1//===- ScheduleDAGVLIW.cpp - SelectionDAG list scheduler for VLIW -*- C++ -*-=//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This implements a top-down list scheduler, using standard algorithms.
10// The basic approach uses a priority queue of available nodes to schedule.
11// One at a time, nodes are taken from the priority queue (thus in priority
12// order), checked for legality to schedule, and emitted if legal.
13//
14// Nodes may not be legal to schedule either due to structural hazards (e.g.
15// pipeline or resource constraints) or because an input to the instruction has
16// not completed execution.
17//
18//===----------------------------------------------------------------------===//
19
20#include "ScheduleDAGSDNodes.h"
21#include "llvm/ADT/Statistic.h"
22#include "llvm/CodeGen/ResourcePriorityQueue.h"
23#include "llvm/CodeGen/ScheduleHazardRecognizer.h"
24#include "llvm/CodeGen/SchedulerRegistry.h"
25#include "llvm/CodeGen/SelectionDAGISel.h"
26#include "llvm/CodeGen/TargetInstrInfo.h"
27#include "llvm/CodeGen/TargetSubtargetInfo.h"
28#include "llvm/Support/Debug.h"
29#include "llvm/Support/ErrorHandling.h"
30#include "llvm/Support/raw_ostream.h"
31using namespace llvm;
32
33#define DEBUG_TYPE "pre-RA-sched"
34
35STATISTIC(NumNoops , "Number of noops inserted");
36STATISTIC(NumStalls, "Number of pipeline stalls");
37
38static RegisterScheduler
39 VLIWScheduler("vliw-td", "VLIW scheduler",
40 createVLIWDAGScheduler);
41
42namespace {
43//===----------------------------------------------------------------------===//
44/// ScheduleDAGVLIW - The actual DFA list scheduler implementation. This
45/// supports / top-down scheduling.
46///
47class ScheduleDAGVLIW : public ScheduleDAGSDNodes {
48private:
49 /// AvailableQueue - The priority queue to use for the available SUnits.
50 ///
51 SchedulingPriorityQueue *AvailableQueue;
52
53 /// PendingQueue - This contains all of the instructions whose operands have
54 /// been issued, but their results are not ready yet (due to the latency of
55 /// the operation). Once the operands become available, the instruction is
56 /// added to the AvailableQueue.
57 std::vector<SUnit*> PendingQueue;
58
59 /// HazardRec - The hazard recognizer to use.
60 ScheduleHazardRecognizer *HazardRec;
61
62 /// AA - AAResults for making memory reference queries.
63 AAResults *AA;
64
65public:
66 ScheduleDAGVLIW(MachineFunction &mf, AAResults *aa,
67 SchedulingPriorityQueue *availqueue)
68 : ScheduleDAGSDNodes(mf), AvailableQueue(availqueue), AA(aa) {
69 const TargetSubtargetInfo &STI = mf.getSubtarget();
70 HazardRec = STI.getInstrInfo()->CreateTargetHazardRecognizer(STI: &STI, DAG: this);
71 }
72
73 ~ScheduleDAGVLIW() override {
74 delete HazardRec;
75 delete AvailableQueue;
76 }
77
78 void Schedule() override;
79
80private:
81 void releaseSucc(SUnit *SU, const SDep &D);
82 void releaseSuccessors(SUnit *SU);
83 void scheduleNodeTopDown(SUnit *SU, unsigned CurCycle);
84 void listScheduleTopDown();
85};
86} // end anonymous namespace
87
88/// Schedule - Schedule the DAG using list scheduling.
89void ScheduleDAGVLIW::Schedule() {
90 LLVM_DEBUG(dbgs() << "********** List Scheduling " << printMBBReference(*BB)
91 << " '" << BB->getName() << "' **********\n");
92
93 // Build the scheduling graph.
94 BuildSchedGraph(AA);
95
96 AvailableQueue->initNodes(SUnits);
97
98 listScheduleTopDown();
99
100 AvailableQueue->releaseState();
101}
102
103//===----------------------------------------------------------------------===//
104// Top-Down Scheduling
105//===----------------------------------------------------------------------===//
106
107/// releaseSucc - Decrement the NumPredsLeft count of a successor. Add it to
108/// the PendingQueue if the count reaches zero. Also update its cycle bound.
109void ScheduleDAGVLIW::releaseSucc(SUnit *SU, const SDep &D) {
110 SUnit *SuccSU = D.getSUnit();
111
112#ifndef NDEBUG
113 if (SuccSU->NumPredsLeft == 0) {
114 dbgs() << "*** Scheduling failed! ***\n";
115 dumpNode(*SuccSU);
116 dbgs() << " has been released too many times!\n";
117 llvm_unreachable(nullptr);
118 }
119#endif
120 assert(!D.isWeak() && "unexpected artificial DAG edge");
121
122 --SuccSU->NumPredsLeft;
123
124 SuccSU->setDepthToAtLeast(SU->getDepth() + D.getLatency());
125
126 // If all the node's predecessors are scheduled, this node is ready
127 // to be scheduled. Ignore the special ExitSU node.
128 if (SuccSU->NumPredsLeft == 0 && SuccSU != &ExitSU) {
129 PendingQueue.push_back(x: SuccSU);
130 }
131}
132
133void ScheduleDAGVLIW::releaseSuccessors(SUnit *SU) {
134 // Top down: release successors.
