[ Please see http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/8152 http://www.ginac.de/pipermail/cln-list/2006-December/000259.html ]
Hello Paul, all, Let's forward your comments and questions to the GCC list, I wasn't aware of this topic being so disruptive: * Paul Eggert wrote on Tue, Dec 19, 2006 at 03:06:52AM CET: > Ralf Wildenhues <[EMAIL PROTECTED]> writes: > > > the newer GCC exploits at -O2 the fact that integer overflow > > produces undefined behavior > > Wheeeoo! That optimization is going to break a _lot_ of GNU > software. (Silently. Oh my.) > > This is a major change. Where is it documented and discussed? I > don't see it listed at either > <http://gcc.gnu.org/gcc-4.2/changes.html> or > <http://gcc.gnu.org/gcc-4.3/changes.html>. > > We tried to do that sort of optimization in the 1990s (back when I > was a GCC contributor), but ran into too many problems in > real-world software. So the optimization got removed. RMS > decided it was too disruptive. > > How about if we report the problem again, and get the optimization > removed from -O2? I don't mind having the optimization available > on request for people who prefer speedy to reliable software, but > it shouldn't be turned on with a mere -O2, as it breaks too much > real-world code like mktime.c, which says: > > /* The code also assumes that signed integer overflow silently wraps > around, but this assumption can't be stated without causing a > diagnostic on some hosts. */ > > The optimization also breaks code that assumes LIA-1 (see Annex H > of the C99 standard). To conform to LIA-1, if signed integer > arithmetic does not wrap around reliably, a signal must be > generated. > > Surely the GCC guys care about LIA-1. After all, gcc has an > -ftrapv option to enable reliable signal generation on signed > overflow. But I'd rather not go the -ftrapv route, since that > will cause other problems. I'd rather have signed integer > overflow silently wrap around, as this is the traditional behavior > and a lot of real-world code assumes this. Is there an option to > the new GCC to specify this? Maybe it's also just an unintended bug I happened to observe (and take for given behavior)? Should I open a bugzilla entry? The testing was done with gcc (GCC) 4.3.0 20061215 and I haven't tested GCC in several months before, so I have no idea when this was introduced. > If not, is there any way to tell the new GCC to disable this > harmful optimization? Maybe we can have 'configure' automatically > generate the appropriate flag to do that. For example, we can > change Autoconf to default to -O1 instead of -O2. I hope we don't > have to be this drastic, though; I'd rather just disable the > optimizations that cause GCC to depart from LIA-1 wraparound > arithmetic. > > This test hangs, > > Does the test hang forever? No, the timeout works. Cheers, Ralf
