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From: "glaw@undo-software.com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/54365] New: ARM optimization bug when pointer arithmetic wraps Date: Fri, 24 Aug 2012 11:57:00 -0000 [thread overview] Message-ID: <bug-54365-4@http.gcc.gnu.org/bugzilla/> (raw) 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: unassigned@gcc.gnu.org ReportedBy: glaw@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!
next reply other threads:[~2012-08-24 11:57 UTC|newest] Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top 2012-08-24 11:57 glaw@undo-software.com [this message] 2012-08-24 18:24 ` [Bug rtl-optimization/54365] " pinskia at gcc dot gnu.org 2012-08-24 18:38 ` glaw@undo-software.com 2012-08-24 18:43 ` jakub at gcc dot gnu.org 2012-08-27 7:39 ` fweimer at redhat dot com 2012-08-30 13:24 ` glaw@undo-software.com 2012-08-30 13:33 ` fweimer at redhat dot com 2012-08-30 13:45 ` jakub at gcc dot gnu.org 2012-08-30 13:56 ` fweimer at redhat dot com
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