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Date: Fri, 17 May 1996 18:48:19 +0200 (MET DST)
Message-Id: <199605171648.SAA04818@plm.simplex.nl>
From: Peter Mutsaers <plm@simplex.nl>
To: nate@sri.MT.net
CC: freebsd-questions@FreeBSD.org
In-reply-to: <199605170437.WAA25870@rocky.sri.MT.net> (message from Nate
	Williams on Thu, 16 May 1996 22:37:54 -0600)
Subject: Re: make world with gcc version 2.7.2 or pgcc 2.7.2.9?
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>> On Thu, 16 May 1996 22:37:54 -0600 you wrote:

  >> Did anyone try to rebuild everything, including the kernel, with gcc
  >> version 2.7.2 or pgcc, and with -O2 or higher?

  NW> I believe Kaleb Keithley rebuilt X with one of them and it didn't work.
  NW> This implies to me it's a compiler bug, as there are quite a few known
  NW> optimizer bugs in gcc 2.7.2 for the x86, and at least one generic
  NW> optimizer bug that was posted on gnu.gcc.bug.

The generic bug was, I think, shown by this example:
#include <stdio.h>
int A[3];
unsigned int b = 3;


void printit(void)
{
int i;
for(i = 0; i < b; i++)
   fprintf(stdout, "A[%d] = %d\n", i, A[i]);
}

int main()
{
int i;
for(i = 0; i < b; i++) *(A+i) = i - 3;

printit();
return 0;
}

gcc 2.7.2 doesn't show correct results. gcc 2.7.2.9 (the current pgcc
release) has this fixed however. It also has fixed the x86 bug that
made it necessary when using -O2 or higher to add -fno-strength-reduce.

  NW> 'probably bogus' and faster code.  Since there are known bugs,
  NW> given a system the size of FreeBSD the possibility of the bug
  NW> being tickled *somewhere* is high.  And, given that the speedups
  NW> for stock gcc aren't stupendous (unlike pgcc which can be quite
  NW> high, but the code generation is suspect in many cases) it's not
  NW> worth it.

Hmm. Do the pentium patches introduce new bugs?

-- 
______________________________________________________________________
Peter Mutsaers       |  Abcoude (Utrecht), |     "Quod licet bovis,
plm@simplex.nl       |  the Netherlands    |      non licet Jovi."