Thanks Ian for your patience. But still I get that I can set several bits with one compare, but how can I use one compare if I need to compare with two different values?
To set a zero bit I should compare with 0 to set an overflow bit I should compare with 65535?
Thanks a lot.
Dmitry.
--- Исходное сообщение ---
От кого: "Ian Lance Taylor" <iant@xxxxxxxxxx>
Кому: "Dmitry" <mittie@xxxxxxx>
Дата: 5 мая 2011, 21:06:29
Тема: Re: Back end question.
> "Dmitry" <mittie@xxxxxxx> writes:
>
> > Yes Ian I understand the whole idea of different CC modes but, what I was asking is how could I load actual values to the separate status bits. I mean in the i386 example you gave "(set (reg:CCC FLAGS_REG) (compare:CCC ..." it sets the only one bit with one set rtx. But if it had CCCZ mode for example (carry + zero bits) it should go like:
> > (set (reg:CCCZ FLAGS_REG) (compare:CCCZ ... - carry bit computation
> > (set (reg:CCCZ FLAGS_REG) (compare:CCCZ ... - zero bit computation
> > or a parallel of some sort?
> > So the question is how could I set the correct values for carry and zero bits? What should go after "(set (reg:CCCZ FLAGS_REG) ..." for two separate status bits?
>
> A set to FLAGS_REG of a compare does not set one bit. It sets whatever
> bits are represented by the mode. So you don't need multiple sets of a
> single register. That doesn't even make sense in gcc's RTL
> representation.
>
> Think about how the value is used: it is used in a conditional
> instruction which is looking at the compare. That conditional can test
> whichever bits the mode permits.
>
> Ian
>
> > --- Исходное сообщение ---
> > От кого: "Ian Lance Taylor" <iant@xxxxxxxxxx>
> > Кому: "Dmitry" <mittie@xxxxxxx>
> > Дата: 5 мая 2011, 19:44:55
> > Тема: Re: Back end question.
> >
> >
> >
> >> "Dmitry" <mittie@xxxxxxx> writes:
> >>
> >> > Yes but as you've said, it's a carry flag setter, but how could this example be transformed if I need to store not only carry flag but also for example a zero flag? Could you write such an example, please?
> >>
> >> You do exactly the same thing, but you define a mode which means that
> >> both the zero flag and the carry flag are valid. Then your test can
> >> look for that mode in the flags register. Don't look at this insn in
> >> isolation--look at it in conjunction with some conditional insn that
> >> tests the value in the flags register. The conditional test is going to
> >> be testing the result of the plus operation compared with zero, in some
> >> mode.
> >>
> >> The i386 backend uses CCmode to represent all the flags, and uses the
> >> other modes to represent subsets of the flags.
> >>
> >> Ian
> >>
> >> > --- Исходное сообщение ---
> >> > От кого: "Ian Lance Taylor" <iant@xxxxxxxxxx>
> >> > Кому: "Dmitry" <mittie@xxxxxxx>
> >> > Дата: 5 мая 2011, 17:41:38
> >> > Тема: Re: Back end question.
> >> >
> >> >
> >> >
> >> >> "Dmitry" <mittie@xxxxxxx> writes:
> >> >>
> >> >> > Ok but if people do not set each status bit individually, how do they set several not related bits in one CC mode and in one set rtx?
> >> >>
> >> >> They represent the collection of bits as a mode, as you've been
> >> >> discussing.
> >> >>
> >> >> E.g., from config/i386/i386.md
> >> >>
> >> >> (define_insn "*<plusminus_insn><mode>3_cc_overflow"
> >> >> [(set (reg:CCC FLAGS_REG)
> >> >> (compare:CCC
> >> >> (plusminus:SWI
> >> >> (match_operand:SWI 1 "nonimmediate_operand" "<comm>0,0")
> >> >> (match_operand:SWI 2 "<general_operand>" "<r><i>,<r>m"))
> >> >> (match_dup 1)))
> >> >> (set (match_operand:SWI 0 "nonimmediate_operand" "=<r>m,<r>")
> >> >> (plusminus:SWI (match_dup 1) (match_dup 2)))]
> >> >> "ix86_binary_operator_ok (<CODE>, <MODE>mode, operands)"
> >> >> "<plusminus_mnemonic>{<imodesuffix>}\t{%2, %0|%0, %2}"
> >> >> [(set_attr "type" "alu")
> >> >> (set_attr "mode" "<MODE>")])
> >> >>
> >> >>
> >> >> Here the CCC mode (defined in i386-modes.def) means that the carry flag
> >> >> is valid.
> >> >>
> >> >> Ian
> >> >>
> >> >>
> >>
> >>
>
>
