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llvm-project/llvm/lib/CodeGen/MachineCombiner.cpp

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//===---- MachineCombiner.cpp - Instcombining on SSA form machine code ----===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// The machine combiner pass uses machine trace metrics to ensure the combined
// instructions do not lengthen the critical path or the resource depth.
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "machine-combiner"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/MachineTraceMetrics.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetSchedule.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetSubtargetInfo.h"
using namespace llvm;
STATISTIC(NumInstCombined, "Number of machineinst combined");
namespace {
class MachineCombiner : public MachineFunctionPass {
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
MCSchedModel SchedModel;
MachineRegisterInfo *MRI;
MachineLoopInfo *MLI; // Current MachineLoopInfo
MachineTraceMetrics *Traces;
MachineTraceMetrics::Ensemble *MinInstr;
TargetSchedModel TSchedModel;
/// True if optimizing for code size.
bool OptSize;
public:
static char ID;
MachineCombiner() : MachineFunctionPass(ID) {
initializeMachineCombinerPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool runOnMachineFunction(MachineFunction &MF) override;
StringRef getPassName() const override { return "Machine InstCombiner"; }
private:
bool doSubstitute(unsigned NewSize, unsigned OldSize);
bool combineInstructions(MachineBasicBlock *);
MachineInstr *getOperandDef(const MachineOperand &MO);
unsigned getDepth(SmallVectorImpl<MachineInstr *> &InsInstrs,
DenseMap<unsigned, unsigned> &InstrIdxForVirtReg,
MachineTraceMetrics::Trace BlockTrace);
unsigned getLatency(MachineInstr *Root, MachineInstr *NewRoot,
MachineTraceMetrics::Trace BlockTrace);
bool
improvesCriticalPathLen(MachineBasicBlock *MBB, MachineInstr *Root,
MachineTraceMetrics::Trace BlockTrace,
SmallVectorImpl<MachineInstr *> &InsInstrs,
SmallVectorImpl<MachineInstr *> &DelInstrs,
DenseMap<unsigned, unsigned> &InstrIdxForVirtReg,
MachineCombinerPattern Pattern);
bool preservesResourceLen(MachineBasicBlock *MBB,
MachineTraceMetrics::Trace BlockTrace,
SmallVectorImpl<MachineInstr *> &InsInstrs,
SmallVectorImpl<MachineInstr *> &DelInstrs);
void instr2instrSC(SmallVectorImpl<MachineInstr *> &Instrs,
SmallVectorImpl<const MCSchedClassDesc *> &InstrsSC);
};
}
char MachineCombiner::ID = 0;
char &llvm::MachineCombinerID = MachineCombiner::ID;
INITIALIZE_PASS_BEGIN(MachineCombiner, "machine-combiner",
"Machine InstCombiner", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_PASS_DEPENDENCY(MachineTraceMetrics)
INITIALIZE_PASS_END(MachineCombiner, "machine-combiner", "Machine InstCombiner",
false, false)
void MachineCombiner::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addPreserved<MachineDominatorTree>();
AU.addRequired<MachineLoopInfo>();
AU.addPreserved<MachineLoopInfo>();
AU.addRequired<MachineTraceMetrics>();
AU.addPreserved<MachineTraceMetrics>();
MachineFunctionPass::getAnalysisUsage(AU);
}
MachineInstr *MachineCombiner::getOperandDef(const MachineOperand &MO) {
MachineInstr *DefInstr = nullptr;
// We need a virtual register definition.
if (MO.isReg() && TargetRegisterInfo::isVirtualRegister(MO.getReg()))
DefInstr = MRI->getUniqueVRegDef(MO.getReg());
// PHI's have no depth etc.
if (DefInstr && DefInstr->isPHI())
DefInstr = nullptr;
return DefInstr;
}
/// Computes depth of instructions in vector \InsInstr.
