llvm dev - Jan 2009 - [LLVMdev] Probably FAQ: can you insert BRCONDs during TargetLowering::LowerOperation?

Chris:

I was trying to avoid using the custom instruction emitter method. Not as if
I didn't look before I asked the question to see if there was an easier way.


-scooter

On Tue, Jan 6, 2009 at 10:07 PM, Chris Lattner <clattner at apple.com>
wrote:
>
> On Jan 6, 2009, at 5:53 PM, Scott Michel wrote:
>
> > It's yet another CellSPU thing: integer division is a long code
> > sequence we have to insert that has branches within it (test for 0
> > divisor, etc.) Can't really emit the sequence of instructions via
> > tblgen (can't have labels or BBs). The other alternative is a
custom
> > instruction emitter.
> >
> > Life could be easier if I could insert BRCONDs and other branches
> > from within target-dependent lowering -- if I had access to the
> > current MBB.
> >
> > Is this possible or just a FAQ?
>
> It isn't currently possible for a selection dag to represent more than
> one machine basic block.  I think Christopher Lamb had some code that
> did some of this, but I don't know how far he got.
>
> The way we currently work around this is with a "custom scheduler
> insertion hook".  For example, see how SELECT_CC_* works on PPC: at
> isel time we claim that we have the operation, then the scheduler
> invokes a virtual function to insert it, which expands out into
> multiple MBBs.
>
> -Chris
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On Jan 6, 2009, at 11:37 PM, Scott Michel wrote:
> Chris:
>
> I was trying to avoid using the custom instruction emitter method.  
> Not as if I didn't look before I asked the question to see if there  
> was an easier way.
Then no, there is no other way.

-Chris

Chris:

It occurred to me this morning that maybe the general solution to  
this particular type of problem is a custom/target-dependent DAG  
inserter pass, analogous to target lowering and DAG legalization. I'm  
not sure if this pass happens before or after legalization. Worth  
considering?

I suppose the best way to discuss this is to just write the code.  
However, some preliminary discussion is probably worthwhile to make  
sure that a target-dependent DAG inserter pass is invoked in the  
right place in the existing pass stream.


-scooter

On Jan 7, 2009, at 12:18 AM, Chris Lattner wrote:
>
> On Jan 6, 2009, at 11:37 PM, Scott Michel wrote:
>
>> Chris:
>>
>> I was trying to avoid using the custom instruction emitter method.
>> Not as if I didn't look before I asked the question to see if there
>> was an easier way.
>
> Then no, there is no other way.
>
> -Chris
>
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