lib/CodeGen/TargetSchedule.cpp

//===-- llvm/Target/TargetSchedule.cpp - Sched Machine Model ----*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a wrapper around MCSchedModel that allows the interface
// to benefit from information currently only available in TargetInstrInfo.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/TargetSchedule.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include "llvm/Support/CommandLine.h"

using namespace llvm;

static cl::opt<bool> EnableSchedModel("schedmodel", cl::Hidden, cl::init(false),
  cl::desc("Use TargetSchedModel for latency lookup"));

static cl::opt<bool> EnableSchedItins("scheditins", cl::Hidden, cl::init(true),
  cl::desc("Use InstrItineraryData for latency lookup"));

void TargetSchedModel::init(const MCSchedModel &sm,
                            const TargetSubtargetInfo *sti,
                            const TargetInstrInfo *tii) {
  SchedModel = sm;
  STI = sti;
  TII = tii;
  STI->initInstrItins(InstrItins);
}

/// If we can determine the operand latency from the def only, without machine
/// model or itinerary lookup, do so. Otherwise return -1.
int TargetSchedModel::getDefLatency(const MachineInstr *DefMI,
                                    bool FindMin) const {

  // Return a latency based on the itinerary properties and defining instruction
  // if possible. Some common subtargets don't require per-operand latency,
  // especially for minimum latencies.
  if (FindMin) {
    // If MinLatency is invalid, then use the itinerary for MinLatency. If no
    // itinerary exists either, then use single cycle latency.
    if (SchedModel.MinLatency < 0
        && !(EnableSchedItins && hasInstrItineraries())) {
      return 1;
    }
    return SchedModel.MinLatency;
  }
  else if (!(EnableSchedModel && hasInstrSchedModel())
           && !(EnableSchedItins && hasInstrItineraries())) {
    return TII->defaultDefLatency(&SchedModel, DefMI);
  }
  // ...operand lookup required
  return -1;
}

/// Return the MCSchedClassDesc for this instruction. Some SchedClasses require
/// evaluation of predicates that depend on instruction operands or flags.
const MCSchedClassDesc *TargetSchedModel::
resolveSchedClass(const MachineInstr *MI) const {

  // Get the definition's scheduling class descriptor from this machine model.
  unsigned SchedClass = MI->getDesc().getSchedClass();
  const MCSchedClassDesc *SCDesc = SchedModel.getSchedClassDesc(SchedClass);

#ifndef NDEBUG
  unsigned NIter = 0;
#endif
  while (SCDesc->isVariant()) {
    assert(++NIter < 6 && "Variants are nested deeper than the magic number");

    SchedClass = STI->resolveSchedClass(SchedClass, MI, this);
    SCDesc = SchedModel.getSchedClassDesc(SchedClass);
  }
  return SCDesc;
}

/// Find the def index of this operand. This index maps to the machine model and
/// is independent of use operands. Def operands may be reordered with uses or
/// merged with uses without affecting the def index (e.g. before/after
/// regalloc). However, an instruction's def operands must never be reordered
/// with respect to each other.
static unsigned findDefIdx(const MachineInstr *MI, unsigned DefOperIdx) {
  unsigned DefIdx = 0;
  for (unsigned i = 0; i != DefOperIdx; ++i) {
    const MachineOperand &MO = MI->getOperand(i);
    if (MO.isReg() && MO.isDef())
      ++DefIdx;
  }
  return DefIdx;
}

/// Find the use index of this operand. This is independent of the instruction's
/// def operands.
static unsigned findUseIdx(const MachineInstr *MI, unsigned UseOperIdx) {
  unsigned UseIdx = 0;
  for (unsigned i = 0; i != UseOperIdx; ++i) {
    const MachineOperand &MO = MI->getOperand(i);
    if (MO.isReg() && MO.isUse())
      ++UseIdx;
  }
  return UseIdx;
}

// Top-level API for clients that know the operand indices.
unsigned TargetSchedModel::computeOperandLatency(
  const MachineInstr *DefMI, unsigned DefOperIdx,
  const MachineInstr *UseMI, unsigned UseOperIdx,
  bool FindMin) const {

  int DefLatency = getDefLatency(DefMI, FindMin);
  if (DefLatency >= 0)
    return DefLatency;

  if (!FindMin && EnableSchedModel && hasInstrSchedModel()) {
    const MCSchedClassDesc *SCDesc = resolveSchedClass(DefMI);
    unsigned DefIdx = findDefIdx(DefMI, DefOperIdx);
    if (DefIdx < SCDesc->NumWriteLatencyEntries) {

      // Lookup the definition's write latency in SubtargetInfo.
      const MCWriteLatencyEntry *WLEntry =
        STI->getWriteLatencyEntry(SCDesc, DefIdx);
      unsigned WriteID = WLEntry->WriteResourceID;
      unsigned Latency = WLEntry->Cycles;
      if (!UseMI)
        return Latency;

      // Lookup the use's latency adjustment in SubtargetInfo.
      const MCSchedClassDesc *UseDesc = resolveSchedClass(UseMI);
      if (UseDesc->NumReadAdvanceEntries == 0)
        return Latency;
      unsigned UseIdx = findUseIdx(UseMI, UseOperIdx);
      return Latency - STI->getReadAdvanceCycles(UseDesc, UseIdx, WriteID);
    }
    // If DefIdx does not exist in the model (e.g. implicit defs), then return
    // unit latency (defaultDefLatency may be too conservative).
    // TODO: For unknown defs, we may want to use the subtarget's model
    // for WAW latency here instead of 1 cycle.
    assert((!SCDesc->isValid() || DefMI->getOperand(DefOperIdx).isImplicit()) &&
           "DefIdx exceeds machine model def operand list");
    return 1;
  }
  assert(EnableSchedItins && hasInstrItineraries() &&
         "operand latency requires itinerary");

  int OperLatency = 0;
  if (UseMI) {
    OperLatency =
      TII->getOperandLatency(&InstrItins, DefMI, DefOperIdx, UseMI, UseOperIdx);
  }
  else {
    unsigned DefClass = DefMI->getDesc().getSchedClass();
    OperLatency = InstrItins.getOperandCycle(DefClass, DefOperIdx);
  }
  if (OperLatency >= 0)
    return OperLatency;

  // No operand latency was found.
  unsigned InstrLatency = TII->getInstrLatency(&InstrItins, DefMI);

  // Expected latency is the max of the stage latency and itinerary props.
  if (!FindMin)
    InstrLatency = std::max(InstrLatency,
                            TII->defaultDefLatency(&SchedModel, DefMI));
  return InstrLatency;
}