1//===- MacroFusion.cpp - Macro Fusion -------------------------------------===//
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/// \file This file contains the implementation of the DAG scheduling mutation
10/// to pair instructions back to back.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/CodeGen/MacroFusion.h"
15#include "llvm/ADT/Statistic.h"
16#include "llvm/CodeGen/MachineInstr.h"
17#include "llvm/CodeGen/ScheduleDAG.h"
18#include "llvm/CodeGen/ScheduleDAGInstrs.h"
19#include "llvm/CodeGen/ScheduleDAGMutation.h"
20#include "llvm/CodeGen/TargetInstrInfo.h"
21#include "llvm/Support/CommandLine.h"
22#include "llvm/Support/Debug.h"
23#include "llvm/Support/raw_ostream.h"
24
25#define DEBUG_TYPE "machine-scheduler"
26
27STATISTIC(NumFused, "Number of instr pairs fused");
28
29using namespace llvm;
30
31static cl::opt<bool> EnableMacroFusion("misched-fusion", cl::Hidden,
32 cl::desc("Enable scheduling for macro fusion."), cl::init(Val: true));
33
34static bool isHazard(const SDep &Dep) {
35 return Dep.getKind() == SDep::Anti || Dep.getKind() == SDep::Output;
36}
37
38static SUnit *getPredClusterSU(const SUnit &SU) {
39 for (const SDep &SI : SU.Preds)
40 if (SI.isCluster())
41 return SI.getSUnit();
42
43 return nullptr;
44}
45
46bool llvm::hasLessThanNumFused(const SUnit &SU, unsigned FuseLimit) {
47 unsigned Num = 1;
48 const SUnit *CurrentSU = &SU;
49 while ((CurrentSU = getPredClusterSU(SU: *CurrentSU)) && Num < FuseLimit) Num ++;
50 return Num < FuseLimit;
51}
52
53bool llvm::fuseInstructionPair(ScheduleDAGInstrs &DAG, SUnit &FirstSU,
54 SUnit &SecondSU) {
55 // Check that neither instr is already paired with another along the edge
56 // between them.
57 for (SDep &SI : FirstSU.Succs)
58 if (SI.isCluster())
59 return false;
60
61 for (SDep &SI : SecondSU.Preds)
62 if (SI.isCluster())
63 return false;
64
65 assert(FirstSU.ParentClusterIdx == InvalidClusterId &&
66 SecondSU.ParentClusterIdx == InvalidClusterId);
67
68 // Though the reachability checks above could be made more generic,
69 // perhaps as part of ScheduleDAGInstrs::addEdge(), since such edges are valid,
70 // the extra computation cost makes it less interesting in general cases.
71
72 // Create a single weak edge between the adjacent instrs. The only effect is
73 // to cause bottom-up scheduling to heavily prioritize the clustered instrs.
74 if (!DAG.addEdge(SuccSU: &SecondSU, PredDep: SDep(&FirstSU, SDep::Cluster)))
75 return false;
76
77 auto &Clusters = DAG.getClusters();
78
79 FirstSU.ParentClusterIdx = Clusters.size();
80 SecondSU.ParentClusterIdx = Clusters.size();
81
82 SmallSet<SUnit *, 8> Cluster{{&FirstSU, &SecondSU}};
83 Clusters.push_back(Elt: Cluster);
84
85 // TODO - If we want to chain more than two instructions, we need to create
86 // artifical edges to make dependencies from the FirstSU also dependent
87 // on other chained instructions, and other chained instructions also
88 // dependent on the dependencies of the SecondSU, to prevent them from being
89 // scheduled into these chained instructions.
90 assert(hasLessThanNumFused(FirstSU, 2) &&
91 "Currently we only support chaining together two instructions");
92
93 // Adjust the latency between both instrs.
94 for (SDep &SI : FirstSU.Succs)
95 if (SI.getSUnit() == &SecondSU)
96 SI.setLatency(0);
97
98 for (SDep &SI : SecondSU.Preds)
99 if (SI.getSUnit() == &FirstSU)
100 SI.setLatency(0);
101
102 LLVM_DEBUG(
103 dbgs() << "Macro fuse: "; DAG.dumpNodeName(FirstSU); dbgs() << " - ";
104 DAG.dumpNodeName(SecondSU); dbgs() << " / ";
105 dbgs() << DAG.TII->getName(FirstSU.getInstr()->getOpcode()) << " - "
106 << DAG.TII->getName(SecondSU.getInstr()->getOpcode()) << '\n';);
107
108 // Make data dependencies from the FirstSU also dependent on the SecondSU to
109 // prevent them from being scheduled between the FirstSU and the SecondSU.
110 if (&SecondSU != &DAG.ExitSU)
111 for (const SDep &SI : FirstSU.Succs) {
112 SUnit *SU = SI.getSUnit();
113 if (SI.isWeak() || isHazard(Dep: SI) ||
114 SU == &DAG.ExitSU || SU == &SecondSU || SU->isPred(N: &SecondSU))
115 continue;
116 LLVM_DEBUG(dbgs() << " Bind "; DAG.dumpNodeName(SecondSU);
117 dbgs() << " - "; DAG.dumpNodeName(*SU); dbgs() << '\n';);
118 DAG.addEdge(SuccSU: SU, PredDep: SDep(&SecondSU, SDep::Artificial));
119 }
120
121 // Make the FirstSU also dependent on the dependencies of the SecondSU to
122 // prevent them from being scheduled between the FirstSU and the SecondSU.
