[Bug tree-optimization/49498] [4.7/4.8 Regression]: gcc.dg/uninit-pred-8_b.c bogus warning line 20

["amker.cheng at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49498

bin.cheng <amker.cheng at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amker.cheng at gmail dot
                   |                            |com

--- Comment #17 from bin.cheng <amker.cheng at gmail dot com> 2012-11-20 07:08:18 UTC ---
Hi,
I spent some time analyzing this bug and I think I understand the problem now.
For below dump file from trunk/cris-elf when compiling the attached k.c:
;; Function foo (foo, funcdef_no=0, decl_uid=1323, cgraph_uid=0)

;; 1 loops found
;;
;; Loop 0
;;  header 0, latch 1
;;  depth 0, outer -1
;;  nodes: 0 1 2 3 4 5 6 7 8 9 10 11
;; 2 succs { 10 3 }
;; 3 succs { 11 4 }
;; 4 succs { 11 }
;; 5 succs { 6 7 }
;; 6 succs { 9 }
;; 7 succs { 6 8 }
;; 8 succs { 9 }
;; 9 succs { 1 }
;; 10 succs { 5 6 }
;; 11 succs { 5 8 }
foo (int n, int l, int m, int r)
{
  int v;
  int g.1;
  int g.0;

  <bb 2>:
  if (n_4(D) <= 9)
    goto <bb 10>;
  else
    goto <bb 3>;

  <bb 3>:
  if (m_5(D) > 100)
    goto <bb 11>;
  else
    goto <bb 4>;

  <bb 4>:
  goto <bb 11>;

  <bb 5>:
  # v_14 = PHI <v_13(11), r_7(D)(10)>
  g.0_9 = g;
  g.1_10 = g.0_9 + 1;
  g = g.1_10;
  if (n_4(D) <= 9)
    goto <bb 6>;
  else
    goto <bb 7>;

  <bb 6>:
  # v_17 = PHI <v_14(5), v_14(7), r_7(D)(10)>
  blah (v_17);
  goto <bb 9>;

  <bb 7>:
  if (m_5(D) > 100)
    goto <bb 6>;
  else
    goto <bb 8>;

  <bb 8>:

  <bb 9>:
  return 0;

  <bb 10>:
  if (m_5(D) != 0)
    goto <bb 5>;
  else
    goto <bb 6>;

  <bb 11>:
  # v_13 = PHI <r_7(D)(3), v_6(D)(4)>
  if (m_5(D) != 0)
    goto <bb 5>;
  else
    goto <bb 8>;

}

There are two flaws in tree-ssa-uninit.c revealing this bug.
1. GCC try to find def_chains from cd_root(which is the closest dominating bb 
for phi_bb) to phi_bb, but only find use_predicates from phi_bb to use_bb. In
general case with canonical CFG, this is fine, but in non-canonical CFG, it's
possible to have ancestor basic block of phi_bb in def_chains which have branch
that never reach to phi_bb, like basic block 10 reported in this PR. In this
scenario the corresponding condition should not be counted in
def_chains(edge<10, 5> in this case).
There are two methods to fix this:
   a) find use predicates from dom(phi_bb), rather than phi_bb in non-canonical
CFGs.
   b) prune branch conditions that are irrelevant to this use/def in
def_chains.
Method a is simpler, but the problem is it results in more dep_chains which
might exceeds the limit MAX_NUM_CHAINS. As for method b), I haven't got any
clue to implement it.

2. When calling is_use_properly_guarded in find_uninit_use, GCC finds
predicates from source basic block if the use_stmt is a phi node. This results
in missing condition at the end of each def_chain. Different from the first
issue, this can be easily fixed.