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From: "wilson at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/13877] [3.3 regression] miscompilation with -O -funroll-loops on powerpc Date: Wed, 10 Mar 2004 23:43:00 -0000 [thread overview] Message-ID: <20040310234313.11436.qmail@sources.redhat.com> (raw) In-Reply-To: <20040127073741.13877.debian-gcc@lists.debian.org> ------- Additional Comments From wilson at gcc dot gnu dot org 2004-03-10 23:43 ------- I can reproduce the problem with gcc-3_3-branch on powerpc-darwin. I can also reproduce the problem with current sources using -O -fold-unroll-loops. The problem is a bad interaction between the old loop unroller and doloop. The loop in question executes 6 times. The initial value of the iterator is 0. The final value is pseudo-reg 118 which holds the value 6 at run time. The old loop unrolling preconditions the loop and then unrolls it four times. Now, at the LOOP_BEG note, the initial value is now 2 at runtime, or (mod (reg 118) (const_int 4)). The loop_info->initial_value field is not changed. Then doloop_optimize runs, and tries to do some of the same calculations as unroll.c. It decides that the loop will run (reg 118) times (i.e. 6 times), because that is final_value minus the initial_value. It sees that the loop increments the iterator by 4 each loop iteration, and thus the loop must execute ceil (r118 / 4) * 4, which at run time means 8. And now we are hosed, as doloop proceeds to make incorrect transformations based on the 8 number. One possible way to fix this is to clear loop_info->initial_value after preconditioning the loop, to indicate that the initial value is now unknown. This disables the doloop optimization, avoiding the bug. Putting the (mod...) expression here seems less useful, as that might confuse other loop optimizers. An even better solution would be to pass enough info from unroll_loops to do_loop_optimize so that the optimization can still occur. I haven't looked at how to do this. This problem does not occur with the new loop unroller because it is a separate pass from loop.c, and does not use the loop_info structure. -- What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |NEW Ever Confirmed| |1 Last reconfirmed|0000-00-00 00:00:00 |2004-03-10 23:43:10 date| | http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13877
next prev parent reply other threads:[~2004-03-10 23:43 UTC|newest] Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top 2004-01-27 7:37 [Bug target/13877] New: " debian-gcc at lists dot debian dot org 2004-01-27 7:38 ` [Bug target/13877] " debian-gcc at lists dot debian dot org 2004-01-27 7:44 ` pinskia at gcc dot gnu dot org 2004-01-27 8:25 ` pinskia at gcc dot gnu dot org 2004-02-15 12:41 ` gdr at gcc dot gnu dot org 2004-03-05 11:38 ` gdr at gcc dot gnu dot org 2004-03-05 22:08 ` doko at cs dot tu-berlin dot de 2004-03-10 23:43 ` wilson at gcc dot gnu dot org [this message] 2004-03-13 1:19 ` cvs-commit at gcc dot gnu dot org 2004-03-13 1:20 ` gdr at gcc dot gnu dot org 2004-03-13 7:04 ` debian-gcc at lists dot debian dot org
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