Re: A plan for eliminating cc0

[Paul Schlie <schlie at comcast dot net>]

> From: Ian Lance Taylor <ian@airs.com>
>> Paul Schlie <schlie@comcast.net> writes:
>> Does GCC truley need to identify/treat condition state registers uniquely
>> from any other value produced as a result of a calculation?
> 
> No, it doesn't.  The change I am proposing removes the unique handling
> of condition state registers, and treats them like other registers.
> The unique handling of condition state registers is historical, and
> arose because of the characteristics of the initial gcc targets (e.g.,
> vax, m68k).
> 
> The idea to do this is not mine; for more background see the
> discussion of cc0 here:
>     http://gcc.gnu.org/wiki/general%20backend%20cleanup

Thank you. After reviewing that reference, a question comes to mind:

Is there any convenient way to reference the newly set register by an
instruction, as opposed to otherwise needing to redundantly re-specify
the operation producing it's value again?

Thereby enabling something like:

(insn xxx [(set (reg: A) (xxx: (reg: B) (reg: C)))
           (set (reg: CC) (newly-set-reg: A))
          ....)

(insn branch-equal (set (pc) (if_then_else
                               (ge: CC 0)
                               (label_ref 23)
                               (pc)))
                    ...)

Thereby enabling an xxx instruction to specify the CC register value being
virtually assigned the result of the instruction's operation (i.e. no code
will actually be generated for assignments to the CC register), upon which
an independently specified conditional branch may be defined to be dependant
upon it (the virtual CC register). Which would seem to be a simple way to
closely approximate the semantics of a global cc-state register?