Path: utzoo!utgpu!cunews!cognos!alzabo!andras
From: andras@alzabo.ocunix.on.ca (Andras Kovacs)
Newsgroups: comp.arch
Subject: Re: Optimising C compiler question
Message-ID: <1991Apr11.003431.24918@alzabo.ocunix.on.ca>
Date: 11 Apr 91 00:34:31 GMT
References: <1991Apr8.193155.3911@vax5.cit.cornell.edu>
Distribution: comp
Organization: Brian's XENIXlings, Ottawa, Canada
Lines: 31

umh@vax5.cit.cornell.edu writes:
>C has provision for register variables, which are supposed to run faster than
>standard variables. How come one never sees register variables in any code?
>Are modern RISC compilers sufficiently good that they automatically make
>sensible choice of register variables? Can I make my code run slower by using
>them?

  My compiler is Norcroft ARM C V1.54A (admittedly an early release). It puts
the first 10 variables into registers and the others are loaded/stored on
demand. Now using 'register' means the same as moving the variable declaration
into the first 10. Would be the compiler better, it would analyze the code and
decide register usage based on that analysis; but then 'register'-ing a var
could have two possible effects:
  1, It is obeyed - but then better if you know what you are doing otherwise
     you can indeed slow down the code, or
  2, Disregarded - because the compiler trusts itself that indeed it knows the
     best allocation scheme.

  I assume that good compilers use the second approach - not out of disregard
to the programmer but either the programmer asks for the right var to be
register and then it is already; or the compiler KNOWS that the register var
would cost execution speed and then what is the point of using it?

  I hope my view is not too simplistic; could someone with actual experience
follow up on the subject?

  Andras
-- 
Andras Kovacs 
andras@alzabo.ocunix.on.ca
Nepean, Ont.