[LLVMdev] Machine LICM and cheap instructions?

[Quentin Colombet]

Hi Hal,

I haven’t looked at the implementation but could it be that we leave room for expensive instructions to be hoisted?
The rational being that hoisting a cheap instruction will increase the register pressure and then we may block expensive instruction (assuming we use some kind of iterative algorithm).

Other than that, I would say that seem rather silly.

Cheers,
Q.
On Jan 8, 2015, at 1:45 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> Hi everyone,
> 
> The MachineLICM pass has a heuristic such that, even in low-register-pressure situations, it will refuse to hoist "cheap" instructions out of loops. By default, when an itinerary is available, this means that all of the defined operands are available in at most 1 cycle. ARM overrides this, and provides this more-customized definition:
> 
> bool ARMBaseInstrInfo::
> hasLowDefLatency(const InstrItineraryData *ItinData,
>                 const MachineInstr *DefMI, unsigned DefIdx) const {
>  if (!ItinData || ItinData->isEmpty())
>    return false;
> 
>  unsigned DDomain = DefMI->getDesc().TSFlags & ARMII::DomainMask;
>  if (DDomain == ARMII::DomainGeneral) {
>    unsigned DefClass = DefMI->getDesc().getSchedClass();
>    int DefCycle = ItinData->getOperandCycle(DefClass, DefIdx);
>    return (DefCycle != -1 && DefCycle <= 2);
>  }
>  return false;
> }
> 
> So it won't hoist instructions that have defined operands ready in at most two cycles for general-domain instructions. Regardless, I don't understand the logic behind this heuristic. High-register-pressure situations are one thing, but why is it ever profitable in low-register-pressure situations not to hoist even "cheap" instructions out of loop bodies?
> 
> Thanks again,
> Hal
> 
> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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