Semantics of PARALLEL that sets and uses CC0

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

Hello,

Forgive me if I overlooked it, but I can't find in the manuals what
the semantics would be of the following define_expand, from avr.md:

(define_expand "cbranchsi4"
  [(parallel [(set (cc0)
                   (compare (match_operand:SI 1 "register_operand" "")
                            (match_operand:SI 2 "nonmemory_operand" "")))
              (clobber (match_scratch:QI 4 ""))])
   (set (pc)
        (if_then_else
              (match_operator 0 "ordered_comparison_operator" [(cc0)
                                                               (const_int 0)])
              (label_ref (match_operand 3 "" ""))
              (pc)))]
 "")

The expander performs a SET of CC0 in the first pattern inside the
PARALLEL, and there is a USE of CC0 in the second pattern of the
PARALLEL.

The manual says this about PARALLELs:

//QUOTE//
``In parallel'' means that first all the values used in the individual
side-effects are computed, and second all the actual side-effects are
performed.  For example,

@smallexample
(parallel [(set (reg:SI 1) (mem:SI (reg:SI 1)))
           (set (mem:SI (reg:SI 1)) (reg:SI 1))])
@end smallexample

@noindent
says unambiguously that the values of hard register 1 and the memory
location addressed by it are interchanged.  In both places where
@code{(reg:SI 1)} appears as a memory address it refers to the value
in register 1 @emph{before} the execution of the insn.
//QUOTE//

Applied to the AVR expander, this seems to indicate that the USE of
CC0 refers to the value of CC0 *before* the SET of CC0 in the first
pattern of the PARALLEL. But I think the intent of the expander is to
use the result of the compare. What am I missing?

This was coded by Paolo in r147425:

147425    bonzini (define_expand "cbranchsi4"
147425    bonzini   [(parallel [(set (cc0)
147425    bonzini                  (compare (match_operand:SI 1
"register_operand" "")
147425    bonzini                           (match_operand:SI 2
"nonmemory_operand" "")))
147425    bonzini             (clobber (match_scratch:QI 4 ""))])
147425    bonzini    (set (pc)
147425    bonzini         (if_then_else
147425    bonzini               (match_operator 0
"ordered_comparison_operator" [(cc0)
147425    bonzini
          (const_int 0)])
147425    bonzini               (label_ref (match_operand 3 "" ""))
147425    bonzini

Paolo, this wouldn't be my first stupid question about RTL, but I
don't understand this expander :-)
Help?

Ciao!
Steven