[Bug middle-end/54838] [4.8 Regression] ICE: in merge_latch_edges, at cfgloop.c:678 with -ftracer

["mpolacek at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54838

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> 2012-11-24 11:53:19 UTC ---
So, in .cse1 we have:

            ENTRY    
             |   
             |   
             2   
             |   
             |   
   +-------- 4 ----------+
   |        / \          |   
   |       /   \         |   
   |      6     5        |   
   |     /\     |\       |   
   |    /  \    | \      |   
   |   7    3   |  8     |   
   |   |    |   |  /\    /   
   +---|----|   | /  \  /
       |      --10    9/  
       |    -/  
      EXIT-/

(3->4 and 9->4 are back edges).  Now, in bypass_block, when we're trying to
bypass BB 4, we iterate over BB 4's incoming edges.  We skip certain edges
(e.g. complex), then we're iterating over reg_use_table (registers used in
insn).  Here we call
set = find_bypass_set (regno, e->src->index);
If set == NULL, we skip to another iteration.  But in this case the set is not
NULL, and we end up with this:
Redirecting fallthru edge 3->4 to 6
JUMP-BYPASS: Proved reg 59 in jump_insn 15 equals constant (const_int 1 [0x1])
Bypass edge from 3->4 to 6
Redirecting fallthru edge 9->4 to 5
JUMP-BYPASS: Proved reg 59 in jump_insn 15 equals constant (const_int 3 [0x3])
Bypass edge from 9->4 to 5
but how can be two different constants in one reg?  The hash table is:
SET hash table (11 buckets, 3 entries)
Index 0 (hash value 4)
  (reg:SI 59 [ D.1735 ]) := (const_int 1 [0x1])
Index 1 (hash value 5)
  (reg/v/f:DI 60 [ b ]) := (const_int 0 [0])
Index 2 (hash value 4)
  (reg:SI 59 [ D.1735 ]) := (const_int 3 [0x3])

redirect_edge_and_branch_force then redirect edges and BB 4 is gone.

I'd say we cannot redirect edges of BBs which have 2 and more incoming back
edges if in the hash table there are more entries with the same hash values,
but the SRC rtx's differ.  I'll post something to ML.