Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!cbatt!ihnp4!cuae2!ltuxa!we53!sw013b!dj3b1!killer!jfh
From: jfh@killer.UUCP
Newsgroups: comp.arch,comp.lang.c
Subject: Re: Optimization vs. the programmer
Message-ID: <761@killer.UUCP>
Date: Thu, 9-Apr-87 16:03:09 EST
Article-I.D.: killer.761
Posted: Thu Apr  9 16:03:09 1987
Date-Received: Sun, 12-Apr-87 01:25:08 EST
References: <479@danews.ATT.COM> <16294@sun.uucp>
Organization: The Unix(tm) Connection, Dallas, Texas
Lines: 35
Xref: utgpu comp.arch:831 comp.lang.c:1558
Summary: But have you seen what the optimizers do?


The request seems fairly obvious to me.  One writer wrote about how
his line
	n = n;
was necasary for the hardware to work.  I have seen compilers drop that
one on the floor.  There was also some discussion about adding a new
storage class to C named 'volatile' or some so that references of this
seemingly weird type aren't ignored.  The device driver code for some
hardware requires you to read from a silo more than one time to get the
desired peice of information.  If I write

	status = *silo;
	status = *silo;

on a real *smart* compiler, the second assignment may never happen.
I might get real clever and write

	status = *silo;
	status = 0;
	status = *silo;

only to find the first two lines dropped.

If I am stupid enough to write

	n = n;

The compiler should be smart enough to code the turkey.  Compilers
are for compiling, program verifiers are for 'picking the peices of
fluff' from your programs.

- John.

Disclaimer:
	I hold you entirely responsible for what I write.