[PATCH][RegAlloc] Make tryInstructionSplit less aggressive.

[Quentin Colombet]

Thanks Jakob for the quick review!

I'll integrate your suggestions and I'll commit after the break so that I am around in case something goes wrong.

Thanks again.

-Quentin
On Dec 23, 2013, at 3:00 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk> wrote:

> 
> On Dec 23, 2013, at 2:35 PM, Quentin Colombet <qcolombet at apple.com> wrote:
> 
>> Hi Jakob,
>> 
>> Here is the reworked patch.
>> 
>> Now the MachineInstr API offers new methods to query the effect of the operand constraints on the register class of a virtual register or on a given register class.
>> The new API is used in MachineRegisterInfo::recomputeRegClass as well as in RAGreedy::tryInstructionSplit.
> 
> Thanks, Quentin.
> 
> This is looking good.
> 
> I think I would prefer to omit the MI::getLargest* functions. You will almost always be looking at multiple instructions, and I’m afraid it can get expensive to call getLargestLegalSuperClass() redundantly for all of them.
> 
> The getRegClassConstraintEffect* functions look great.
> 
> +  const TargetRegisterClass *SuperRC = TRI->getLargestLegalSuperClass(CurRC);
> +  // Split around every non-copy instruction if this split will relax
> +  // the constraints on the virtual register.
> +  // Otherwise, splitting just inserts uncoalescable copies that do not help
> +  // the allocation.
>   for (unsigned i = 0; i != Uses.size(); ++i) {
>     if (const MachineInstr *MI = Indexes->getInstructionFromIndex(Uses[i]))
> -      if (MI->isFullCopy()) {
> +      if (MI->isFullCopy() ||
> +          SuperRC == MI->getLargestRegClassForVReg(VirtReg.reg, TII, TRI, MRI,
> +                                                   /* ExploreBundle */ true)) {
> 
> Here, I would just pass SuperRC to MI->getRegClassConstraintEffectForVReg() instead.
> 
> Also, don’t compare the register classes by equality, but check that the returned register class actually has fewer allocatable registers. See for example GPR/rGPR on ARM. RegisterClassInfo::getNumAllocatableRegs() is very fast.
> 
> +  if (ExploreBundle && (isBundle() || isBundled())) {
> +    MachineBasicBlock::const_instr_iterator I = this;
> +    MachineBasicBlock::const_instr_iterator E = getParent()->instr_end();
> +    while (CurRC && ++I != E && I->isInsideBundle())
> +      // Accumulate the constraints through the instructions.
> +      CurRC = I->getRegClassConstraintEffectForVRegImpl(Reg, CurRC, TII, TRI);
> +  }
> 
> See MIBundleOperands.
> 
> Go ahead and commit with those changes.
> 
> Thanks,
> /jakob
>