135 for (SDep &Succ : SU->Succs) {
136 assert(!Succ.isAssignedRegDep() &&
137 "The list-td scheduler doesn't yet support physreg dependencies!");
138
139 releaseSucc(SU, D: Succ);
140 }
141}
142
143/// scheduleNodeTopDown - Add the node to the schedule. Decrement the pending
144/// count of its successors. If a successor pending count is zero, add it to
145/// the Available queue.
146void ScheduleDAGVLIW::scheduleNodeTopDown(SUnit *SU, unsigned CurCycle) {
147 LLVM_DEBUG(dbgs() << "*** Scheduling [" << CurCycle << "]: ");
148 LLVM_DEBUG(dumpNode(*SU));
149
150 Sequence.push_back(x: SU);
151 assert(CurCycle >= SU->getDepth() && "Node scheduled above its depth!");
152 SU->setDepthToAtLeast(CurCycle);
153
154 releaseSuccessors(SU);
155 SU->isScheduled = true;
156 AvailableQueue->scheduledNode(SU);
157}
158
159/// listScheduleTopDown - The main loop of list scheduling for top-down
160/// schedulers.
161void ScheduleDAGVLIW::listScheduleTopDown() {
162 unsigned CurCycle = 0;
163
164 // Release any successors of the special Entry node.
165 releaseSuccessors(SU: &EntrySU);
166
167 // All leaves to AvailableQueue.
168 for (SUnit &SU : SUnits) {
169 // It is available if it has no predecessors.
170 if (SU.Preds.empty()) {
171 AvailableQueue->push(U: &SU);
172 SU.isAvailable = true;
173 }
174 }
175
176 // While AvailableQueue is not empty, grab the node with the highest
177 // priority. If it is not ready put it back. Schedule the node.
178 std::vector<SUnit*> NotReady;
179 Sequence.reserve(n: SUnits.size());
180 while (!AvailableQueue->empty() || !PendingQueue.empty()) {
181 // Check to see if any of the pending instructions are ready to issue. If
182 // so, add them to the available queue.
183 for (unsigned i = 0, e = PendingQueue.size(); i != e; ++i) {
184 if (PendingQueue[i]->getDepth() == CurCycle) {
185 AvailableQueue->push(U: PendingQueue[i]);
186 PendingQueue[i]->isAvailable = true;
187 PendingQueue[i] = PendingQueue.back();
188 PendingQueue.pop_back();
189 --i; --e;
190 }
191 else {
192 assert(PendingQueue[i]->getDepth() > CurCycle && "Negative latency?");
193 }
194 }
195
196 // If there are no instructions available, don't try to issue anything, and
197 // don't advance the hazard recognizer.
198 if (AvailableQueue->empty()) {
199 // Reset DFA state.
200 AvailableQueue->scheduledNode(nullptr);
201 ++CurCycle;
202 continue;
203 }
204
205 SUnit *FoundSUnit = nullptr;
206
207 bool HasNoopHazards = false;
208 while (!AvailableQueue->empty()) {
209 SUnit *CurSUnit = AvailableQueue->pop();
210
211 ScheduleHazardRecognizer::HazardType HT =
212 HazardRec->getHazardType(CurSUnit, Stalls: 0/*no stalls*/);
213 if (HT == ScheduleHazardRecognizer::NoHazard) {
214 FoundSUnit = CurSUnit;
215 break;
216 }
217
218 // Remember if this is a noop hazard.
219 HasNoopHazards |= HT == ScheduleHazardRecognizer::NoopHazard;
220
221 NotReady.push_back(x: CurSUnit);
222 }
223
224 // Add the nodes that aren't ready back onto the available list.
225 if (!NotReady.empty()) {
226 AvailableQueue->push_all(Nodes: NotReady);
227 NotReady.clear();
228 }
229
230 // If we found a node to schedule, do it now.
231 if (FoundSUnit) {
232 scheduleNodeTopDown(SU: FoundSUnit, CurCycle);
233 HazardRec->EmitInstruction(FoundSUnit);
234
235 // If this is a pseudo-op node, we don't want to increment the current
236 // cycle.
237 if (FoundSUnit->Latency) // Don't increment CurCycle for pseudo-ops!
238 ++CurCycle;
239 } else if (!HasNoopHazards) {
240 // Otherwise, we have a pipeline stall, but no other problem, just advance
241 // the current cycle and try again.
242 LLVM_DEBUG(dbgs() << "*** Advancing cycle, no work to do\n");
243 HazardRec->AdvanceCycle();
244 ++NumStalls;
245 ++CurCycle;
246 } else {
247 // Otherwise, we have no instructions to issue and we have instructions
248 // that will fault if we don't do this right. This is the case for
249 // processors without pipeline interlocks and other cases.
250 LLVM_DEBUG(dbgs() << "*** Emitting noop\n");
251 HazardRec->EmitNoop();
252 Sequence.push_back(x: nullptr); // NULL here means noop
253 ++NumNoops;
254 ++CurCycle;
255 }
256 }
257
258#ifndef NDEBUG
259 VerifyScheduledSequence(/*isBottomUp=*/false);
260#endif
261}
262
263//===----------------------------------------------------------------------===//
264// Public Constructor Functions
265//===----------------------------------------------------------------------===//
266
267/// createVLIWDAGScheduler - This creates a top-down list scheduler.
268ScheduleDAGSDNodes *llvm::createVLIWDAGScheduler(SelectionDAGISel *IS,
269 CodeGenOptLevel) {
270 return new ScheduleDAGVLIW(*IS->MF, IS->AA, new ResourcePriorityQueue(IS));
271}
272