///
/// \param InsInstrs is a vector of machine instructions
/// \param InstrIdxForVirtReg is a dense map of virtual register to index
/// of defining machine instruction in \p InsInstrs
/// \param BlockTrace is a trace of machine instructions
///
/// \returns Depth of last instruction in \InsInstrs ("NewRoot")
unsigned
MachineCombiner::getDepth(SmallVectorImpl<MachineInstr *> &InsInstrs,
DenseMap<unsigned, unsigned> &InstrIdxForVirtReg,
MachineTraceMetrics::Trace BlockTrace) {
SmallVector<unsigned, 16> InstrDepth;
assert(TSchedModel.hasInstrSchedModelOrItineraries() &&
"Missing machine model\n");
// For each instruction in the new sequence compute the depth based on the
// operands. Use the trace information when possible. For new operands which
// are tracked in the InstrIdxForVirtReg map depth is looked up in InstrDepth
for (auto *InstrPtr : InsInstrs) { // for each Use
unsigned IDepth = 0;
DEBUG(dbgs() << "NEW INSTR ";
InstrPtr->print(dbgs(), TII);
dbgs() << "\n";);
for (const MachineOperand &MO : InstrPtr->operands()) {
// Check for virtual register operand.
if (!(MO.isReg() && TargetRegisterInfo::isVirtualRegister(MO.getReg())))
continue;
if (!MO.isUse())
continue;
unsigned DepthOp = 0;
unsigned LatencyOp = 0;
DenseMap<unsigned, unsigned>::iterator II =
InstrIdxForVirtReg.find(MO.getReg());
if (II != InstrIdxForVirtReg.end()) {
// Operand is new virtual register not in trace
assert(II->second < InstrDepth.size() && "Bad Index");
MachineInstr *DefInstr = InsInstrs[II->second];
assert(DefInstr &&
"There must be a definition for a new virtual register");
DepthOp = InstrDepth[II->second];
LatencyOp = TSchedModel.computeOperandLatency(
DefInstr, DefInstr->findRegisterDefOperandIdx(MO.getReg()),
InstrPtr, InstrPtr->findRegisterUseOperandIdx(MO.getReg()));
} else {
MachineInstr *DefInstr = getOperandDef(MO);
if (DefInstr) {
DepthOp = BlockTrace.getInstrCycles(*DefInstr).Depth;
LatencyOp = TSchedModel.computeOperandLatency(
DefInstr, DefInstr->findRegisterDefOperandIdx(MO.getReg()),
InstrPtr, InstrPtr->findRegisterUseOperandIdx(MO.getReg()));
}
}
IDepth = std::max(IDepth, DepthOp + LatencyOp);
}
InstrDepth.push_back(IDepth);
}
unsigned NewRootIdx = InsInstrs.size() - 1;
return InstrDepth[NewRootIdx];
}
/// Computes instruction latency as max of latency of defined operands.
///
/// \param Root is a machine instruction that could be replaced by NewRoot.
/// It is used to compute a more accurate latency information for NewRoot in
/// case there is a dependent instruction in the same trace (\p BlockTrace)
/// \param NewRoot is the instruction for which the latency is computed
/// \param BlockTrace is a trace of machine instructions
///
/// \returns Latency of \p NewRoot
unsigned MachineCombiner::getLatency(MachineInstr *Root, MachineInstr *NewRoot,
MachineTraceMetrics::Trace BlockTrace) {
assert(TSchedModel.hasInstrSchedModelOrItineraries() &&
"Missing machine model\n");
// Check each definition in NewRoot and compute the latency
unsigned NewRootLatency = 0;
for (const MachineOperand &MO : NewRoot->operands()) {
// Check for virtual register operand.
if (!(MO.isReg() && TargetRegisterInfo::isVirtualRegister(MO.getReg())))
continue;
if (!MO.isDef())
continue;
// Get the first instruction that uses MO
MachineRegisterInfo::reg_iterator RI = MRI->reg_begin(MO.getReg());
RI++;
MachineInstr *UseMO = RI->getParent();
unsigned LatencyOp = 0;
if (UseMO && BlockTrace.isDepInTrace(*Root, *UseMO)) {
LatencyOp = TSchedModel.computeOperandLatency(
NewRoot, NewRoot->findRegisterDefOperandIdx(MO.getReg()), UseMO,
UseMO->findRegisterUseOperandIdx(MO.getReg()));
} else {
LatencyOp = TSchedModel.computeInstrLatency(NewRoot);
}
NewRootLatency = std::max(NewRootLatency, LatencyOp);
}
return NewRootLatency;
}
/// The combiner's goal may differ based on which pattern it is attempting
/// to optimize.
enum class CombinerObjective {
MustReduceDepth, // The data dependency chain must be improved.
