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//===- ModuloSchedule.h - Software pipeline schedule expansion ------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// Software pipelining (SWP) is an instruction scheduling technique for loops
// that overlaps loop iterations and exploits ILP via compiler transformations.
// There are multiple methods for analyzing a loop and creating a schedule.
// An example algorithm is Swing Modulo Scheduling (implemented by the
// MachinePipeliner). The details of how a schedule is arrived at are irrelevant
// for the task of actually rewriting a loop to adhere to the schedule, which
// is what this file does.
// A schedule is, for every instruction in a block, a Cycle and a Stage. Note
// that we only support single-block loops, so "block" and "loop" can be used
// interchangably.
// The Cycle of an instruction defines a partial order of the instructions in
// the remapped loop. Instructions within a cycle must not consume the output
// of any instruction in the same cycle. Cycle information is assumed to have
// been calculated such that the processor will execute instructions in
// lock-step (for example in a VLIW ISA).
// The Stage of an instruction defines the mapping between logical loop
// iterations and pipelined loop iterations. An example (unrolled) pipeline
// may look something like:
// I0[0] Execute instruction I0 of iteration 0
// I1[0], I0[1] Execute I0 of iteration 1 and I1 of iteration 1
// I1[1], I0[2]
// I1[2], I0[3]
// In the schedule for this unrolled sequence we would say that I0 was scheduled
// in stage 0 and I1 in stage 1:
// loop:
// [stage 0] x = I0
// [stage 1] I1 x (from stage 0)
// And to actually generate valid code we must insert a phi:
// loop:
// x' = phi(x)
// x = I0
// I1 x'
// This is a simple example; the rules for how to generate correct code given
// an arbitrary schedule containing loop-carried values are complex.
// Note that these examples only mention the steady-state kernel of the
// generated loop; prologs and epilogs must be generated also that prime and
// flush the pipeline. Doing so is nontrivial.
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineLoopUtils.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include <deque>
#include <vector>
namespace llvm {
class MachineBasicBlock;
class MachineInstr;
class LiveIntervals;
/// Represents a schedule for a single-block loop. For every instruction we
/// maintain a Cycle and Stage.
class ModuloSchedule {
/// The block containing the loop instructions.
MachineLoop *Loop;
/// The instructions to be generated, in total order. Cycle provides a partial
/// order; the total order within cycles has been decided by the schedule
/// producer.
std::vector<MachineInstr *> ScheduledInstrs;
/// The cycle for each instruction.
DenseMap<MachineInstr *, int> Cycle;
/// The stage for each instruction.
DenseMap<MachineInstr *, int> Stage;
/// The number of stages in this schedule (Max(Stage) + 1).
int NumStages;
/// Create a new ModuloSchedule.
/// \arg ScheduledInstrs The new loop instructions, in total resequenced
/// order.
/// \arg Cycle Cycle index for all instructions in ScheduledInstrs. Cycle does
/// not need to start at zero. ScheduledInstrs must be partially ordered by
/// Cycle.
/// \arg Stage Stage index for all instructions in ScheduleInstrs.
ModuloSchedule(MachineFunction &MF, MachineLoop *Loop,
std::vector<MachineInstr *> ScheduledInstrs,
DenseMap<MachineInstr *, int> Cycle,
DenseMap<MachineInstr *, int> Stage)
: Loop(Loop), ScheduledInstrs(ScheduledInstrs), Cycle(std::move(Cycle)),
Stage(std::move(Stage)) {
NumStages = 0;
for (auto &KV : this->Stage)
NumStages = std::max(NumStages, KV.second);
/// Return the single-block loop being scheduled.
