http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54365
Bug #: 54365
Summary: ARM optimization bug when pointer arithmetic wraps
Classification: Unclassified
Product: gcc
Version: 4.6.3
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: g...@undo-software.com
Created attachment 28077
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28077
Simple program showing the bug.
Apologies if I've got the wrong component - it was a bit of a guess!
The attached program shows a fairly simple, but I think nasty, bug in GCC on
ARM. It seems to be present on at least versions 4.4.5, 4.5.1 and 4.6.3. If
compiled without optimisation, it behaves as I expect: adding 4 to a pointer
such that it wraps gives a pointer that is less than the original. With -O2,
that is not the case. I note that the problem does not occur if I use integer
types.
Transcript follows (.c file attached separately and in transcript below for
convenience):
$ cat compilerbug.c
#include <stdio.h>
int
main( void)
{
unsigned char* addr = (unsigned char*)0xfffffffe;
unsigned len = 4;
if ( addr+len < addr)
{
printf( "it wraps\n");
}
else
{
printf( "no wrap\n");
}
return 0;
}
$ gcc --version
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 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.
$ gcc compilerbug.c
$ ./a.out
it wraps
$ # As expected. Now let's try with optimisations.
$ gcc -O2 compilerbug.c
$ ./a.out
no wrap
$ # Oh dear!
