Newsgroups: comp.arch
Path: utzoo!utgpu!cunews!bnrgate!bigsur!bcarh680!coopteam
From: coopteam@bcarh680.bnr.ca (Jean Laliberte)
Subject: Re: Optimising C compiler question
Message-ID: <1991Apr15.134417.24380@bigsur.uucp>
Sender: news@bigsur.uucp
Organization: Bell Northern Research, Ottawa, Canada
References: <1991Apr8.193155.3911@vax5.cit.cornell.edu> <1991Apr11.003431.24918@alzabo.ocunix.on.ca> <9526@cognos.UUCP>
Distribution: comp
Date: Mon, 15 Apr 91 13:44:17 GMT

In article
    <9526@cognos.UUCP>
    jimp@cognos.UUCP (Jim Patterson)
writes:
>I don't think a compiler can ever know absolutely that a given variable
>should be in a register whereas another should not, for "performance".
>It can't do anything more than guess at the actual code dynamics, and
>therefore would be allocating registers based on assumptions which
>could be totally erronous. Here's an example which should illustrate
>this. 
>
>[description deleted]
>
>for (i=0 ; i<10000; ++i) {
>    e=malloc(sizeof(element));
>    if (!e) {
>       for (j=0; j<10000; ++j)
>          if (j < i)   
>              free(table[j]);
>       getout(); /* Leave routine now */
>    }
>    table[i] = e;
>}
>
Is there some reason register allocation couldn't also be dynamic? In
this example, i would be stored in the register for the most part,
but when the if() statement evaluates to true, 'store i' and 'load j'
instructions would be executed, and j would then be the register
variable (when the if() condition is over, i would be put back in the
register). I think this would be faster than choosing one variable
or the other.
>-- 
>Jim Patterson                              Cognos Incorporated
>UUCP:uunet!mitel!cunews!cognos!jimp        P.O. BOX 9707    
>PHONE:(613)738-1440 x6112                  3755 Riverside Drive
>NOT a Jays fan (not even a fan)            Ottawa, Ont  K1G 3Z4

no signature.
std disclaimer.
using a 6809 which *does* only allow one register variable.