[Bug tree-optimization/51721] -Warray-bounds false positives and inconsistencies

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51721

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-12-31 13:42:55 UTC ---
This is a common problem with the -Warray-bounds warning, first jump threading
(during vrp1) optimizes it into just a single s == 17 check, followed by
a[11] = 0; b[11] = 0; c[17] = 0; d[11] = 0; if true and a[s] = 0; etc. if false
(well, at the end of vrp1 the constants aren't in the array refs yet, but they
are propagated there afterwards), and as no optimization figures out the weird
if (s >> 1 == 0) check (if (s < 2) would DTRT) to determine that s is not 17,
vrp2 warns about those accesses.
Perhaps for -Warray-bounds (at least if not -Warray-bounds=2 or similar) we
shouldn't warn on code that has been jump threaded, anyway, I don't think that
is solvable for 4.7 easily.

What we perhaps could do more easily for this testcase (and could improve code
too) is during VRP for:
<bb 2>:
  D.1716_2 = s_1(D) >> 1;
  if (D.1716_2 == 0)
    goto <bb 3>;
  else
    goto <bb 12>;
(or any other constant after >>, both signed and unsigned right shift, and ==
or !=) insert ASSERT_EXPRs into both bbs, saying that the SSA_NAME in rhs1 of
the
shift is in/out of second ==/!= operand << rhs2 of shift, -""- + ((1 << rhs2) -
1) range.  In this case it would be ASSERT_EXPRs that s_1(D) <= 1 at the start
of bb 3 (and if bb 12 had only one predecessor, also that s_1(D) > 1 at bb 12
start).  Richard, what do you think about that?