Date: Thu, 10 Aug 1995 17:49:34 -0600 From: Nate Williams <nate@trout.sri.MT.net> To: hackers@FreeBSD.org Subject: Serious GCC optimization bug on x86 Message-ID: <199508102349.RAA02602@trout.sri.MT.net>
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From: davis@space.mit.edu (John E. Davis)
Newsgroups: comp.os.linux.development.system,comp.os.linux.development.apps
Subject: Warning to those who use gcc
Date: 8 Aug 1995 18:35:43 GMT
Organization: Center for Space Research
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Hi,
If you use gcc to compile the kernel or any other program under Linux
with gcc, do not use -O2 optimization. If you do, then add the
-fno-strength-reduce flags. The reason is that gcc -O2 generates the WRONG
code for very simple things. I am sure you do not want your kernel to have
bugs caused by a buggy compiler. For all I know, some problems that people
report may be a result of this bug. The bug that I am referring to is in
gcc 2.6.3 as well as in 2.7.0. It is probably also in 2.5.4 but I do not
know for sure. Can anyone verify this?
Here is a simple program that illustrates the bug. Compile it with and
without -O2 and compare. Then compile it with -O2 -fno-strength-reduce.
The bug has been reported to the gcc folks.
-------------------------------
#include <stdio.h>
static int Order_List[100] =
{
0, 1, 2, 3, 4, 5, 6, 7, 8, 9
};
static unsigned int Num_Orders = 10;
static void print_it (void)
{
int i;
for (i = 0; i < Num_Orders; i++)
{
fprintf (stdout, "%d %d\n", i, Order_List[i]);
}
}
int main (void)
{
unsigned int i;
for (i = 0; i < Num_Orders; i++)
{
Order_List[i] = (int) i - 10;
}
print_it ();
return 0;
}
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