Re: Back end question.

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

"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
>> 
>>