Default // The critical path must not be lengthened.
};
static CombinerObjective getCombinerObjective(MachineCombinerPattern P) {
// TODO: If C++ ever gets a real enum class, make this part of the
// MachineCombinerPattern class.
switch (P) {
case MachineCombinerPattern::REASSOC_AX_BY:
case MachineCombinerPattern::REASSOC_AX_YB:
case MachineCombinerPattern::REASSOC_XA_BY:
case MachineCombinerPattern::REASSOC_XA_YB:
return CombinerObjective::MustReduceDepth;
default:
return CombinerObjective::Default;
}
}
/// The DAGCombine code sequence ends in MI (Machine Instruction) Root.
/// The new code sequence ends in MI NewRoot. A necessary condition for the new
/// sequence to replace the old sequence is that it cannot lengthen the critical
/// path. The definition of "improve" may be restricted by specifying that the
/// new path improves the data dependency chain (MustReduceDepth).
bool MachineCombiner::improvesCriticalPathLen(
MachineBasicBlock *MBB, MachineInstr *Root,
MachineTraceMetrics::Trace BlockTrace,
SmallVectorImpl<MachineInstr *> &InsInstrs,
SmallVectorImpl<MachineInstr *> &DelInstrs,
DenseMap<unsigned, unsigned> &InstrIdxForVirtReg,
MachineCombinerPattern Pattern) {
assert(TSchedModel.hasInstrSchedModelOrItineraries() &&
"Missing machine model\n");
// NewRoot is the last instruction in the \p InsInstrs vector.
unsigned NewRootIdx = InsInstrs.size() - 1;
MachineInstr *NewRoot = InsInstrs[NewRootIdx];
// Get depth and latency of NewRoot and Root.
unsigned NewRootDepth = getDepth(InsInstrs, InstrIdxForVirtReg, BlockTrace);
unsigned RootDepth = BlockTrace.getInstrCycles(*Root).Depth;
DEBUG(dbgs() << "DEPENDENCE DATA FOR " << Root << "\n";
dbgs() << " NewRootDepth: " << NewRootDepth << "\n";
dbgs() << " RootDepth: " << RootDepth << "\n");
// For a transform such as reassociation, the cost equation is
// conservatively calculated so that we must improve the depth (data
// dependency cycles) in the critical path to proceed with the transform.
// Being conservative also protects against inaccuracies in the underlying
// machine trace metrics and CPU models.
if (getCombinerObjective(Pattern) == CombinerObjective::MustReduceDepth)
return NewRootDepth < RootDepth;
// A more flexible cost calculation for the critical path includes the slack
// of the original code sequence. This may allow the transform to proceed
// even if the instruction depths (data dependency cycles) become worse.
unsigned NewRootLatency = getLatency(Root, NewRoot, BlockTrace);
unsigned RootLatency = 0;
for (auto I : DelInstrs)
RootLatency += TSchedModel.computeInstrLatency(I);
unsigned RootSlack = BlockTrace.getInstrSlack(*Root);
DEBUG(dbgs() << " NewRootLatency: " << NewRootLatency << "\n";
dbgs() << " RootLatency: " << RootLatency << "\n";
dbgs() << " RootSlack: " << RootSlack << "\n";
dbgs() << " NewRootDepth + NewRootLatency = "
<< NewRootDepth + NewRootLatency << "\n";
dbgs() << " RootDepth + RootLatency + RootSlack = "
<< RootDepth + RootLatency + RootSlack << "\n";);
unsigned NewCycleCount = NewRootDepth + NewRootLatency;
unsigned OldCycleCount = RootDepth + RootLatency + RootSlack;
return NewCycleCount <= OldCycleCount;
}
/// helper routine to convert instructions into SC
void MachineCombiner::instr2instrSC(
SmallVectorImpl<MachineInstr *> &Instrs,
SmallVectorImpl<const MCSchedClassDesc *> &InstrsSC) {
for (auto *InstrPtr : Instrs) {
unsigned Opc = InstrPtr->getOpcode();
unsigned Idx = TII->get(Opc).getSchedClass();
const MCSchedClassDesc *SC = SchedModel.getSchedClassDesc(Idx);
InstrsSC.push_back(SC);
}
}
/// True when the new instructions do not increase resource length
bool MachineCombiner::preservesResourceLen(
MachineBasicBlock *MBB, MachineTraceMetrics::Trace BlockTrace,
SmallVectorImpl<MachineInstr *> &InsInstrs,
SmallVectorImpl<MachineInstr *> &DelInstrs) {
if (!TSchedModel.hasInstrSchedModel())
return true;
// Compute current resource length
//ArrayRef<const MachineBasicBlock *> MBBarr(MBB);
SmallVector <const MachineBasicBlock *, 1> MBBarr;
MBBarr.push_back(MBB);
unsigned ResLenBeforeCombine = BlockTrace.getResourceLength(MBBarr);
// Deal with SC rather than Instructions.
