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From: cik@l.cc.purdue.edu (Herman Rubin)
Newsgroups: comp.lang.c
Subject: Re: Does your compiler get this program right?
Summary: If the hardware instructions are there, it is ONE instruction
Keywords: float, increment
Message-ID: <1025@l.cc.purdue.edu>
Date: 24 Nov 88 12:13:35 GMT
References: <2298@cbnews.ATT.COM> <4082@cs.utexas.edu>
Organization: Purdue University Statistics Department
Lines: 35

In article <4082@cs.utexas.edu>, meyering@cs.utexas.edu (Jim Meyering) writes:
> In article <2298@cbnews.ATT.COM> lvc@cbnews.ATT.COM (Lawrence V. Cipriani) writes:
> 	>A friend of mine found a bug in his C compiler.  He found
> 
> It's not a bug.

		[Much deleted]

> You might be interested to know that while the Sun3/os3.2
> (or an HP, don't remember which) C compiler produced code
> that gave the "correct" results for your code, when I replaced
> that statement by the two:
> 
> 	*f += *g++;      or      *f = *f + *g++;
> 	f++;                     f++;
> 
> I found that the size of the object code was actually reduced.
> Chalk one up for readability *and* efficiency.

On a machine for which the ++ notation is hardware and on which there
is memory-memory addition, this should be done by ONE operation, not
even two, such as

	ADDF2	(rG)+,(rF)+

where rF is the register holding the address  f, and rG g.  For different
architectures, different numbers of instructions.  Is there a compiler for
the VAX which could make the optimization above?  I would expect any 
competent human programmer to do it.


-- 
Herman Rubin, Dept. of Statistics, Purdue Univ., West Lafayette IN47907
Phone: (317)494-6054
hrubin@l.cc.purdue.edu (Internet, bitnet, UUCP)