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Subject: [Bug middle-end/55283] New: EON performance regression at -O2 due to loop unrolling changes
Date: Mon, 12 Nov 2012 10:44:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55283

             Bug #: 55283
           Summary: EON performance regression at -O2 due to loop
                    unrolling changes
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: hubicka@gcc.gnu.org


EON regressed by about 20% at -O2 due to

2012-11-06  Jan Hubicka  <jh@suse.cz>

        * cfgloopanal.c (get_loop_hot_path): New function.
        * tree-ssa-lop-ivcanon.c (struct loop_size): Add CONSTANT_IV,
        NUM_NON_PURE_CALLS_ON_HOT_PATH, NUM_PURE_CALLS_ON_HOT_PATH,
        NUM_BRANCHES_ON_HOT_PATH.
        (tree_estimate_loop_size): Compute the new values.
        (try_unroll_loop_completely): Disable unrolling of loops with only
        calls or too many branches.
        (tree_unroll_loops_completely): Deal also with outer loops of hot
loops.
        * cfgloop.h (get_loop_hot_path): Declare.
        * params.def (PARAM_MAX_PEEL_BRANCHES): New parameters.
        * invoke.texi (max-peel-branches): Document.

This seems to be bad cost model on array accesses because we unroll only for
size at -O2