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//===---- LatencyPriorityQueue.cpp - A latency-oriented priority queue ----===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file implements the LatencyPriorityQueue class, which is a
// SchedulingPriorityQueue that schedules using latency information to
// reduce the length of the critical path through the basic block.
#include "llvm/CodeGen/LatencyPriorityQueue.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "scheduler"
bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
// The isScheduleHigh flag allows nodes with wraparound dependencies that
// cannot easily be modeled as edges with latencies to be scheduled as
// soon as possible in a top-down schedule.
if (LHS->isScheduleHigh && !RHS->isScheduleHigh)
return false;
if (!LHS->isScheduleHigh && RHS->isScheduleHigh)
return true;
unsigned LHSNum = LHS->NodeNum;
unsigned RHSNum = RHS->NodeNum;
// The most important heuristic is scheduling the critical path.
unsigned LHSLatency = PQ->getLatency(LHSNum);
unsigned RHSLatency = PQ->getLatency(RHSNum);
if (LHSLatency < RHSLatency) return true;
if (LHSLatency > RHSLatency) return false;
// After that, if two nodes have identical latencies, look to see if one will
// unblock more other nodes than the other.
unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum);
unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum);
if (LHSBlocked < RHSBlocked) return true;
if (LHSBlocked > RHSBlocked) return false;
// Finally, just to provide a stable ordering, use the node number as a
// deciding factor.
return RHSNum < LHSNum;
/// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
/// of SU, return it, otherwise return null.
SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
SUnit *OnlyAvailablePred = nullptr;
for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
I != E; ++I) {
SUnit &Pred = *I->getSUnit();
if (!Pred.isScheduled) {
// We found an available, but not scheduled, predecessor. If it's the
// only one we have found, keep track of it... otherwise give up.
if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
return nullptr;
OnlyAvailablePred = &Pred;
return OnlyAvailablePred;
void LatencyPriorityQueue::push(SUnit *SU) {
// Look at all of the successors of this node. Count the number of nodes that
// this node is the sole unscheduled node for.
unsigned NumNodesBlocking = 0;
for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
I != E; ++I) {
if (getSingleUnscheduledPred(I->getSUnit()) == SU)
NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
// scheduledNode - As nodes are scheduled, we look to see if there are any
// successor nodes that have a single unscheduled predecessor. If so, that
// single predecessor has a higher priority, since scheduling it will make
// the node available.
void LatencyPriorityQueue::scheduledNode(SUnit *SU) {
for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
I != E; ++I) {
/// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
/// scheduled. If SU is not itself available, then there is at least one
/// predecessor node that has not been scheduled yet. If SU has exactly ONE
/// unscheduled predecessor, we want to increase its priority: it getting
/// scheduled will make this node available, so it is better than some other
/// node of the same priority that will not make a node available.
void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
if (SU->isAvailable) return; // All preds scheduled.
SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
if (!OnlyAvailablePred || !OnlyAvailablePred->isAvailable) return;
// Okay, we found a single predecessor that is available, but not scheduled.
// Since it is available, it must be in the priority queue. First remove it.
// Reinsert the node into the priority queue, which recomputes its
// NumNodesSolelyBlocking value.
SUnit *LatencyPriorityQueue::pop() {
if (empty()) return nullptr;
std::vector<SUnit *>::iterator Best = Queue.begin();
for (std::vector<SUnit *>::iterator I = std::next(Queue.begin()),
E = Queue.end(); I != E; ++I)
if (Picker(*Best, *I))
Best = I;
SUnit *V = *Best;
if (Best != std::prev(Queue.end()))
std::swap(*Best, Queue.back());
return V;
void LatencyPriorityQueue::remove(SUnit *SU) {
assert(!Queue.empty() && "Queue is empty!");
std::vector<SUnit *>::iterator I = find(Queue, SU);
assert(I != Queue.end() && "Queue doesn't contain the SU being removed!");
if (I != std::prev(Queue.end()))
std::swap(*I, Queue.back());
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void LatencyPriorityQueue::dump(ScheduleDAG *DAG) const {
dbgs() << "Latency Priority Queue\n";
dbgs() << " Number of Queue Entries: " << Queue.size() << "\n";
for (const SUnit *SU : Queue) {
dbgs() << " ";