[Bug tree-optimization/109791] -Wstringop-overflow warning with -O3 and _GLIBCXX_USE_CXX11_ABI=0

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109791

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
So the first pass that makes things difficult is reassoc which transforms

  _51 = (unsigned long) &MEM <char[3]> [(void *)&str + 2B];
  _4 = (unsigned long) __i_44;
  _65 = _51 - _4;
  _48 = _65 + 18446744073709551615;
  _46 = _48 > 13;
  if (_46 != 0)

to

  _51 = (unsigned long) &MEM <char[3]> [(void *)&str + 2B];
  _4 = (unsigned long) __i_44;
  _12 = -_4;
  _119 = _51 + 18446744073709551615;
  _48 = _119 - _4;
  _46 = _48 > 13;
  if (_46 != 0)

(also leaving garbage around).  That's probably because of the PHI biasing
and __i_44 being defined by a PHI.  niter analysis produces

    # of iterations (((unsigned long) &MEM <char[3]> [(void *)&str + 2B] -
(unsigned long) __i_44) + 18446744073709551615) / 2, bounded by
9223372036854775807

but doesn't expand __i_44 (not sure if we'd fold the thing then).  The
gimplifier folds stmts w/o following SSA edges when we eventually
re-gimplify those expressions for insertion.  If we expand offsetting of
invariant bases we get instead

# of iterations ((unsigned long) &MEM <char[3]> [(void *)&str + 2B] - (unsigned
long) (&str + (_69 + 1))) / 2, bounded by 9223372036854775807

but that's still not simplified.  I think ptr_difference_const should
handle this - of course the difference here isn't const...

/* Try folding difference of addresses.  */ 
(simplify
 (minus (convert ADDR_EXPR@0) (convert @1))
 (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
  (with { poly_int64 diff; } 
   (if (ptr_difference_const (@0, @1, &diff))
    { build_int_cst_type (type, diff); }))))
(simplify 
 (minus (convert @0) (convert ADDR_EXPR@1))
 (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
  (with { poly_int64 diff; }
   (if (ptr_difference_const (@0, @1, &diff))
    { build_int_cst_type (type, diff); })))) 

it works fine when adding

(simplify
 (minus (convert ADDR_EXPR@0) (convert (pointer_plus @1 @2)))
 (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
  (with { poly_int64 diff; }
   (if (ptr_difference_const (@0, @1, &diff))
    (minus { build_int_cst_type (type, diff); } (convert @2))))))

then we get

    # of iterations (1 - (unsigned long) _69) / 2, bounded by
9223372036854775807

and the diagnostic is gone.