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From: "steven at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/30521] "if (i == n) ++i;" or "i += i == n;"? Date: Sat, 22 Nov 2008 10:05:00 -0000 [thread overview] Message-ID: <20081122100430.32282.qmail@sourceware.org> (raw) In-Reply-To: <bug-30521-315@http.gcc.gnu.org/bugzilla/> ------- Comment #6 from steven at gcc dot gnu dot org 2008-11-22 10:04 ------- Ah, now I see what Pinski meant at comment #2. Before CSE, we still have the original code for f1: unsigned int f(unsigned int i, unsigned int n) { i.20 = i + 1; if (i.20 == n) i.20 = i.20 + 1; return i.20; } After CSE (but before the first if-conversion pass) it's been transformed to: unsigned int f(unsigned int i, unsigned int n) { i.20 = i + 1; if (i.20 == n) i.20 = i + 2; return i.20; } The form of the code before CSE is caught in noce_try_addcc. The second form obviously not (because we don't know that the value of i.20 is "i + 1"). So this comes down to a pass ordering problem. Or, one could argue that CSE should not perform this transformation if (say) "i.20 = i + 1" is not made dead code by the transformation to "i.20 = i + 2". -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30521
next prev parent reply other threads:[~2008-11-22 10:05 UTC|newest] Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top 2007-01-21 0:32 [Bug c++/30521] New: " sigra at home dot se 2007-01-21 8:54 ` [Bug middle-end/30521] " pinskia at gcc dot gnu dot org 2007-01-22 0:29 ` pinskia at gcc dot gnu dot org 2008-08-27 4:40 ` bonzini at gnu dot org 2008-08-27 4:40 ` bonzini at gnu dot org 2008-11-22 9:53 ` steven at gcc dot gnu dot org 2008-11-22 10:05 ` steven at gcc dot gnu dot org [this message] 2008-11-22 10:17 ` bonzini at gnu dot org [not found] <bug-30521-4@http.gcc.gnu.org/bugzilla/> 2012-03-04 1:45 ` pinskia at gcc dot gnu.org
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