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From: "mpolacek at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/54838] [4.8 Regression] ICE: in merge_latch_edges, at cfgloop.c:678 with -ftracer
Date: Sun, 04 Nov 2012 12:49:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54838

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> 2012-11-04 12:49:27 UTC ---
I think the problem is that we somehow arrive at this:
  loop_1 (header = 2, multiple latches, niter = )
  {
    bb_2 (preds = {bb_0 }, succs = {bb_4 bb_3 })
    {
(note 5 0 2 2 [bb 2] NOTE_INSN_BASIC_BLOCK)
(insn 2 5 3 2 (set (reg/v/f:DI 60 [ b ])
        (reg:DI 5 di [ b ])) ./pr54838.c:5 63 {*movdi_internal_rex64}
     (nil))
(insn 3 2 4 2 (set (reg/v/f:DI 61 [ c ])
        (reg:DI 4 si [ c ])) ./pr54838.c:5 63 {*movdi_internal_rex64}
     (nil))
(note 4 3 7 2 NOTE_INSN_FUNCTION_BEG)
(insn 7 4 14 2 (set (reg:SI 59 [ D.1735 ])
        (mem:SI (reg/v/f:DI 61 [ c ]) [2 *c_3(D)+0 S4 A32])) 65
{*movsi_internal}
     (nil))
(insn 14 7 15 2 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg:SI 59 [ D.1735 ])
            (const_int 1 [0x1]))) ./pr54838.c:7 7 {*cmpsi_1}
     (nil))
(jump_insn 15 14 38 2 (set (pc)
        (if_then_else (eq (reg:CCZ 17 flags)
                (const_int 0 [0]))
            (label_ref:DI 20)
            (pc))) ./pr54838.c:7 595 {*jcc_1}
     (expr_list:REG_BR_PROB (const_int 3333 [0xd05])
        (nil))
 -> 20)

    }
  }
, i.e. we have a loop which contains only the header.  The rest of BBs are
there, but in loop_0.  When we call disambiguate_loops_with_multiple_latches,
loop->latch is NULL on that loop, so we end up calling merge_latch_edges, but
that ICEs because VEC_length (edge, latches) is of course 0.