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Re: Back end question.
"Dmitry" <mittie@ukr.net> 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
> --- ÐÑÑÐÐÐÐÐ ÑÐÐÐÑÐÐÐÐ ---
> ÐÑ ÐÐÐÐ: "Ian Lance Taylor" <iant@google.com>
> ÐÐÐÑ: "Dmitry" <mittie@ukr.net>
> ÐÐÑÐ: 5 ÐÐÑ 2011, 19:44:55
> ÐÐÐÐ: Re: Back end question.
>
>
>
>> "Dmitry" <mittie@ukr.net> 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
>>
>> > --- ÐÑÑÐÐÐÐÐ ÑÐÐÐÑÐÐÐÐ ---
>> > ÐÑ ÐÐÐÐ: "Ian Lance Taylor" <iant@google.com>
>> > ÐÐÐÑ: "Dmitry" <mittie@ukr.net>
>> > ÐÐÑÐ: 5 ÐÐÑ 2011, 17:41:38
>> > ÐÐÐÐ: Re: Back end question.
>> >
>> >
>> >
>> >> "Dmitry" <mittie@ukr.net> 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
>> >>
>> >>
>>
>>