Path: utzoo!attcan!uunet!cs.utexas.edu!news-server.csri.toronto.edu!turing.toronto.edu!west
From: west@turing.toronto.edu (Tom West)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Microsoft C 6.0 BAD code generation
Message-ID: <1990Jun13.162147.11744@jarvis.csri.toronto.edu>
Date: 13 Jun 90 20:21:47 GMT
References: <j7c6203r7@tmsoft.uucp>
Organization: CSRI, University of Toronto
Lines: 56

In article <j7c6203r7@tmsoft.uucp> mshiels@tmsoft.UUCP (Michael A. Shiels) writes:
>One of my co workers was trying to start using Microsoft C 6.0 but found out
>that it's just like Microsfot C 5.1.  You have to disable most of the
>optimizations so it will generate correct code.  Here's his very simple sample
>code which doesn't have anything to do with loops but is broken when you 
>specify loop optimizations.

  Unfortunately, I can confirm this.  I have a bug into MS at the moment where
MSC 6.0 will produce code that eventually crashes the machine.  The specifics 
in this case are that if you eliminate the -Oe, it works, or if you 
eliminate the register declaration(!!) in the register long y line, it 
works!  Strange behaviour for an option that supposedly ignores the
register keyword when turned on.  (Specifically, MSC 6.0 doesn't generate
enough FP stack pops and eventually overflows the FP stack).  This bug is
into MS along with another two, but this is the only one that generates bad
code.  This is scary!.  My project is 300k long so I *can't* exhaustively test
it for code generation bugs!  What am I to do?   Unfortunately, I trusted new
technology too much and put in features found only in 6.0.  Now it looks like 
I'll pay for it.
  On the other hand, the poor tech support contact at MS is trying to do his 
best.  It's just there isn't a hell of a lot he can do on circumstances like
this.

---------------------------
cl -Oe main.c
---------------------------
#include <stdio.h>

void TL1 ();

_cdecl main()
{
    for (;;)
    {
	long right = 2;
	double product = 1.0;
	if (right < 0)
	{
	    long x;
	    for (x = right; x <= -1; x++) product /= (double) 3.0;
	}
	else
	{
	    register long y;
	    for (y = right; y >= 1; y--) product *= (double) 3.0;
	};
	TL1 (product);
	printf("%lf\n", product);
    }
}

void TL1 (value)
double  value;
{
}
---------------------------