MachineLoop *getLoop() const { return Loop; }
/// Return the number of stages contained in this schedule, which is the
/// largest stage index + 1.
int getNumStages() const { return NumStages; }
/// Return the first cycle in the schedule, which is the cycle index of the
/// first instruction.
int getFirstCycle() { return Cycle[ScheduledInstrs.front()]; }
/// Return the final cycle in the schedule, which is the cycle index of the
/// last instruction.
int getFinalCycle() { return Cycle[ScheduledInstrs.back()]; }
/// Return the stage that MI is scheduled in, or -1.
int getStage(MachineInstr *MI) {
auto I = Stage.find(MI);
return I == Stage.end() ? -1 : I->second;
/// Return the cycle that MI is scheduled at, or -1.
int getCycle(MachineInstr *MI) {
auto I = Cycle.find(MI);
return I == Cycle.end() ? -1 : I->second;
/// Return the rescheduled instructions in order.
ArrayRef<MachineInstr *> getInstructions() { return ScheduledInstrs; }
void dump() { print(dbgs()); }
void print(raw_ostream &OS);
/// The ModuloScheduleExpander takes a ModuloSchedule and expands it in-place,
/// rewriting the old loop and inserting prologs and epilogs as required.
class ModuloScheduleExpander {
using InstrChangesTy = DenseMap<MachineInstr *, std::pair<unsigned, int64_t>>;
using ValueMapTy = DenseMap<unsigned, unsigned>;
using MBBVectorTy = SmallVectorImpl<MachineBasicBlock *>;
using InstrMapTy = DenseMap<MachineInstr *, MachineInstr *>;
ModuloSchedule &Schedule;
MachineFunction &MF;
const TargetSubtargetInfo &ST;
MachineRegisterInfo &MRI;
const TargetInstrInfo *TII;
LiveIntervals &LIS;
MachineBasicBlock *BB;
MachineBasicBlock *Preheader;
MachineBasicBlock *NewKernel = nullptr;
std::unique_ptr<TargetInstrInfo::PipelinerLoopInfo> LoopInfo;
/// Map for each register and the max difference between its uses and def.
/// The first element in the pair is the max difference in stages. The
/// second is true if the register defines a Phi value and loop value is
/// scheduled before the Phi.
std::map<unsigned, std::pair<unsigned, bool>> RegToStageDiff;
/// Instructions to change when emitting the final schedule.
InstrChangesTy InstrChanges;
void generatePipelinedLoop();
void generateProlog(unsigned LastStage, MachineBasicBlock *KernelBB,
ValueMapTy *VRMap, MBBVectorTy &PrologBBs);
void generateEpilog(unsigned LastStage, MachineBasicBlock *KernelBB,
ValueMapTy *VRMap, MBBVectorTy &EpilogBBs,
MBBVectorTy &PrologBBs);
void generateExistingPhis(MachineBasicBlock *NewBB, MachineBasicBlock *BB1,
MachineBasicBlock *BB2, MachineBasicBlock *KernelBB,
ValueMapTy *VRMap, InstrMapTy &InstrMap,
unsigned LastStageNum, unsigned CurStageNum,
bool IsLast);
void generatePhis(MachineBasicBlock *NewBB, MachineBasicBlock *BB1,
MachineBasicBlock *BB2, MachineBasicBlock *KernelBB,
ValueMapTy *VRMap, InstrMapTy &InstrMap,
unsigned LastStageNum, unsigned CurStageNum, bool IsLast);
void removeDeadInstructions(MachineBasicBlock *KernelBB,
MBBVectorTy &EpilogBBs);
void splitLifetimes(MachineBasicBlock *KernelBB, MBBVectorTy &EpilogBBs);
void addBranches(MachineBasicBlock &PreheaderBB, MBBVectorTy &PrologBBs,
MachineBasicBlock *KernelBB, MBBVectorTy &EpilogBBs,
ValueMapTy *VRMap);
bool computeDelta(MachineInstr &MI, unsigned &Delta);
void updateMemOperands(MachineInstr &NewMI, MachineInstr &OldMI,
unsigned Num);
MachineInstr *cloneInstr(MachineInstr *OldMI, unsigned CurStageNum,
unsigned InstStageNum);
MachineInstr *cloneAndChangeInstr(MachineInstr *OldMI, unsigned CurStageNum,
unsigned InstStageNum);
void updateInstruction(MachineInstr *NewMI, bool LastDef,
unsigned CurStageNum, unsigned InstrStageNum,
ValueMapTy *VRMap);
MachineInstr *findDefInLoop(unsigned Reg);
unsigned getPrevMapVal(unsigned StageNum, unsigned PhiStage, unsigned LoopVal,
unsigned LoopStage, ValueMapTy *VRMap,
MachineBasicBlock *BB);
void rewritePhiValues(MachineBasicBlock *NewBB, unsigned StageNum,
ValueMapTy *VRMap, InstrMapTy &InstrMap);
void rewriteScheduledInstr(MachineBasicBlock *BB, InstrMapTy &InstrMap,
unsigned CurStageNum, unsigned PhiNum,
MachineInstr *Phi, unsigned OldReg,
unsigned NewReg, unsigned PrevReg = 0);
bool isLoopCarried(MachineInstr &Phi);
/// Return the max. number of stages/iterations that can occur between a
/// register definition and its uses.
