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From: kilgallen@eisner.decus.org (Larry Kilgallen)
Subject: Re: GNAT & GCC performace (bad news)
Date: 1999/12/06
Message-ID: <1999Dec6.073729.1@eisner>#1/1
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References: <B46CA6A1.2991%Harald.Schmidt@tomcat.de>
 <38473D90.68D8F47@acenet.com.au> <1999Dec3.103807.1@eisner>
 <82fgh2$mt3$1@nnrp1.deja.com>
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Organization: LJK Software
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Newsgroups: comp.lang.ada
Date: 1999-12-06T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <82fgh2$mt3$1@nnrp1.deja.com>, Robert Dewar <dewar@gnat.com> writes:

> But there is a huge difference between emitting assembly
> language, which is just a 1-1 shorthand for machine language,
> and is therefore obviously equivalent from a performance
> point of view to emitting machine language (*)
> 
> and emitting C, where there are two significant problems:
> 
> 1. It may be impossible to emit efficient code for a given
> construct in the intermediate high level language (e.g.
> exceptions, compiler checks, various other things ...)
> 
> 2. The C compiler will introduce inefficiencies of its own
> for the generated code.

The C compiler may also introduce optimizations specific to
the machine that were not provided by the earlier compiler.
If you read the propaganda for the Averstar Ada-to-C project,
an aim seems to be to leverage effort machine manufacturers
are already putting into optimizing their C compilers.  Of
course there are other optimizations that can only be done
well with the full Ada semantics.

On a theoretical basis, it is possible to do anything in
machine language one can do in a higher level language.
However I believe that optimizing for various versions of
a particular chip is higher priority for the folks pushing
that chip than they are for the folks trying to provide a
multi-platform compiler (regardless of whether it emits
assembly language or C code).

Larry Kilgallen