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From: "zackw at panix dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/48580] missed optimization: integer overflow checks Date: Tue, 12 Apr 2011 20:40:00 -0000 [thread overview] Message-ID: <bug-48580-4-OXbDfFobdK@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-48580-4@http.gcc.gnu.org/bugzilla/> 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.
next prev parent reply other threads:[~2011-04-12 20:40 UTC|newest] Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-04-12 18:36 [Bug rtl-optimization/48580] New: " zackw at panix dot com 2011-04-12 20:18 ` [Bug rtl-optimization/48580] " joseph at codesourcery dot com 2011-04-12 20:40 ` zackw at panix dot com [this message] 2011-04-12 20:52 ` joseph at codesourcery dot com 2011-04-12 21:03 ` zackw at panix dot com 2011-04-12 21:04 ` zackw at panix dot com 2011-04-12 21:10 ` joseph at codesourcery dot com 2011-04-12 21:16 ` joseph at codesourcery dot com 2011-04-13 12:11 ` [Bug middle-end/48580] " rguenth at gcc dot gnu.org 2011-04-13 12:46 ` joseph at codesourcery dot com 2011-04-13 17:44 ` svfuerst at gmail dot com 2011-06-20 9:47 ` jsm28 at gcc dot gnu.org 2011-10-05 12:44 ` jules at gcc dot gnu.org 2011-10-05 13:08 ` jules at gcc dot gnu.org 2011-10-05 15:20 ` joseph at codesourcery dot com 2013-02-02 14:03 ` Martin.vGagern at gmx dot net 2013-02-02 17:02 ` noloader at gmail dot com 2013-02-02 18:54 ` Martin.vGagern at gmx dot net 2013-02-02 21:59 ` zackw at panix dot com 2013-02-02 22:08 ` Martin.vGagern at gmx dot net 2013-05-19 13:04 ` glisse at gcc dot gnu.org 2014-08-24 7:06 ` Martin.vGagern at gmx dot net 2021-08-15 11:45 ` pinskia at gcc dot gnu.org 2021-08-15 11:49 ` pinskia at gcc dot gnu.org 2021-10-22 22:18 ` pinskia at gcc dot gnu.org 2023-08-09 22:29 ` pinskia at gcc dot gnu.org
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