unsigned getStagesForReg(int Reg, unsigned CurStage) {
std::pair<unsigned, bool> Stages = RegToStageDiff[Reg];
if ((int)CurStage > Schedule.getNumStages() - 1 && Stages.first == 0 &&
return 1;
return Stages.first;
/// The number of stages for a Phi is a little different than other
/// instructions. The minimum value computed in RegToStageDiff is 1
/// because we assume the Phi is needed for at least 1 iteration.
/// This is not the case if the loop value is scheduled prior to the
/// Phi in the same stage. This function returns the number of stages
/// or iterations needed between the Phi definition and any uses.
unsigned getStagesForPhi(int Reg) {
std::pair<unsigned, bool> Stages = RegToStageDiff[Reg];
if (Stages.second)
return Stages.first;
return Stages.first - 1;
/// Create a new ModuloScheduleExpander.
/// \arg InstrChanges Modifications to make to instructions with memory
/// operands.
/// FIXME: InstrChanges is opaque and is an implementation detail of an
/// optimization in MachinePipeliner that crosses abstraction boundaries.
ModuloScheduleExpander(MachineFunction &MF, ModuloSchedule &S,
LiveIntervals &LIS, InstrChangesTy InstrChanges)
: Schedule(S), MF(MF), ST(MF.getSubtarget()), MRI(MF.getRegInfo()),
TII(ST.getInstrInfo()), LIS(LIS),
InstrChanges(std::move(InstrChanges)) {}
/// Performs the actual expansion.
void expand();
/// Performs final cleanup after expansion.
void cleanup();
/// Returns the newly rewritten kernel block, or nullptr if this was
/// optimized away.
MachineBasicBlock *getRewrittenKernel() { return NewKernel; }
/// A reimplementation of ModuloScheduleExpander. It works by generating a
/// standalone kernel loop and peeling out the prologs and epilogs.
class PeelingModuloScheduleExpander {
ModuloSchedule &Schedule;
MachineFunction &MF;
const TargetSubtargetInfo &ST;
MachineRegisterInfo &MRI;
const TargetInstrInfo *TII;
LiveIntervals *LIS;
/// The original loop block that gets rewritten in-place.
MachineBasicBlock *BB;
/// The original loop preheader.
MachineBasicBlock *Preheader;
/// All prolog and epilog blocks.
SmallVector<MachineBasicBlock *, 4> Prologs, Epilogs;
/// For every block, the stages that are produced.
DenseMap<MachineBasicBlock *, BitVector> LiveStages;
/// For every block, the stages that are available. A stage can be available
/// but not produced (in the epilog) or produced but not available (in the
/// prolog).
DenseMap<MachineBasicBlock *, BitVector> AvailableStages;
/// When peeling the epilogue keep track of the distance between the phi
/// nodes and the kernel.
