Expanding instructions with condition codes inter-deps

["Paulo J. Matos" <paulo at matos-sorge dot com>]

Hi,

To negate a double word (HImode) register, I used to take the 
instruction all the way to assembly generation and then expand into 
three assembly instructions like so:
  xor  %t0, #ffff ; invert bits in top word of op0
  nadd %b0, #0    ; negate bottom bits of op0
  addc %t0, #0    ; add carry to top bits of op0

The interesting thing about this sequence is that the carry added to the 
top bits of op0, is the carry generated by the previous instruction.

If I instead of taking the neghi rule all the way to assembly, and 
instead expand it I get:
(define_expand "neghi2"
  [(set (match_operand:HI 0 "register_operand")
        (neg:HI (match_operand:HI 1 "general_operand")))
   (clobber (reg:CC RCC))]
  ""
{
    rtx op0_low = gen_lowpart(QImode, operands[0]);
    rtx op0_high = gen_highpart(QImode, operands[0]);
    emit_move_insn(operands[0], operands[1]);
    emit_insn(gen_xorqi3(op0_high, op0_high, GEN_INT(-1)));
    emit_insn(gen_negqi2(op0_low, op0_low));
    emit_insn(gen_addc_internal(op0_high, op0_high, const0_rtx));
    DONE;
})


addc_internal looks like:
(define_insn "addc_internal"
  [(set (match_operand:QI 0 "nonimmediate_operand" "=c")
        (plus:QI
          (plus:QI
            (ltu:QI (reg:CC RCC) (const_int 0))
            (match_operand:QI 1 "nonimmediate_operand" "%0"))
          (match_operand:QI 2 "general_operand" "cwmi")))
   (clobber (reg:CC RCC))]
  ""
  "addc\\t%0,%f2")


The problem is that GCC sometimes moves nadd and addc around. It might 
swap their order or insert instructions in-between that set the carry 
flag. This ends up generating code which won't work properly.

What's the best way to instruct GCC during neghi2 expansion that the 
carry read by addc internal is the one generated by negqi2 and so no 
instructions setting carry should be inserted between them and that 
their order should not be changed? (for example, it would be ok to 
output negqi2, xorqi3 and addc_internal since xorqi3 only sets N and Z, 
not the Carry bit).

Cheers,

--
PMatos