[LLVMdev] FP emulation (continued)

[Roman Levenstein]

Hi Chris,

--- Chris Lattner <sabre at nondot.org> wrote:
> On Wed, 29 Nov 2006, Roman Levenstein wrote:
> > Thanks, this is a good idea.
> >
> > But I cannot figure out how to make a machine function pass run
> > _BEFORE_ the RA. I guess I'm missing something very obvious.
> 
> In your target's TargetMachine::addInstSelector method, add it to the
> pass  mgr right after your isel.

Thanks a lot! This is exactly what I could not understand.  
 
>>  And BTW, it seems to me that currently new RA passes are not
>> allowed to derive from the existing ones. If it is correct, why so?
>> Wouldn't it be nice?
> 
> I'm not sure what you mean.  We don't expose linscan through a public

> header, but a pass in the same .cpp file could subclass it.  

> We haven't had a need to do this yet, so we don't have the provisions

> to do it.

OK, I see. I just had the idea that it could be useful, if someone
defines a target-specific RA, which is a slight modification of an
existing one, like the linear scan RA. Let's say it just executes some
target-specific actions before and after the existing register
allocator. Then you probably don't want to put these target-specific
bits into the same file as the existing allocator. It would be much
cleaner to define in a  separate source file a new RA that in some
sense "derives" from the existing RA (either using inheritance or by
having a class member that is of a known existing RA class). Such a new
RA would do some pre/post RA actions in its runOnMachineFunction()
method and delegate a real RA job to the "parent" register allocator.

Thanks again for your hint,
 -Roman
 


 
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