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From: jlg@lanl.gov (Jim Giles)
Newsgroups: comp.lang.misc
Subject: Re: Aggressive optimization
Message-ID: <4699@lanl.gov>
Date: 1 Nov 90 22:33:35 GMT
References: <PCG.90Nov1171210@odin.cs.aber.ac.uk>
Organization: Los Alamos Natl Lab, Los Alamos, N.M.
Lines: 22

From article <PCG.90Nov1171210@odin.cs.aber.ac.uk>, by pcg@cs.aber.ac.uk (Piercarlo Grandi):
> [...]
> In a way or another this implies that each language implies a virtual
> machine. The task of the code generator is to map the virtual machine
> onto the real machine as well as it can, which means faithfully
> (semantics) and efficiently (pragmatics).

Unfortunately for your argument, there are few languages which have
definitions of this kind.  The definition of a language usually
implies a _class_ of virtual machines.  The code generator is allowed
to map any of these onto the real hardware.  Any optimizer which
generates code which faithfully implements anything within the class
of acceptable behaviours is correct.  If an optimizer leaves the
class of acceptable behaviours, it is _broken_.

You are trying to limit the compiler to code transformations that
_you_ regard as straightforward.  In truth, the compiler is only
limited by the language definition.  If you don't like some of the
optimizations allowed by the language definition, then lobby for
the definition to be changed.

J. Giles