Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!ccut!kogwy!new1!roger
From: roger@zuken.co.jp (Roger Meunier)
Newsgroups: comp.sys.hp
Subject: Re: C Compiler Optimizer
Message-ID: <ROGER.90Aug29091647@rd11.zuken.co.jp>
Date: 29 Aug 90 14:16:47 GMT
References: <59169@bbn.BBN.COM>
Sender: news@new1.zuken.co.jp
Organization: ZUKEN Inc. Yokohama, JAPAN
Lines: 85
In-reply-to: ahill@bbn.com's message of 27 Aug 90 19:55:03 GMT

In article <59169@bbn.BBN.COM> ahill@bbn.com (Alan R. Hill) writes:

 >	  We have experienced problems of varying symptoms while using
 >the optimizer switch of the HP-UX 7.0 C compiler.  Rather than 
 >extensively analyzing these problems we have opted to compile without
 >optimizing.  Has anyone else noticed bad behavior with the optimizer?

Indeed we have.  The code given below optimizes cleanly under +O1, but
the -O optimizer does manage registers properly.  It doesn't handle
cases where side affects may be introduced by function calls.

---------------------------  Cut Here  ------------------------------

#include <stdio.h>

void	reg_and_stack();
void	doit();
void	doitagain();

main()
{
	int	parms[10];

	parms[0] = -1;
	reg_and_stack(parms);
}

void	reg_and_stack(parms)
int*	parms;
{
	int	id;
	int	args[30];
	short	cnt;

	cnt = 0;

	if (parms) {
		doit(parms,&args[0],&cnt);
		if (parms[0] & 0x0020)	id = parms[1];
	}

	args[cnt] = 1;	cnt++;
	args[cnt] = 2;	cnt++;
	args[cnt] = 3;	cnt++;

	doitagain(&args[0],&cnt);
}

void	doit(parms,args,cnt)
int*	parms;
int	args[];
short*	cnt;
{
	printf("doit: on entry, cnt = %d\n",*cnt);
	*cnt = 2;
	printf("doit: on exit, cnt = %d\n",*cnt);
}

void	doitagain(args,cnt)
int	args[];
short*	cnt;
{
	printf("doitagain: on entry, cnt = %d\n",*cnt);
}
---------------------------  Cut Here  ------------------------------

% cc -o reg_stack +O1 reg_stack.c
% reg_stack
doit: on entry, cnt = 0
doit: on exit, cnt = 2
doitagain: on entry, cnt = 5
                           ^
% cc -o reg_stack -O reg_stack.c
% reg_stack
doit: on entry, cnt = 0
doit: on exit, cnt = 2
doitagain: on entry, cnt = 3
                           ^

A quick look at the assembly listing produced by the -O optimizer shows
that while the stack version of cnt was set to 2 by doit(), the stack
version is overwritten by the register version, which still contained
the pre-doit() value of 0.
--
Roger Meunier @ Zuken, Inc.  Yokohama, Japan	(roger@zuken.co.jp)