SmallVector<const MCSchedClassDesc *, 16> InsInstrsSC;
SmallVector<const MCSchedClassDesc *, 16> DelInstrsSC;
instr2instrSC(InsInstrs, InsInstrsSC);
instr2instrSC(DelInstrs, DelInstrsSC);
ArrayRef<const MCSchedClassDesc *> MSCInsArr = makeArrayRef(InsInstrsSC);
ArrayRef<const MCSchedClassDesc *> MSCDelArr = makeArrayRef(DelInstrsSC);
// Compute new resource length.
unsigned ResLenAfterCombine =
BlockTrace.getResourceLength(MBBarr, MSCInsArr, MSCDelArr);
DEBUG(dbgs() << "RESOURCE DATA: \n";
dbgs() << " resource len before: " << ResLenBeforeCombine
<< " after: " << ResLenAfterCombine << "\n";);
return ResLenAfterCombine <= ResLenBeforeCombine;
}
/// \returns true when new instruction sequence should be generated
/// independent if it lengthens critical path or not
bool MachineCombiner::doSubstitute(unsigned NewSize, unsigned OldSize) {
if (OptSize && (NewSize < OldSize))
return true;
if (!TSchedModel.hasInstrSchedModelOrItineraries())
return true;
return false;
}
static void insertDeleteInstructions(MachineBasicBlock *MBB, MachineInstr &MI,
SmallVector<MachineInstr *, 16> InsInstrs,
SmallVector<MachineInstr *, 16> DelInstrs,
MachineTraceMetrics *Traces) {
for (auto *InstrPtr : InsInstrs)
MBB->insert((MachineBasicBlock::iterator)&MI, InstrPtr);
for (auto *InstrPtr : DelInstrs)
InstrPtr->eraseFromParentAndMarkDBGValuesForRemoval();
++NumInstCombined;
Traces->invalidate(MBB);
Traces->verifyAnalysis();
}
/// Substitute a slow code sequence with a faster one by
/// evaluating instruction combining pattern.
/// The prototype of such a pattern is MUl + ADD -> MADD. Performs instruction
/// combining based on machine trace metrics. Only combine a sequence of
/// instructions when this neither lengthens the critical path nor increases
/// resource pressure. When optimizing for codesize always combine when the new
/// sequence is shorter.
bool MachineCombiner::combineInstructions(MachineBasicBlock *MBB) {
bool Changed = false;
DEBUG(dbgs() << "Combining MBB " << MBB->getName() << "\n");
auto BlockIter = MBB->begin();
// Check if the block is in a loop.
const MachineLoop *ML = MLI->getLoopFor(MBB);
while (BlockIter != MBB->end()) {
auto &MI = *BlockIter++;
DEBUG(dbgs() << "INSTR "; MI.dump(); dbgs() << "\n";);
SmallVector<MachineCombinerPattern, 16> Patterns;
// The motivating example is:
//
// MUL Other MUL_op1 MUL_op2 Other
// \ / \ | /
// ADD/SUB => MADD/MSUB
// (=Root) (=NewRoot)
// The DAGCombine code always replaced MUL + ADD/SUB by MADD. While this is
// usually beneficial for code size it unfortunately can hurt performance
// when the ADD is on the critical path, but the MUL is not. With the
// substitution the MUL becomes part of the critical path (in form of the
// MADD) and can lengthen it on architectures where the MADD latency is
// longer than the ADD latency.