DenseMap<MachineInstr *, unsigned> PhiNodeLoopIteration;
/// CanonicalMIs and BlockMIs form a bidirectional map between any of the
/// loop kernel clones.
DenseMap<MachineInstr *, MachineInstr *> CanonicalMIs;
DenseMap<std::pair<MachineBasicBlock *, MachineInstr *>, MachineInstr *>
/// State passed from peelKernel to peelPrologAndEpilogs().
std::deque<MachineBasicBlock *> PeeledFront, PeeledBack;
/// Illegal phis that need to be deleted once we re-link stages.
SmallVector<MachineInstr *, 4> IllegalPhisToDelete;
PeelingModuloScheduleExpander(MachineFunction &MF, ModuloSchedule &S,
LiveIntervals *LIS)
: Schedule(S), MF(MF), ST(MF.getSubtarget()), MRI(MF.getRegInfo()),
TII(ST.getInstrInfo()), LIS(LIS) {}
void expand();
/// Runs ModuloScheduleExpander and treats it as a golden input to validate
/// aspects of the code generated by PeelingModuloScheduleExpander.
void validateAgainstModuloScheduleExpander();
/// Converts BB from the original loop body to the rewritten, pipelined
/// steady-state.
void rewriteKernel();
/// Peels one iteration of the rewritten kernel (BB) in the specified
/// direction.
MachineBasicBlock *peelKernel(LoopPeelDirection LPD);
// Delete instructions whose stage is less than MinStage in the given basic
// block.
void filterInstructions(MachineBasicBlock *MB, int MinStage);
// Move instructions of the given stage from sourceBB to DestBB. Remap the phi
// instructions to keep a valid IR.
void moveStageBetweenBlocks(MachineBasicBlock *DestBB,
MachineBasicBlock *SourceBB, unsigned Stage);
/// Peel the kernel forwards and backwards to produce prologs and epilogs,
/// and stitch them together.
void peelPrologAndEpilogs();
/// All prolog and epilog blocks are clones of the kernel, so any produced
/// register in one block has an corollary in all other blocks.
Register getEquivalentRegisterIn(Register Reg, MachineBasicBlock *BB);
/// Change all users of MI, if MI is predicated out
/// (LiveStages[MI->getParent()] == false).
void rewriteUsesOf(MachineInstr *MI);
/// Insert branches between prologs, kernel and epilogs.
void fixupBranches();
/// Create a poor-man's LCSSA by cloning only the PHIs from the kernel block
/// to a block dominated by all prologs and epilogs. This allows us to treat
/// the loop exiting block as any other kernel clone.
MachineBasicBlock *CreateLCSSAExitingBlock();
/// Helper to get the stage of an instruction in the schedule.
unsigned getStage(MachineInstr *MI) {
if (CanonicalMIs.count(MI))
MI = CanonicalMIs[MI];
return Schedule.getStage(MI);
/// Helper function to find the right canonical register for a phi instruction
/// coming from a peeled out prologue.
Register getPhiCanonicalReg(MachineInstr* CanonicalPhi, MachineInstr* Phi);
/// Target loop info before kernel peeling.
std::unique_ptr<TargetInstrInfo::PipelinerLoopInfo> Info;
/// Expander that simply annotates each scheduled instruction with a post-instr
/// symbol that can be consumed by the ModuloScheduleTest pass.
/// The post-instr symbol is a way of annotating an instruction that can be
/// roundtripped in MIR. The syntax is:
/// MYINST %0, post-instr-symbol <mcsymbol Stage-1_Cycle-5>
class ModuloScheduleTestAnnotater {
MachineFunction &MF;
ModuloSchedule &S;
ModuloScheduleTestAnnotater(MachineFunction &MF, ModuloSchedule &S)
: MF(MF), S(S) {}
/// Performs the annotation.
void annotate();
} // end namespace llvm