[Bug middle-end/49283] pointless warning with -Wstrict-overflow

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49283

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-06-06 10:38:33 UTC ---
1) Line 2026 contains the expression
  (p < buff + sizeof buff - 1)

GCC only looks at local tuple stmts when emitting the warning, so I suppose
it sees sth like

  p = &buff[...];
  p = p + 3;
  (p < buff + sizeof buff - 1)

and then optimizes the last two stmts as

  p + 3 < buff + sizeof buff - 1

moving the + 3 from the LHS and combining it with the constant offset
on the RHS.  That is only valid if p + 3 does not get outside of buff
which GCC doesn't see (it would also be invalid, but that is what the
option tries to catch - so, catch22).