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From: barrett@jhunix.HCF.JHU.EDU (Dan Barrett)
Newsgroups: comp.sys.amiga.programmer
Subject: Manx 5.0d bug (-sr optimization error)
Message-ID: <7536@jhunix.HCF.JHU.EDU>
Date: 11 Feb 91 04:56:09 GMT
Organization: The Johns Hopkins University - HCF
Lines: 53


	I have found a bug in Manx 5.0d.  The optimizer can produce
incorrect code with the -sr option ("compiler picks which local variables
get assigned to registers").  Enclosed is a minimal code example that
illustrates the bug.  Please someone verify this for me.  I have an
Amiga 1000 running 1.3.2.

This prints the right answer:

	Compile:	cc bug.c
	Link:		ln bug.o -lc
	Run:		1> bug
			Did you know that 3276476 + 8 - 1 == 3276483?

This prints the wrong answer:

	Compile:	cc -sr bug.c
	Link:		ln bug.o -lc
	Run:		1> bug
			Did you know that 3276480 + 8 - 1 == 3276488?

The problem is in the address arithmetic:

	p = buf + strlen(buf) - 1;

The "- 1" is ignored by the compiler.  It doesn't matter what the constant
is, as long as it is NOT stored in a variable.

	Code is below my signature.  I hope I saved somebody some 
frustration by hunting this down!!

                                                        Dan

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| Dan Barrett, Department of Computer Science      Johns Hopkins University |
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#include <stdio.h>
#include <string.h>

main()
{
	char *p;
	char buf[BUFSIZ];

	strcpy(buf, "Anything");		/* Any string. */
	p = buf + strlen(buf) - 1;

	printf("Did you know that %ld + %d - 1 == %ld?\n",
		buf, strlen(buf), p);
}