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From: db@lfcs.ed.ac.uk (Dave Berry)
Newsgroups: comp.lang.misc
Subject: Query re optimising compilers
Message-ID: <561@etive.ed.ac.uk>
Date: 22 Jul 88 18:54:38 GMT
Sender: news@etive.ed.ac.uk
Reply-To: db@lfcs.ed.ac.uk (Dave Berry)
Organization: Laboratory for the Foundations of Computer Science, Edinburgh U
Lines: 39

Background:

Several languages leave the order of evaluation of the arguments of
operators undefined.  This allows compilers to optimise expressions
containing such operators.

Question:

Does anyone know of any compilers which produce code that decides the order
of evaluation at run-time?   My guess is that most of these optimisations
are made at compile-time.

Would it be sensible to optimise the following code:

a := function ();
b := (complicated expression) * a;

to the equivalent of:

a:= function ();
if (a == 0)
  b := 0;
else
  (complicated expression) * a;


Assuming:
  the result returned by "function" is a very small integer (including 0),
	and isn't known at compile-time
  "complicated_expression" has no side-effects and would take much longer
	to evaluate than a comparison with 0.
  the time taken to compile a program isn't considered.


I'd like to be able to reference such a compiler.  However, I expect that
cases like this occur too infrequently to be worth bothering with.

 Dave Berry.		  "May you have interesting weather"
 db@lfcs.ed.ac.uk	   	   -- Ancient British curse.