Re: Back end question.

[Ian Lance Taylor <iant at google dot com>]

"Dmitry" <mittie@ukr.net> writes:

> I see. Ok let's say it this way. If I have an move instruction that if
> a destination operand is an accumulator can set some status bits, like
> zero or sign. How can I use this feature with the compiler? For
> example, after "mov" to the accumulator the compiler could use the
> status bits from that "mov" with the branch insn (without a seperate
> compare).

You have to change the move instruction to be a PARALLEL of two SETS,
one to set the destination register and one to set the status register.

It may help to read the "Condition Code Status" section in the internals
manual.

Ian

> --- ÐÑÑÐÐÐÐÐ ÑÐÐÐÑÐÐÐÐ ---
>  ÐÑ ÐÐÐÐ: "Ian Lance Taylor" <iant@google.com> 
>  ÐÐÐÑ: "Dmitry" <mittie@ukr.net> 
>  ÐÐÑÐ: 4 ÐÐÑ 2011, 17:29:03 
>  ÐÐÐÐ: Re: Back end question. 
>  
>
>
>> "Dmitry" <mittie@ukr.net> writes:
>> 
>> > Ok. But what if the compiler uses that unnecessary clobber for a
>> > branch or another CC dependent insn before reload, and if I remove
>> > that clobber in a define_split after reload, could I end up with a
>> > corrupted code?
>> 
>> I'm sorry, I don't understand what you are trying to say. The compiler
>> can't use a clobber. Simply removing a clobber can't cause corrupted
>> code, because the compiler was already assuming that the value in the
>> register was unpredictable.
>> 
>> Ian
>> 
>> > --- ÐÑÑÐÐÐÐÐ ÑÐÐÐÑÐÐÐÐ ---
>> > ÐÑ ÐÐÐÐ: "Ian Lance Taylor" <iant@google.com> 
>> > ÐÐÐÑ: "Dmitry" <mittie@ukr.net> 
>> > ÐÐÑÐ: 29 ÐÐÑÐÐÑ 2011, 19:52:44 
>> > ÐÐÐÐ: Re: Back end question. 
>> > 
>> >
>> >
>> >> "Dmitry" <mittie@ukr.net> writes:
>> >> 
>> >> > I have one more question. If I have, for example, a define_insn rtx which matches a pattern generated by previous define_expand rtx. define_expand and define_split combination produces either a simple "set" rtx or "set" plus clobber of CC reg. And if in define_insn I specify a constraint that requires such an operand type which should generate a CC clobber, but initially there could be a combination of operands that does not clobber a CC and also does not satisfy constraints, so compiler must reload one or two operands in appropriate registers which satisfies constraints. In the case those appropriate registers should clobber a CC does a compiler add a suitable clobber rtx as written in define_split, or it just changes the registers names without any knowledge of a necessary clobber?
>> >> 
>> >> If I understand your question correctly, then the compiler is not going
>> >> to add a clobber. Reload does not change the shape of the instruction.
>> >> It just shuffles registers to match the constraints. If you have
>> >> constraints which require a clobber, then the insn pattern should have a
>> >> clobber. You can use define_split to remove an unnecessary clobber
>> >> after reload is complete.
>> >> 
>> >> Ian
>> >> 
>> >> 
>> 
>>