Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!wuarchive!zaphod.mps.ohio-state.edu!sdd.hp.com!decwrl!shelby!unix!garth!fouts@bozeman.bozeman.ingr.UUCP (Martin Fouts)
From: fouts@bozeman.bozeman.ingr.UUCP (Martin Fouts)
Newsgroups: comp.os.msdos.programmer
Subject: Optimization  (Was Re: TC++ bug)
Message-ID: <648@garth.UUCP>
Date: 6 Aug 90 16:11:17 GMT
References: <2557@mindlink.UUCP> <2111@cernvax.UUCP>
Sender: fouts@garth.UUCP
Organization: INTERGRAPH (APD) -- Palo Alto, CA
Lines: 34
In-reply-to: antoniop@cernvax.UUCP's message of 23 Jul 90 11:15:31 GMT


Having spent some time debugging optimizers, I would like to point out
that changing the optimization level changes the semantics of the
language being accepted by the compiler and therefor the code
generated.

A good compiler will document the new assumptions made about code so
that a compiler user will be able to determine if they are appropriate
for the code being optimized, but all compilers generate code for
slightly different languages when the optimizer is on than when it is
off.

Consider the pre ANSI handling of 'volatile' variables.  Turning
register optimization on is telling the compiler that you do not use
volatile variables and that it can determine the liveness of automatic
variables by looking at the local source code.  This will break
applications which depend on 'volatile' variables which are set
outside of the source code.

The "Bugs" that ensue usually fall into the niches left by the
standards writer for the compiler writer to implement.  Often a
compiler will handle an implementation defined behavior differently
when the optimizer is on than when it is off.   This is not a bug, but
a feature, since the standard allows the compiler writer to do so.
--
Martin Fouts

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