Re: Back end question.

["Dmitry" <mittie at ukr dot net>]

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?
Dmitry.

--- Исходное сообщение ---
 От кого: "Ian Lance Taylor" <iant@google.com> 
 Кому: "Dmitry" <mittie@ukr.net> 
 Дата: 29 апреля 2011, 19:52:44 
 Тема: 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
> 
>