* Loop condition question. (GCC bug?)
@ 2011-03-10 2:05 Tetsuo Handa
0 siblings, 0 replies; only message in thread
From: Tetsuo Handa @ 2011-03-10 2:05 UTC (permalink / raw)
To: linux-kernel
I was trying to figure out what happens to "for (i = 0; i < n i++)" loop when
"i is int" and "sizeof(i) == 4" and "n is unsigned long" and "sizeof(n) == 8"
and "n is larger than INT_MAX" (in order to determine whether such comparison
can cause "int i" to become negative value).
----- Start of source code -----
#include <linux/module.h>
#include <linux/sched.h>
static int __init test_init(void)
{
int i;
unsigned long n = 2147483649UL;
for (i = 2147483646; i < n; i++) {
printk(KERN_INFO "*** i=%d\n", i);
if (signal_pending(current))
break;
}
return -EINVAL;
}
module_init(test_init);
MODULE_LICENSE("GPL");
----- End of source code -----
# gcc --version
gcc (GCC) 4.5.1 20100924 (Red Hat 4.5.1-4)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
# uname -a
Linux localhost.localdomain 2.6.35.11-83.fc14.x86_64 #1 SMP Mon Feb 7 07:06:44 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux
I got a strange behavior around the boundary.
Above program prints
*** i=2147483647
*** i=2147483647
*** i=2147483647
*** i=2147483647
line forever.
If the loop is "for (i = 2147483645; i < n; i++)" instead of
"for (i = 2147483646; i < n; i++)", it prints only
*** i=2147483645
*** i=2147483646
*** i=2147483647
lines.
If I use "volatile int i;" instead of "int i;", the
"for (i = 2147483646; i < n; i++)" loop prints only
*** i=2147483646
*** i=2147483647
lines.
Is this a compiler bug?
Also, is the "for (i = 0; i < n; i++)" loop guaranteed to stop at
i == 2147483647 if n is larger than 2147483648?
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2011-03-10 2:05 Loop condition question. (GCC bug?) Tetsuo Handa
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