Re: Expanding instructions with condition codes inter-deps

[Richard Henderson <rth@redhat.com>]

On 10/21/2011 10:15 AM, Paulo J. Matos wrote:
> So I have implemented the nadd and addc as:
> 
> (define_insn "negqi2"
>   [(set (match_operand:QI 0 "register_operand" "=c")
>         (neg:QI (match_operand:QI 1 "register_operand" "0")))
>    (set (reg:CC_C RCC) (eq (match_dup 1) (const_int 0)))
>    (clobber (reg:CC RCC))]
>   ""
> {
>     operands[2] = const0_rtx;
>     return  "nadd\\t%0,%2";
> })

There are lots of parts of the compiler that don't optimize well when an
insn has more than one output.  For the normal insn, just clobber the flags;
don't include a second SET.

> (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")))
>    (use (reg:CC_C RCC))
>    (clobber (reg:CC RCC))]
>   ""
>   "addc\\t%0,%f2")

You don't need the USE, because you mention RCC inside the LTU.

> (define_insn "*addc_internal_flags"

Likewise.

> A couple of things to note:
> * negqi (which generates the nadd x, y equivalent to -x + y) has a
> set RCC in C mode followed by a clobber. The set in C mode doesn't
> show up in the _flags variant which is used only for the compare-elim
> since it doesn't really matter and it already contains a set RCC
> anyway.

Surely the NADD insn is simply a normal subtract (with reversed operands).
You shouldn't *need* to implement NEG at all, as the middle-end will let
NEG expand via MINUS.

Just so you know...

> * is this enough for GCC to understand that anything that clobbers
> RCC or specifically touches the RCC in C mode shouldn't go in between
> these two instructions?

Yes.

> Also, do I need to specify in the RCC
> clobber, exactly which flags are clobbered, or should I use a set
> instead?

No, the compiler will assume the entire register is changed, no matter
what CCmode you place there.

> * in the case of using sets, it was easy in the case of the negqi of
> findind the source of the set RCC, however, it's not so easy for the
> general case. Is unspec the answer? Is unspec the way of saying:
> "hey, I am setting RCC in Cmode here, you shouldn't bother about the
> value that I put there. Just know that RCC is going to be set."

You can often just use (compare x y) as well, assuming that the flags
are set "appropriately".  GCC doesn't assume anything about the 
contents of the intermediate CCmode object, but does assume that

  (lt (compare x y) (const_int 0))

produces the same value as

  (lt x y)

But, yes, if there's no obvious comparison, then unspec is ok.


r~