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* [Bug c/52560] New: if (r == -1) causes 'assuming signed overflow does not occur when simplifying conditional to constant' @ 2012-03-12 11:19 rjones at redhat dot com 2012-03-12 12:30 ` [Bug c/52560] " jim at meyering dot net ` (2 more replies) 0 siblings, 3 replies; 4+ messages in thread From: rjones at redhat dot com @ 2012-03-12 11:19 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52560 Bug #: 52560 Summary: if (r == -1) causes 'assuming signed overflow does not occur when simplifying conditional to constant' Classification: Unclassified Product: gcc Version: 4.7.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned@gcc.gnu.org ReportedBy: rjones@redhat.com Here is the reproducer: wget 'http://oirase.annexia.org/strict-overflow-warning.i.xz' unxz strict-overflow-warning.i.xz gcc -std=gnu99 -O2 -Wstrict-overflow -c strict-overflow-warning.i The warning is: inspect_fs_unix.c: In function ‘check_fstab’: inspect_fs_unix.c:1075:6: warning: assuming signed overflow does not occur when simplifying conditional to constant [-Wstrict-overflow] However the code doesn't look like anything should be simplified, or a warning: n_app_md_devices = map_app_md_devices (g, &app_map); if (n_app_md_devices == -1) goto error; where map_app_md_devices is a function that returns an int: static int map_app_md_devices (guestfs_h *g, Hash_table **map); and n_app_md_devices is also an int. I've tried this on several versions of gcc: gcc (GCC) 4.7.0 20120308 (Red Hat 4.7.0-0.19) Copyright (C) 2012 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. (for further build info, go to: http://koji.fedoraproject.org/koji/buildinfo?buildID=305760 and click 'build logs') Same thing with this gcc from Ubuntu 11.10: $ gcc --version gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1 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. We think this first started happening in gcc 4.5.1. ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c/52560] if (r == -1) causes 'assuming signed overflow does not occur when simplifying conditional to constant' 2012-03-12 11:19 [Bug c/52560] New: if (r == -1) causes 'assuming signed overflow does not occur when simplifying conditional to constant' rjones at redhat dot com @ 2012-03-12 12:30 ` jim at meyering dot net 2012-03-12 13:23 ` [Bug tree-optimization/52560] " rguenth at gcc dot gnu.org 2012-03-12 16:31 ` rjones at redhat dot com 2 siblings, 0 replies; 4+ messages in thread From: jim at meyering dot net @ 2012-03-12 12:30 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52560 --- Comment #1 from jim at meyering dot net 2012-03-12 12:30:20 UTC --- Created attachment 26877 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26877 50-line reproducer ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/52560] if (r == -1) causes 'assuming signed overflow does not occur when simplifying conditional to constant' 2012-03-12 11:19 [Bug c/52560] New: if (r == -1) causes 'assuming signed overflow does not occur when simplifying conditional to constant' rjones at redhat dot com 2012-03-12 12:30 ` [Bug c/52560] " jim at meyering dot net @ 2012-03-12 13:23 ` rguenth at gcc dot gnu.org 2012-03-12 16:31 ` rjones at redhat dot com 2 siblings, 0 replies; 4+ messages in thread From: rguenth at gcc dot gnu.org @ 2012-03-12 13:23 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52560 Richard Guenther <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Keywords| |missed-optimization Status|UNCONFIRMED |NEW Last reconfirmed| |2012-03-12 Component|c |tree-optimization Ever Confirmed|0 |1 --- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-03-12 13:22:32 UTC --- It certainly inlines the function, does not figures out the loop does not run and then computes n's value-range as is [0, +INF(OVF)] and thus when simplifying the return value comparison against -1 it says it assumes that n++ does not wrap. Looks ok to me, though it is all because of dead code and thus a missed VRP of some sort. ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/52560] if (r == -1) causes 'assuming signed overflow does not occur when simplifying conditional to constant' 2012-03-12 11:19 [Bug c/52560] New: if (r == -1) causes 'assuming signed overflow does not occur when simplifying conditional to constant' rjones at redhat dot com 2012-03-12 12:30 ` [Bug c/52560] " jim at meyering dot net 2012-03-12 13:23 ` [Bug tree-optimization/52560] " rguenth at gcc dot gnu.org @ 2012-03-12 16:31 ` rjones at redhat dot com 2 siblings, 0 replies; 4+ messages in thread From: rjones at redhat dot com @ 2012-03-12 16:31 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52560 --- Comment #3 from Richard W.M. Jones <rjones at redhat dot com> 2012-03-12 16:30:45 UTC --- I see that this is actually a bug in our code. I pushed the following fix to libguestfs: https://github.com/libguestfs/libguestfs/commit/d66dd2260c724bdfe57a8595aac37c8e9173cee5 ^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2012-03-12 16:31 UTC | newest] Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2012-03-12 11:19 [Bug c/52560] New: if (r == -1) causes 'assuming signed overflow does not occur when simplifying conditional to constant' rjones at redhat dot com 2012-03-12 12:30 ` [Bug c/52560] " jim at meyering dot net 2012-03-12 13:23 ` [Bug tree-optimization/52560] " rguenth at gcc dot gnu.org 2012-03-12 16:31 ` rjones at redhat dot com
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