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From: "rth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug optimization/12941] [tree-ssa] builtin-bitops-1.c miscompilation Date: Fri, 23 Jan 2004 02:19:00 -0000 [thread overview] Message-ID: <20040123021940.28391.qmail@sources.redhat.com> (raw) In-Reply-To: <20031107144745.12941.falk@debian.org> ------- Additional Comments From rth at gcc dot gnu dot org 2004-01-23 02:19 ------- This is not loop, but combine. Its line of reasoning goes 1) (2 & (1 << (63 - i))) != 0) 2) = (((2 >> (63 - i)) & 1) != 0) 3) = (((2 >> ~i) & 1) != 0) # From SHIFT_COUNT_TRUNCATED 4) = ~i == 1 # Since only 2 >> 1 & 1 != 0 5) = i == -2 Of course, I is never negative; the correct result is I == 62. Clearly the reasoning in step 4 is flawed if SHIFT_COUNT_TRUNCATED is set. I've not yet located the exact place in combine that makes this leap... -- What |Removed |Added ---------------------------------------------------------------------------- AssignedTo|unassigned at gcc dot gnu |rth at gcc dot gnu dot org |dot org | Status|NEW |ASSIGNED Summary|[tree-ssa] loop |[tree-ssa] builtin-bitops- |miscompilation |1.c miscompilation http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12941
next prev parent reply other threads:[~2004-01-23 2:19 UTC|newest] Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top 2003-11-07 14:47 [Bug optimization/12941] New: [tree-ssa] loop miscompilation falk at debian dot org 2003-11-07 15:49 ` [Bug optimization/12941] " pinskia at gcc dot gnu dot org 2003-11-07 17:54 ` falk at debian dot org 2003-11-07 17:58 ` pinskia at gcc dot gnu dot org 2003-11-18 5:39 ` pinskia at gcc dot gnu dot org 2003-11-18 9:23 ` falk dot hueffner at student dot uni-tuebingen dot de 2003-11-18 9:35 ` pinskia at gcc dot gnu dot org 2003-12-01 4:38 ` pinskia at gcc dot gnu dot org 2003-12-01 14:22 ` falk at debian dot org 2004-01-23 2:19 ` rth at gcc dot gnu dot org [this message] 2004-01-23 20:27 ` [Bug optimization/12941] [tree-ssa] builtin-bitops-1.c miscompilation cvs-commit at gcc dot gnu dot org 2004-01-23 22:38 ` rth at gcc dot gnu dot org 2004-01-27 13:05 ` cvs-commit at gcc dot gnu dot org 2004-01-30 19:57 ` jbuck at gcc dot gnu dot org 2004-01-30 20:03 ` rth at redhat dot com 2004-01-30 23:25 ` jbuck at gcc dot gnu dot org
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