[Bug middle-end/128004] wrong code with -frounding-math: 1.0 + -1.0 is folded to +0.0 under FE_DOWNWARD

[roger at nextmovesoftware dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128004

Roger Sayle <roger at nextmovesoftware dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |roger at 
nextmovesoftware dot com
                 CC|                            |roger at nextmovesoftware dot 
com
             Status|NEW                         |ASSIGNED

--- Comment #2 from Roger Sayle <roger at nextmovesoftware dot com> ---
Created attachment 65908
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=65908&action=edit
proposed patch

Here's my proposed patch.  The same flaw affects both the tree optimizers and
the RTL optimizers (i.e. fold-const.cc and simplify-rtx.cc).