Path: utzoo!utgpu!watmath!clyde!mcdchg!chinet!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!bu-cs!purdue!decwrl!sun!pitstop!sundc!seismo!uunet!munnari!munmurra!lean
From: lean@munmurra.mu.oz (Giles Lean)
Newsgroups: comp.lang.c++
Subject: Re: C++ : A free lunch ?
Message-ID: <715@munmurra.mu.oz>
Date: 29 Oct 88 04:36:41 GMT
References: <17558@gatech.edu>
Organization: Comp Sci, Melbourne Uni, Australia
Lines: 44

Continuing the C++ efficiency debate,

in article <17558@gatech.edu>, snm@horus.gatech.edu (Sathis Menon) says:
>In article <77300014@p.cs.uiuc.edu> johnson@p.cs.uiuc.edu writes:
>>
>>C++ is just as fast as C (within + or - 2%).  We have written an
>>operating system (Choices) in C++ and are getting very good performance
>>figures from it.  There is essentially no performance overhead in
>>using C++.

> The distributed OS project that I am working on (Clouds Project at
> Georgia Tech) is using g++ (GNU C++) to implement the kernel and
> system objects (essential system services). Unfortunately, the
> optimizer that we got from GNU folks assumed a little too much,
> which doesn't go hand in hand with low level coding. Hence, we don't
> optimize the code right now and the code generated is worse than C code
> (by checking the assembler listing, of course).

Firstly, judging the quality of code output by an optimising compiler
before the optimising pass is not sensible for assessing the efficiency
of a language. The compiler may have offloaded even basic optimisations
onto the optimiser.

Further, comparing the output from two compilers as a language comparison
only makes sense if the compilers are somehow "equivalent" - i.e. both
generating the "best" code possible.  (Both being very basic generating
naive code is not appropriate for an efficiency comparison.)
What was the C compiler you compared g++ with?

> OK., you are getting good performance figures from your OS using C++, but
> did you have a parallel implementation in C to compare it?. Are you
> implying that the "very good performance figures" that you mention
> are attibuted to C++ (at least partly)?

The question is still open - I can't see that you can claim that your
g++ code is worse than C code, but only that in this instance the g++
optimiser prevents you using C++ to its fullest.

Vote (1) for disableable features in optimisers!


Giles Lean			My own opinions, which have been wrong before,
lean@munnari.oz 		  and I don't have an employer until February!
University of Melbourne