123 if (&FirstSU != &DAG.EntrySU) {
124 for (const SDep &SI : SecondSU.Preds) {
125 SUnit *SU = SI.getSUnit();
126 if (SI.isWeak() || isHazard(Dep: SI) || &FirstSU == SU || FirstSU.isSucc(N: SU))
127 continue;
128 LLVM_DEBUG(dbgs() << " Bind "; DAG.dumpNodeName(*SU); dbgs() << " - ";
129 DAG.dumpNodeName(FirstSU); dbgs() << '\n';);
130 DAG.addEdge(SuccSU: &FirstSU, PredDep: SDep(SU, SDep::Artificial));
131 }
132 // ExitSU comes last by design, which acts like an implicit dependency
133 // between ExitSU and any bottom root in the graph. We should transfer
134 // this to FirstSU as well.
135 if (&SecondSU == &DAG.ExitSU) {
136 for (SUnit &SU : DAG.SUnits) {
137 if (SU.Succs.empty())
138 DAG.addEdge(SuccSU: &FirstSU, PredDep: SDep(&SU, SDep::Artificial));
139 }
140 }
141 }
142
143 ++NumFused;
144 return true;
145}
146
147namespace {
148
149/// Post-process the DAG to create cluster edges between instrs that may
150/// be fused by the processor into a single operation.
151class MacroFusion : public ScheduleDAGMutation {
152 std::vector<MacroFusionPredTy> Predicates;
153 bool FuseBlock;
154 bool scheduleAdjacentImpl(ScheduleDAGInstrs &DAG, SUnit &AnchorSU);
155
156public:
157 MacroFusion(ArrayRef<MacroFusionPredTy> Predicates, bool FuseBlock)
158 : Predicates(Predicates.begin(), Predicates.end()), FuseBlock(FuseBlock) {
159 }
160
161 void apply(ScheduleDAGInstrs *DAGInstrs) override;
162
163 bool shouldScheduleAdjacent(const TargetInstrInfo &TII,
164 const TargetSubtargetInfo &STI,
165 const MachineInstr *FirstMI,
166 const MachineInstr &SecondMI);
167};
168
169} // end anonymous namespace
170
171bool MacroFusion::shouldScheduleAdjacent(const TargetInstrInfo &TII,
172 const TargetSubtargetInfo &STI,
173 const MachineInstr *FirstMI,
174 const MachineInstr &SecondMI) {
175 return llvm::any_of(Range&: Predicates, P: [&](MacroFusionPredTy Predicate) {
176 return Predicate(TII, STI, FirstMI, SecondMI);
177 });
178}
179
180void MacroFusion::apply(ScheduleDAGInstrs *DAG) {
181 if (FuseBlock)
182 // For each of the SUnits in the scheduling block, try to fuse the instr in
183 // it with one in its predecessors.
184 for (SUnit &ISU : DAG->SUnits)
185 scheduleAdjacentImpl(DAG&: *DAG, AnchorSU&: ISU);
186
187 if (DAG->ExitSU.getInstr())
188 // Try to fuse the instr in the ExitSU with one in its predecessors.
189 scheduleAdjacentImpl(DAG&: *DAG, AnchorSU&: DAG->ExitSU);
190}
191
192/// Implement the fusion of instr pairs in the scheduling DAG,
193/// anchored at the instr in AnchorSU..
194bool MacroFusion::scheduleAdjacentImpl(ScheduleDAGInstrs &DAG, SUnit &AnchorSU) {
195 const MachineInstr &AnchorMI = *AnchorSU.getInstr();
196 const TargetInstrInfo &TII = *DAG.TII;
197 const TargetSubtargetInfo &ST = DAG.MF.getSubtarget();
198
199 // Check if the anchor instr may be fused.
200 if (!shouldScheduleAdjacent(TII, STI: ST, FirstMI: nullptr, SecondMI: AnchorMI))
201 return false;
202
203 // Explorer for fusion candidates among the dependencies of the anchor instr.
204 for (SDep &Dep : AnchorSU.Preds) {
205 // Ignore dependencies other than data or strong ordering.
206 if (Dep.isWeak() || isHazard(Dep))
207 continue;
208
209 SUnit &DepSU = *Dep.getSUnit();
210 if (DepSU.isBoundaryNode())
211 continue;
212
213 // Only chain two instructions together at most.
214 const MachineInstr *DepMI = DepSU.getInstr();
215 if (!hasLessThanNumFused(SU: DepSU, FuseLimit: 2) ||
216 !shouldScheduleAdjacent(TII, STI: ST, FirstMI: DepMI, SecondMI: AnchorMI))
217 continue;
218
219 if (fuseInstructionPair(DAG, FirstSU&: DepSU, SecondSU&: AnchorSU))
220 return true;
221 }
222
223 return false;
224}
225
226std::unique_ptr<ScheduleDAGMutation>
227llvm::createMacroFusionDAGMutation(ArrayRef<MacroFusionPredTy> Predicates,
228 bool BranchOnly) {
229 if (EnableMacroFusion)
230 return std::make_unique<MacroFusion>(args&: Predicates, args: !BranchOnly);
231 return nullptr;
232}
233