Re: Use of FLAGS_REGNUM clashes with generates insn

["Paulo J. Matos" <paulo@matos-sorge.com>]

On Fri, 23 Sep 2011 09:30:48 -0400, amylaar wrote:

> Hiding the flags register would mean it is not represented in the rtl at
> all.  You can have combined compare-branch instructions. Of course,
> going that route would mean that the model you present to GCC is even
> further from the hardware than one that uses cc0.
>

Got it! That seems that it would go against the whole point of replacing 
cc0 for CC_REGNUM in my specific case. Oh well...
 
>> What I currently have in mind is to have a backend macro listing all
>> the move for which a move clobber CC_REG, then whenever GCC generates a
>> move, it queries the macro to know if the move requires clobbering and
>> emits the clobber if required. However, I am unsure how deep the rabbit
>> hole goes.
> 
> Oh, so you do have variants that can do without the clobber. 

Actually I don't... My explanation was supposed to be referring to a 
general solution. In my case, the macro would list all moves since all 
moves clobber CC.

> If you can
> make all the reloads without introducing explicit flag clobbers, that it
> should work.

Unfortunately I can't.

> But you can't just pull a flag clobber out of thin air.  

Understood.

> You should have
> some way to generate valid code when the flags register is unavailable /
> must be saved.  Then you can use peephole2 to add flag clobbers where
> the flags register is available.
> 
> Or you can use machine_dependent_reorg or another machine-specific pass
> inserted with the pass manager to rewrite clobber-free instructions into
> ones that have a hardware equivalent; but you must make sure that your
> data flow remains sound in the process.

I think your last suggestion of having a pass to rewrite the clobber free 
instructions into one with a hardware equivalent seems the one to go for 
me. 

Thanks for the suggestions,

-- 
PMatos