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From: aglew@urbsdc.Urbana.Gould.COM
Newsgroups: comp.arch
Subject: Re: register save/restore
Message-ID: <28200229@urbsdc>
Date: 5 Nov 88 21:32:00 GMT
References: <3300037@m.cs.uiuc.edu>
Lines: 22
Nf-ID: #R:m.cs.uiuc.edu:3300037:urbsdc:28200229:000:1147
Nf-From: urbsdc.Urbana.Gould.COM!aglew    Nov  5 15:32:00 1988


>I haven't seen anyone mention the idea of mixing caller-saves and callee-saves
>methods: the caller hands to the callee a mask of live registers; the callee
>ANDs this with a mask of the registers it uses and saves the registers whose
>corresponding bits are set.  The mask the callee hands to the routines it calls
>is the OR of these two masks. (I hope that's clear).
>
>	Amos Shapir				amos@nsc.com
>National Semiconductor (Israel) P.O.B. 3007, Herzlia 46104, Israel
>Tel. +972 52 522261  TWX: 33691, fax: +972-52-558322
>34 48 E / 32 10 N			(My other cpu is a NS32532)

There has been a paper on this; sorry, memory fails.

The best argument I heard against this sort of operation is that it makes
high performance instruction dispatch difficult - you can't dispatch 
instructions after the SAVE-MASK until the mask has been computed, because
you don't know which registers are used. In general, instructions that
do not have static register addressing imply an instruction dispatch stall.

This is, of course, not a question on the current generation of microprocessors,
which do not really use any advanced instruction dispatch techniques.