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| 1 | +//======----------- WindowScheduler.cpp - window scheduler -------------======// |
| 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 | +// An implementation of the Window Scheduling software pipelining algorithm. |
| 10 | +// |
| 11 | +// The concept of the window algorithm was first unveiled in Steven Muchnick's |
| 12 | +// book, "Advanced Compiler Design And Implementation", and later elaborated |
| 13 | +// upon in Venkatraman Govindaraju's report, "Implementation of Software |
| 14 | +// Pipelining Using Window Scheduling". |
| 15 | +// |
| 16 | +// The window algorithm can be perceived as a modulo scheduling algorithm with a |
| 17 | +// stage count of 2. It boasts a higher scheduling success rate in targets with |
| 18 | +// severe resource conflicts when compared to the classic Swing Modulo |
| 19 | +// Scheduling (SMS) algorithm. To align with the LLVM scheduling framework, we |
| 20 | +// have enhanced the original window algorithm. The primary steps are as |
| 21 | +// follows: |
| 22 | +// |
| 23 | +// 1. Instead of duplicating the original MBB twice as mentioned in the |
| 24 | +// literature, we copy it three times, generating TripleMBB and the |
| 25 | +// corresponding TripleDAG. |
| 26 | +// |
| 27 | +// 2. We establish a scheduling window on TripleMBB and execute list scheduling |
| 28 | +// within it. |
| 29 | +// |
| 30 | +// 3. After multiple list scheduling, we select the best outcome and expand it |
| 31 | +// into the final scheduling result. |
| 32 | +// |
| 33 | +// To cater to the needs of various targets, we have developed the window |
| 34 | +// scheduler in a form that is easily derivable. We recommend employing this |
| 35 | +// algorithm in targets with severe resource conflicts, and it can be utilized |
| 36 | +// either before or after the Register Allocator (RA). |
| 37 | +// |
| 38 | +// The default implementation provided here is before RA. If it is to be used |
| 39 | +// after RA, certain critical algorithm functions will need to be derived. |
| 40 | +// |
| 41 | +//===----------------------------------------------------------------------===// |
| 42 | +#ifndef LLVM_CODEGEN_WINDOWSCHEDULER_H |
| 43 | +#define LLVM_CODEGEN_WINDOWSCHEDULER_H |
| 44 | + |
| 45 | +#include "llvm/CodeGen/MachineLoopInfo.h" |
| 46 | +#include "llvm/CodeGen/MachineRegisterInfo.h" |
| 47 | +#include "llvm/CodeGen/MachineScheduler.h" |
| 48 | +#include "llvm/CodeGen/ScheduleDAGInstrs.h" |
| 49 | +#include "llvm/CodeGen/TargetSubtargetInfo.h" |
| 50 | + |
| 51 | +namespace llvm { |
| 52 | + |
| 53 | +enum WindowSchedulingFlag { |
| 54 | + WS_Off, /// Turn off window algorithm. |
| 55 | + WS_On, /// Use window algorithm after SMS algorithm fails. |
| 56 | + WS_Force /// Use window algorithm instead of SMS algorithm. |
| 57 | +}; |
| 58 | + |
| 59 | +/// The main class in the implementation of the target independent window |
| 60 | +/// scheduler. |
| 61 | +class WindowScheduler { |
| 62 | +protected: |
| 63 | + MachineSchedContext *Context = nullptr; |
| 64 | + MachineFunction *MF = nullptr; |
| 65 | + MachineBasicBlock *MBB = nullptr; |
| 66 | + MachineLoop &Loop; |
| 67 | + const TargetSubtargetInfo *Subtarget = nullptr; |
| 68 | + const TargetInstrInfo *TII = nullptr; |
| 69 | + const TargetRegisterInfo *TRI = nullptr; |
| 70 | + MachineRegisterInfo *MRI = nullptr; |
| 71 | + |
| 72 | + /// To innovatively identify the dependencies between MIs across two trips, we |
| 73 | + /// construct a DAG for a new MBB, which is created by copying the original |
| 74 | + /// MBB three times. We refer to this new MBB as 'TripleMBB' and the |
| 75 | + /// corresponding DAG as 'TripleDAG'. |
| 76 | + /// If the dependencies are more than two trips, we avoid applying window |
| 77 | + /// algorithm by identifying successive phis in the old MBB. |
| 78 | + std::unique_ptr<ScheduleDAGInstrs> TripleDAG; |
| 79 | + /// OriMIs keeps the MIs removed from the original MBB. |
| 80 | + SmallVector<MachineInstr *> OriMIs; |
| 81 | + /// TriMIs keeps the MIs of TripleMBB, which is used to restore TripleMBB. |
| 82 | + SmallVector<MachineInstr *> TriMIs; |
| 83 | + /// TriToOri keeps the mappings between the MI clones in TripleMBB and their |
| 84 | + /// original MI. |
| 85 | + DenseMap<MachineInstr *, MachineInstr *> TriToOri; |
| 86 | + /// OriToCycle keeps the mappings between the original MI and its issue cycle. |
| 87 | + DenseMap<MachineInstr *, int> OriToCycle; |
| 88 | + /// SchedResult keeps the result of each list scheduling, and the format of |
| 89 | + /// the tuple is <MI pointer, Cycle, Stage, Order ID>. |
| 90 | + SmallVector<std::tuple<MachineInstr *, int, int, int>, 256> SchedResult; |
| 91 | + /// SchedPhiNum records the number of phi in the original MBB, and the |
