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Re: Semantics of PARALLEL that sets and uses CC0

From: Paolo Bonzini <bonzini at gnu dot org>
To: Ulrich Weigand <uweigand at de dot ibm dot com>
Cc: Steven Bosscher <stevenb dot gcc at gmail dot com>, GCC Mailing List <gcc at gcc dot gnu dot org>, eric dot weddington at atmel dot com
Date: Mon, 09 Aug 2010 13:53:01 -0400
Subject: Re: Semantics of PARALLEL that sets and uses CC0
References: <201008091739.o79HdVSW013616@d12av02.megacenter.de.ibm.com>

On 08/09/2010 01:39 PM, Ulrich Weigand wrote:

(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 second pattern of the PARALLEL is just a CLOBBER.

The USE of CC0 happens in a completely separate second INSN that is
emitted by this define_expand.  (Note that as opposed to define_insn,
define_expand can emit more than a single insn.)

Yeah, that's the case. What my patch did was just to combine the PARALLEL that the old cmpsi patterns produced (cc0 set), and the SET of the old bCC patterns (cc0 use).

In fact, having a cc0 set not _followed_ by a cc0 use would totally break every other RTL pass.

Paolo

References:
- Re: Semantics of PARALLEL that sets and uses CC0
  - From: Ulrich Weigand

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