[Bug tree-optimization/55424] New: [4.8 Regression]gcc.dg/uninit-pred-8_b.c bogus warning line 23 on ARM/Cortex-M0/-Os

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55424

             Bug #: 55424
           Summary: [4.8 Regression]gcc.dg/uninit-pred-8_b.c bogus warning
                    line 23 on ARM/Cortex-M0/-Os
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: amker.cheng@gmail.com


The test case require optimization level "-O2" and it passes on ARM/cortex-m0
with "-O2", but the failure with "-Os" does reveal potential bug in
tree-ssa-uninit.c

Test command line:
arm-none-eabi-gcc ./uninit-pred-8_b.c  -fno-diagnostics-show-caret  
-Wuninitialized -fno-tree-dominator-opts -S    -mthumb -mcpu=cortex-m0 -Os -o
uninit-pred-8_b.s
The warning info:
.../trunk-orig/gcc/gcc/testsuite/gcc.dg/uninit-pred-8_b.c: In function 'foo':
.../trunk-orig/gcc/gcc/testsuite/gcc.dg/uninit-pred-8_b.c:23:11: warning: 'v'
may be used uninitialized in this function [-Wmaybe-uninitialized]
.../trunk-orig/gcc/gcc/testsuite/gcc.dg/uninit-pred-8_b.c: In function 'foo_2':
.../trunk-orig/gcc/gcc/testsuite/gcc.dg/uninit-pred-8_b.c:42:11: warning: 'v'
may be used uninitialized in this function [-Wmaybe-uninitialized]

This failure occurs after checking in r193687. The patch prefers to generate
branches on ARM/cortex-m0.

After investigating tree dump of tree-ssa-uninit.c, I think:

tree-ssa-uninit.c computes control dependent chain for uses/def of variable and
checks whether each use is guarded by def. It has a upper bound on the number
of control dependent chains(MAX_NUM_CHAINS==8) and just retreat to false
warning if the number of chains exceeds MAX_NUM_CHAINS. In our scenario, the
number of chains exceeds MAX_NUM_CHAINS because we prefer short circuit now,
resulting in false warning information. These false warning cannot be fully
removed if the MAX_NUM_CHAINS exists, but we can improve it in following way:
There are lots of invalid control dependent chains computed in
tree-ssa-uninit.c now and should be pruned. I have already implemented a quick
fix and it works for our scenario.

I am not sure it should be fixed in this way, so please comments if you have
any opinions.

Thanks

Dump of tree-ssa-uninit.c:

;; Function foo (foo, funcdef_no=0, decl_uid=4065, cgraph_uid=0)


Use in stmt v_24 = PHI <v_25(8), v_1(9)>
is guarded by :
 (.NOT.) if (m_6(D) != 0)
Operand defs of phi v_1 = PHI <v_9(D)(6), r_7(D)(4), r_7(D)(2), r_7(D)(5)>
is guarded by :
 (.NOT.) if (n_5(D) <= 9)
(.AND.)
 (.NOT.) if (m_6(D) > 100)
(.AND.)
if (r_7(D) <= 19)
(.OR.)
if (n_5(D) <= 9)
(.OR.)
 (.NOT.) if (n_5(D) <= 9)
(.AND.)
 (.NOT.) if (m_6(D) > 100)
(.AND.)
 (.NOT.) if (r_7(D) <= 19)
(.AND.)
if (l_8(D) != 0)
foo (int n, int l, int m, int r)
{
  int v;
  int g.1;
  int g.0;

  <bb 2>:
  if (n_5(D) <= 9)
    goto <bb 7>;
  else
    goto <bb 3>;

  <bb 3>:
  if (m_6(D) > 100)
    goto <bb 18>;
  else
    goto <bb 4>;

  <bb 4>:
  if (r_7(D) <= 19)
    goto <bb 7>;
  else
    goto <bb 5>;

  <bb 5>:
  if (l_8(D) != 0)
    goto <bb 7>;
  else
    goto <bb 6>;

  <bb 6>:

  <bb 7>:
  # v_1 = PHI <v_9(D)(6), r_7(D)(4), r_7(D)(2), r_7(D)(5)>
  if (m_6(D) != 0)
    goto <bb 8>;
  else
    goto <bb 9>;

  <bb 8>:
  # v_25 = PHI <v_1(7), v_22(18)>
  g.0_11 = g;
  g.1_12 = g.0_11 + 1;
  g = g.1_12;
  goto <bb 10>;

  <bb 9>:
  bar ();

  <bb 10>:
  # v_24 = PHI <v_25(8), v_1(9)>
  if (n_5(D) <= 9)
    goto <bb 14>;
  else
    goto <bb 11>;

  <bb 11>:
  if (m_6(D) > 100)
    goto <bb 14>;
  else
    goto <bb 12>;

  <bb 12>:
  if (r_7(D) <= 19)
    goto <bb 14>;
  else
    goto <bb 16>;

  <bb 13>:
  if (m_6(D) > 100)
    goto <bb 15>;
  else
    goto <bb 16>;

  <bb 14>:
  blah (v_24);
  if (n_5(D) <= 9)
    goto <bb 15>;
  else
    goto <bb 13>;

  <bb 15>:
  blah (v_24);
  goto <bb 17>;

  <bb 16>:
  if (r_7(D) <= 9)
    goto <bb 15>;
  else
    goto <bb 17>;

  <bb 17>:
  return 0;

  <bb 18>:
  # v_22 = PHI <r_7(D)(3)>
  goto <bb 8>;

}