| 92 | + /// scheduling starts with MI after phis. |
| 93 | + unsigned SchedPhiNum = 0; |
| 94 | + /// SchedInstrNum records the MIs involved in scheduling in the original MBB, |
| 95 | + /// excluding debug instructions. |
| 96 | + unsigned SchedInstrNum = 0; |
| 97 | + /// BestII and BestOffset record the characteristics of the best scheduling |
| 98 | + /// result and are used together with SchedResult as the final window |
| 99 | + /// scheduling result. |
| 100 | + unsigned BestII = UINT_MAX; |
| 101 | + unsigned BestOffset = 0; |
| 102 | + /// BaseII is the II obtained when the window offset is SchedPhiNum. This |
| 103 | + /// offset is the initial position of the sliding window. |
| 104 | + unsigned BaseII = 0; |
| 105 | + |
| 106 | +public: |
| 107 | + WindowScheduler(MachineSchedContext *C, MachineLoop &ML); |
| 108 | + virtual ~WindowScheduler() {} |
| 109 | + |
| 110 | + bool run(); |
| 111 | + |
| 112 | +protected: |
| 113 | + /// Two types of ScheduleDAGs are needed, one for creating dependency graphs |
| 114 | + /// only, and the other for list scheduling as determined by the target. |
| 115 | + virtual ScheduleDAGInstrs * |
| 116 | + createMachineScheduler(bool OnlyBuildGraph = false); |
| 117 | + /// Initializes the algorithm and determines if it can be executed. |
| 118 | + virtual bool initialize(); |
| 119 | + /// Add some related processing before running window scheduling. |
| 120 | + virtual void preProcess(); |
| 121 | + /// Add some related processing after running window scheduling. |
| 122 | + virtual void postProcess(); |
| 123 | + /// Back up the MIs in the original MBB and remove them from MBB. |
| 124 | + void backupMBB(); |
| 125 | + /// Erase the MIs in current MBB and restore the original MIs. |
| 126 | + void restoreMBB(); |
| 127 | + /// Make three copies of the original MBB to generate a new TripleMBB. |
| 128 | + virtual void generateTripleMBB(); |
| 129 | + /// Restore the order of MIs in TripleMBB after each list scheduling. |
| 130 | + virtual void restoreTripleMBB(); |
| 131 | + /// Give the folding position in the window algorithm, where different |
| 132 | + /// heuristics can be used. It determines the performance and compilation time |
| 133 | + /// of the algorithm. |
| 134 | + virtual SmallVector<unsigned> getSearchIndexes(unsigned SearchNum, |
| 135 | + unsigned SearchRatio); |
| 136 | + /// Calculate MIs execution cycle after list scheduling. |
| 137 | + virtual int calculateMaxCycle(ScheduleDAGInstrs &DAG, unsigned Offset); |
| 138 | + /// Calculate the stall cycle between two trips after list scheduling. |
| 139 | + virtual int calculateStallCycle(unsigned Offset, int MaxCycle); |
| 140 | + /// Analyzes the II value after each list scheduling. |
| 141 | + virtual unsigned analyseII(ScheduleDAGInstrs &DAG, unsigned Offset); |
| 142 | + /// Phis are scheduled separately after each list scheduling. |
| 143 | + virtual void schedulePhi(int Offset, unsigned &II); |
| 144 | + /// Get the final issue order of all scheduled MIs including phis. |
| 145 | + DenseMap<MachineInstr *, int> getIssueOrder(unsigned Offset, unsigned II); |
| 146 | + /// Update the scheduling result after each list scheduling. |
| 147 | + virtual void updateScheduleResult(unsigned Offset, unsigned II); |
| 148 | + /// Check whether the final result of window scheduling is valid. |
| 149 | + virtual bool isScheduleValid() { return BestOffset != SchedPhiNum; } |
| 150 | + /// Using the scheduling infrastructure to expand the results of window |
| 151 | + /// scheduling. It is usually necessary to add prologue and epilogue MBBs. |
| 152 | + virtual void expand(); |
| 153 | + /// Update the live intervals for all registers used within MBB. |
| 154 | + virtual void updateLiveIntervals(); |
| 155 | + /// Estimate a II value at which all MIs will be scheduled successfully. |
| 156 | + int getEstimatedII(ScheduleDAGInstrs &DAG); |
| 157 | + /// Gets the iterator range of MIs in the scheduling window. |
| 158 | + iterator_range<MachineBasicBlock::iterator> getScheduleRange(unsigned Offset, |
| 159 | + unsigned Num); |
| 160 | + /// Get the issue cycle of the new MI based on the cycle of the original MI. |
| 161 | + int getOriCycle(MachineInstr *NewMI); |
| 162 | + /// Get the original MI from which the new MI is cloned. |
| 163 | + MachineInstr *getOriMI(MachineInstr *NewMI); |
| 164 | + /// Get the scheduling stage, where the stage of the new MI is identical to |
| 165 | + /// the original MI. |
| 166 | + unsigned getOriStage(MachineInstr *OriMI, unsigned Offset); |
| 167 | + /// Gets the register in phi which is generated from the current MBB. |
| 168 | + Register getAntiRegister(MachineInstr *Phi); |
| 169 | +}; |
| 170 | +} // namespace llvm |
| 171 | +#endif |
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