http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48580
--- Comment #2 from Zack Weinberg <zackw at panix dot com> 2011-04-12 20:40:41 UTC --- (In reply to comment #1) > > Two signed integers given that they are known to be positive, anyway. > This may return unexpected results if either or both arguments are > negative or zero. ... > (If the function gets called with one constant operand, you can make it > inline and use __builtin_constant_p to replace a divide with a range check > on the other operand. That's only useful for some cases of overflow > checks, of course.) In the code that this is cut down from, both arguments are known to be strictly positive, but neither is constant. (They're only signed for historical reasons, I think, but it would be a huge amount of work to change that.) > I sort of think GCC should have built-in functions exposing C and C++ > interfaces for: each basic arithmetic operation, defined to wrap; each > basic arithmetic operation, defined to saturate; each basic arithmetic > operation, defined to have undefined overflow; each basic arithmetic > operation, with a separate overflow flag being set; each basic arithmetic > operation, defined to trap on overflow. All of these for both signed and > unsigned and for any desired number of bits (up to the maximum number > supported for arithmetic, so generally 1-64 bits on 32-bit configurations > and 1-128 bits on 64-bit configurations); except for the defined-to-trap > case, all would still have undefined behavior on division by 0. You could > then have optimizations mapping generic C idioms to such built-in > operations where the target supports efficient code for the operations. > But this rather relies on the no-undefined-overflow work being finished > first so that some of the required operations actually exist inside GCC, > before they can easily be exposed to the user. So you see this as more of a tree-level than an RTL-level missed optimization, then? Your plan sounds fine to me, although I might look for a less ambitious but more likely to get done soon approach, personally.
