1 //===---- LatencyPriorityQueue.cpp - A latency-oriented priority queue ----===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements the LatencyPriorityQueue class, which is a
11 // SchedulingPriorityQueue that schedules using latency information to
12 // reduce the length of the critical path through the basic block.
14 //===----------------------------------------------------------------------===//
16 #define DEBUG_TYPE "scheduler"
17 #include "llvm/CodeGen/LatencyPriorityQueue.h"
18 #include "llvm/Support/Debug.h"
21 bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
22 unsigned LHSNum = LHS->NodeNum;
23 unsigned RHSNum = RHS->NodeNum;
25 // The most important heuristic is scheduling the critical path.
26 unsigned LHSLatency = PQ->getLatency(LHSNum);
27 unsigned RHSLatency = PQ->getLatency(RHSNum);
28 if (LHSLatency < RHSLatency) return true;
29 if (LHSLatency > RHSLatency) return false;
31 // After that, if two nodes have identical latencies, look to see if one will
32 // unblock more other nodes than the other.
33 unsigned LHSBlocked = PQ->getNumSolelyBlockNodes(LHSNum);
34 unsigned RHSBlocked = PQ->getNumSolelyBlockNodes(RHSNum);
35 if (LHSBlocked < RHSBlocked) return true;
36 if (LHSBlocked > RHSBlocked) return false;
38 // Finally, just to provide a stable ordering, use the node number as a
40 return LHSNum < RHSNum;
44 /// getSingleUnscheduledPred - If there is exactly one unscheduled predecessor
45 /// of SU, return it, otherwise return null.
46 SUnit *LatencyPriorityQueue::getSingleUnscheduledPred(SUnit *SU) {
47 SUnit *OnlyAvailablePred = 0;
48 for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
50 SUnit &Pred = *I->getSUnit();
51 if (!Pred.isScheduled) {
52 // We found an available, but not scheduled, predecessor. If it's the
53 // only one we have found, keep track of it... otherwise give up.
54 if (OnlyAvailablePred && OnlyAvailablePred != &Pred)
56 OnlyAvailablePred = &Pred;
60 return OnlyAvailablePred;
63 void LatencyPriorityQueue::push_impl(SUnit *SU) {
64 // Look at all of the successors of this node. Count the number of nodes that
65 // this node is the sole unscheduled node for.
66 unsigned NumNodesBlocking = 0;
67 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
69 if (getSingleUnscheduledPred(I->getSUnit()) == SU)
71 NumNodesSolelyBlocking[SU->NodeNum] = NumNodesBlocking;
77 // ScheduledNode - As nodes are scheduled, we look to see if there are any
78 // successor nodes that have a single unscheduled predecessor. If so, that
79 // single predecessor has a higher priority, since scheduling it will make
80 // the node available.
81 void LatencyPriorityQueue::ScheduledNode(SUnit *SU) {
82 for (SUnit::const_succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
84 AdjustPriorityOfUnscheduledPreds(I->getSUnit());
87 /// AdjustPriorityOfUnscheduledPreds - One of the predecessors of SU was just
88 /// scheduled. If SU is not itself available, then there is at least one
89 /// predecessor node that has not been scheduled yet. If SU has exactly ONE
90 /// unscheduled predecessor, we want to increase its priority: it getting
91 /// scheduled will make this node available, so it is better than some other
92 /// node of the same priority that will not make a node available.
93 void LatencyPriorityQueue::AdjustPriorityOfUnscheduledPreds(SUnit *SU) {
94 if (SU->isAvailable) return; // All preds scheduled.
96 SUnit *OnlyAvailablePred = getSingleUnscheduledPred(SU);
97 if (OnlyAvailablePred == 0 || !OnlyAvailablePred->isAvailable) return;
99 // Okay, we found a single predecessor that is available, but not scheduled.
100 // Since it is available, it must be in the priority queue. First remove it.
101 remove(OnlyAvailablePred);
103 // Reinsert the node into the priority queue, which recomputes its
104 // NumNodesSolelyBlocking value.
105 push(OnlyAvailablePred);