[Bug target/65689] [AArch64] S constraint fails for inline asm at -O0

["alalaw01 at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65689

--- Comment #1 from alalaw01 at gcc dot gnu.org ---
Problem stems from parse_input_constraint (in stmt.c):

    if (reg_class_for_constraint (cn) != NO_REGS
        || insn_extra_address_constraint (cn))
      *allows_reg = true;
    else if (insn_extra_memory_constraint (cn))
      *allows_mem = true;
    else
      {
        /* Otherwise we can't assume anything about the nature of
           the constraint except that it isn't purely registers.
           Treat it like "g" and hope for the best.  */
        *allows_reg = true;
        *allows_mem = true;
      }

which causes expand_asm_operands to use (reg/f:DI ...), which fails the
definition of the S constraint. If instead parse_input_constraint set both
allows_reg and allows_mem to false (as it does for e.g. an "i" constraint, via
a special-case), expand_asm_operands would follow the register to its
definition:

(const:DI (plus:DI (symbol_ref:DI ("test") [flags 0x3] <function_decl
0x7fb7c60300 test>)
        (const_int 4 [0x4])))

(as also happens with -O), which satisfies the S constraint.

One solution could be to generalize the special case in parse_input_constraint.