[Bug c/101090] incorrect -Wunused-value warning on remquo with constant values

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101090

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
   Last reconfirmed|                            |2021-06-16
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We warn about

(gdb) p expr
$2 = <compound_expr 0x7ffff669cbb8>
(gdb) p debug_generic_expr (expr)
quo = 0;, 1.0e+0

which is folded from the call via do_mpfr_remquo:

              /* Dereference the quo pointer argument.  */
              arg_quo = build_fold_indirect_ref (arg_quo);
              /* Proceed iff a valid pointer type was passed in.  */
              if (TYPE_MAIN_VARIANT (TREE_TYPE (arg_quo)) == integer_type_node)
                {
                  /* Set the value. */
                  tree result_quo
                    = fold_build2 (MODIFY_EXPR, TREE_TYPE (arg_quo), arg_quo,
                                   build_int_cst (TREE_TYPE (arg_quo),
                                                  integer_quo));
                  TREE_SIDE_EFFECTS (result_quo) = 1;
                  /* Combine the quo assignment with the rem.  */
                  result = non_lvalue (fold_build2 (COMPOUND_EXPR, type,
                                                    result_quo, result_rem));

but we should probably set TREE_NO_WARNING on the COMPOUND_EXPR.