike@vlsivie.tuwien.ac.at (Michael K. Gschwind)
Newsgroups: comp.arch
Subject: Re: register save
Message-ID: <2376@tuvie.UUCP>
Date: 13 Mar 91 08:33:20 GMT
References: <1991Mar11.192116.1974@dgbt.doc.ca>
Sender: news@tuvie.UUCP
Organization: Vienna University of Technology
Lines: 38

In article <1991Mar11.192116.1974@dgbt.doc.ca> don@dgbt.doc.ca (Donald McLachlan) writes:
>
>	The idea of saving the return address in a register, rather than
>on the stack sounds nice to me, but ...
It is nice AND saves a lot of cycles ...
>
>	This requires that the calling function knows that the called
>function is a leaf function, not very practical from a high level language
>point of view as far as I can tell.
Simply not true. As you yourself explain later.
>	A further optimisation I could see for this is any function that
>performs a call could save the "return register" on the stack once at
>invocation, and restore it prior to returning. (gee isn't this what is done
>now:-)
Exactly.
>
>	Now that all the mechanics are out of the way (the way I see them)
>only one question remains. HOW MUCH DOES THIS ACTUALLY SAVE???
>I interpret this as ... What is the ratio of calls to "leaf functions"
>versus calls to "non-leaf functions"?
Dynamically, about 50% of all functions called are leaf-functions. 
Assuming you save 2 memory accesses AND 2 stack manipulations, this is 
a quite interesting optimization. Also, the CALL instruction becomes
much simpler. Just move the PC to some register and some register to the
PC. - No stack handling involved. Everything is much smoother (and
simpler to design - read my thesis ;-)

			bye,
				mike



Michael K. Gschwind, Dept. of VLSI-Design, Vienna University of Technology
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