[Bug middle-end/103439] New: genemit emits dead code

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103439

            Bug ID: 103439
           Summary: genemit emits dead code
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

Patterns like

(define_expand "nearbyinthf2"
  [(match_operand:HF 0 "register_operand")
   (match_operand:HF 1 "nonimmediate_operand")]
  "TARGET_AVX512FP16"
{
  emit_insn (gen_sse4_1_roundhf2 (operands[0],
                                  operands[1],
                                  GEN_INT (ROUND_MXCSR | ROUND_NO_EXC)));
  DONE;
})

generate

/* /home/rguenther/src/trunk/gcc/config/i386/i386.md:18636 */
rtx
gen_nearbyinthf2 (rtx operand0,
        rtx operand1)
{
  rtx_insn *_val = 0;
  start_sequence ();
  {
    rtx operands[2];
    operands[0] = operand0;
    operands[1] = operand1;
#define FAIL _Pragma ("GCC error \"nearbyinthf2 cannot FAIL\"") (void)0
#define DONE return (_val = get_insns (), end_sequence (), _val)
#line 18640 "/home/rguenther/src/trunk/gcc/config/i386/i386.md"
{
  emit_insn (gen_sse4_1_roundhf2 (operands[0],
                                  operands[1],
                                  GEN_INT (ROUND_MXCSR | ROUND_NO_EXC)));
  DONE;
}
#undef DONE
#undef FAIL
    operand0 = operands[0];
    (void) operand0;
    operand1 = operands[1];
    (void) operand1;
  }
  emit (operand0, true);
  emit (operand1, false);
  _val = get_insns ();
  end_sequence ();
  return _val;
}

where the code after DONE is not reachable.  With a proposed
-Wunreachable-code-return this diagnoses most expander patterns.

I'm not sure if there are valid cases where we have a mix of a direct
RTL pattern and manual expansion, so where the { } part falls thru.