//
// For each instruction we check if it can be the root of a combiner
// pattern. Then for each pattern the new code sequence in form of MI is
// generated and evaluated. When the efficiency criteria (don't lengthen
// critical path, don't use more resources) is met the new sequence gets
// hooked up into the basic block before the old sequence is removed.
//
// The algorithm does not try to evaluate all patterns and pick the best.
// This is only an artificial restriction though. In practice there is
// mostly one pattern, and getMachineCombinerPatterns() can order patterns
// based on an internal cost heuristic.
if (!TII->getMachineCombinerPatterns(MI, Patterns))
continue;
for (auto P : Patterns) {
SmallVector<MachineInstr *, 16> InsInstrs;
SmallVector<MachineInstr *, 16> DelInstrs;
DenseMap<unsigned, unsigned> InstrIdxForVirtReg;
if (!MinInstr)
MinInstr = Traces->getEnsemble(MachineTraceMetrics::TS_MinInstrCount);
Traces->verifyAnalysis();
TII->genAlternativeCodeSequence(MI, P, InsInstrs, DelInstrs,
InstrIdxForVirtReg);
unsigned NewInstCount = InsInstrs.size();
unsigned OldInstCount = DelInstrs.size();
// Found pattern, but did not generate alternative sequence.
// This can happen e.g. when an immediate could not be materialized
// in a single instruction.
if (!NewInstCount)
continue;
bool SubstituteAlways = false;
if (ML && TII->isThroughputPattern(P))
SubstituteAlways = true;
// Substitute when we optimize for codesize and the new sequence has
// fewer instructions OR
// the new sequence neither lengthens the critical path nor increases
// resource pressure.
if (SubstituteAlways || doSubstitute(NewInstCount, OldInstCount)) {
insertDeleteInstructions(MBB, MI, InsInstrs, DelInstrs, Traces);
// Eagerly stop after the first pattern fires.
Changed = true;
break;
} else {
// Calculating the trace metrics may be expensive,
// so only do this when necessary.
MachineTraceMetrics::Trace BlockTrace = MinInstr->getTrace(MBB);
if (improvesCriticalPathLen(MBB, &MI, BlockTrace, InsInstrs, DelInstrs,
InstrIdxForVirtReg, P) &&
preservesResourceLen(MBB, BlockTrace, InsInstrs, DelInstrs)) {
insertDeleteInstructions(MBB, MI, InsInstrs, DelInstrs, Traces);
// Eagerly stop after the first pattern fires.
Changed = true;
break;
}
// Cleanup instructions of the alternative code sequence. There is no
// use for them.
MachineFunction *MF = MBB->getParent();
for (auto *InstrPtr : InsInstrs)
MF->DeleteMachineInstr(InstrPtr);
}
InstrIdxForVirtReg.clear();
}
}
return Changed;
}
bool MachineCombiner::runOnMachineFunction(MachineFunction &MF) {
const TargetSubtargetInfo &STI = MF.getSubtarget();
TII = STI.getInstrInfo();
TRI = STI.getRegisterInfo();
SchedModel = STI.getSchedModel();
TSchedModel.init(SchedModel, &STI, TII);
MRI = &MF.getRegInfo();
MLI = &getAnalysis<MachineLoopInfo>();
Traces = &getAnalysis<MachineTraceMetrics>();
MinInstr = nullptr;
OptSize = MF.getFunction()->optForSize();
DEBUG(dbgs() << getPassName() << ": " << MF.getName() << '\n');
if (!TII->useMachineCombiner()) {
DEBUG(dbgs() << " Skipping pass: Target does not support machine combiner\n");
return false;
}
bool Changed = false;
// Try to combine instructions.
for (auto &MBB : MF)
Changed |= combineInstructions(&MBB);
return Changed;
}
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