[LLVMdev] LiveInterval Splitting & SubRegisters

[Evan Cheng]

On Jan 23, 2008, at 2:12 PM, David Greene <dag at cray.com> wrote:

> On Wednesday 23 January 2008 02:01, Evan Cheng wrote:
>
>>> Can you explain the basic mechanics of the live interval splitting
>>> code?
>
>> 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.
>
> Ah, got it.  Thanks.
>
>>> 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.
>
> Yep, I saw that.  Does the subregh index in the MachineOperand tell  
> the
> asm printer what name to output for a register operand?  How else is
> it used?

There is a table in MRegistrInfo that maps a register to its  
subregisters. I think that's the only use now.

>
>
>>> 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.
>
> Ok, I see where that is happening (the coalescer sets the subreg in
> the MachineOperand).  I'm worried here about the case where an
> EXTRACT_SUBREG targeting two virtual registers is coalesced.
>
> How does the LiveInterval information change?  Since the coalescer
> always coalesces to the superregister, I'm guesing that LiveIntervals
> will have an (extended) interval for the superregister and NO interval
> foir the subregister after coalescing is done (since the subregister
> was coalesced away).  Then when

Yep, you got it.

> the AsmPrinter comes along, it
> sees the subregister set in the MachineOperand and does the
> Right Thing(tm) even though the MachineOperand register is in
> fact the superregister.  Correct?  Any other stuff happening besides
> this?

Nope. Thais it.

Evan
>
>
> Thanks for your help!
>
>                                             -Dave
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