[Bug tree-optimization/82135] Missed constant propagation through possible unsigned wraparound, with std::align() variable pointer, constant everything else.

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82135

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
This PR is a bit messy, please minimize your examples...
Looking at the dse2 dump (before reassoc messes things up):

  __intptr_2 = (const long unsigned int) voidp_9(D);
  _3 = __intptr_2 + 63;
  __aligned_4 = _3 & 18446744073709551552;
  __diff_5 = __aligned_4 - __intptr_2;
  _6 = __diff_5 + 64;
  if (_6 > 1024)

IIUC, essentially, you would like gcc to realize that __diff_5 is in [0,63], so
the condition is always false.

If aligned was not reused, we could simplify ((x+63)&-64)-x to 63&-x, but we
don't want to do it in general. Maybe we could add a very special case in VRP
(or CCP for nonzero bits)...
(we could also add if(__diff>__align)__builtin__unreachable() in <memory> but
that's getting really specific)