[LLVMdev] register scavenger

[Reed Kotler]

I think I answered my own question.

When you use the scavenged register, you set it to kill and now it's 
available as a free register for further needs , until it has to be 
restored.

So that is the reason for chosing the register needed the furthest away 
from the original  place; because no further emergency slot 
manimupatation will be needed as long as possible.

On 11/10/2012 02:17 PM, Reed Kotler wrote:
> I'm confused as to the logic used in the register scavenger when it
> cannot find a free register.
>
> I would think that it would want to free up the emergency spill slot
> immediately after it's use, because otherwise there is a chance of
> needing to use the emergency slot again and not be able to.
>
> Instead it tries to restore it only right before register it is freeing up.
>
> Maybe I'm misunderstanding this code.
>
>    // If the target knows how to save/restore the register, let it do so;
>    // otherwise, use the emergency stack spill slot.
>    if (!TRI->saveScavengerRegister(*MBB, I, UseMI, RC, SReg)) {
>      // Spill the scavenged register before I.
>      assert(ScavengingFrameIndex >= 0 &&
>             "Cannot scavenge register without an emergency spill slot!");
>      TII->storeRegToStackSlot(*MBB, I, SReg, true, ScavengingFrameIndex,
> RC,TRI);
>      MachineBasicBlock::iterator II = prior(I);
>      TRI->eliminateFrameIndex(II, SPAdj, this);
>
>      // Restore the scavenged register before its use (or first
> terminator).
>      TII->loadRegFromStackSlot(*MBB, UseMI, SReg, ScavengingFrameIndex,
> RC, TRI);
>      II = prior(UseMI);
>      TRI->eliminateFrameIndex(II, SPAdj, this);
>    }