[LLVMdev] LiveInterval Splitting & SubRegisters

[Evan Cheng]

On Jan 22, 2008, at 12:23 PM, David Greene wrote:

> Evan,
>
> Can you explain the basic mechanics of the live interval splitting  
> code?
> Is it all in LiveIntervalAnalysis.cpp under addIntervalsForSpills  
> and child
> routines?  What is it trying to do?

It's splitting live intervals that span multiple basic blocks. That  
is, when an interval is spilled, it introduce a single reload per  
basic block and retarget all the uses to use the result of the single  
reload. It does not (yet) split intra-bb intervals.

>
> Also, in the ancient subregister coalescing code, there used to be  
> an update
> of the SSARegMap to point subregisters to the superregister they were
> coalesced to (IIRC).  That has since gone away.  I used to use that  
> in my
> code to return the correct live interval for a virtual register in  
> the case
> where a subregister extract was coalesced.

Right. That has been removed because it's surprisingly painful to  
maintain that information. Now subreg index lives on MachineOperand.

>
> That information appears to now be in RegSubIdxMap, which is local to
> runOnMachineFunction, so it's lost after the coalescer runs.  Do I  
> not need
> to worry about this information anymore?

That's used at the end of coalescing to rewrite virtual registers.  
It's purely local information.

Evan

>
> Thanks.
>
>                                      -Dave
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