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

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//===---- ReachingDefAnalysis.cpp - Reaching Def Analysis ---*- C++ -*-----===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/SmallSet.h"
#include "llvm/CodeGen/LivePhysRegs.h"
#include "llvm/CodeGen/ReachingDefAnalysis.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
#define DEBUG_TYPE "reaching-deps-analysis"
char ReachingDefAnalysis::ID = 0;
INITIALIZE_PASS(ReachingDefAnalysis, DEBUG_TYPE, "ReachingDefAnalysis", false,
true)
void ReachingDefAnalysis::enterBasicBlock(
const LoopTraversal::TraversedMBBInfo &TraversedMBB) {
MachineBasicBlock *MBB = TraversedMBB.MBB;
unsigned MBBNumber = MBB->getNumber();
assert(MBBNumber < MBBReachingDefs.size() &&
"Unexpected basic block number.");
MBBReachingDefs[MBBNumber].resize(NumRegUnits);
// Reset instruction counter in each basic block.
CurInstr = 0;
// Set up LiveRegs to represent registers entering MBB.
// Default values are 'nothing happened a long time ago'.
if (LiveRegs.empty())
LiveRegs.assign(NumRegUnits, ReachingDefDefaultVal);
// This is the entry block.
if (MBB->pred_empty()) {
for (const auto &LI : MBB->liveins()) {
for (MCRegUnitIterator Unit(LI.PhysReg, TRI); Unit.isValid(); ++Unit) {
// Treat function live-ins as if they were defined just before the first
// instruction. Usually, function arguments are set up immediately
// before the call.
LiveRegs[*Unit] = -1;
MBBReachingDefs[MBBNumber][*Unit].push_back(LiveRegs[*Unit]);
}
}
LLVM_DEBUG(dbgs() << printMBBReference(*MBB) << ": entry\n");
return;
}
// Try to coalesce live-out registers from predecessors.
for (MachineBasicBlock *pred : MBB->predecessors()) {
assert(unsigned(pred->getNumber()) < MBBOutRegsInfos.size() &&
"Should have pre-allocated MBBInfos for all MBBs");
const LiveRegsDefInfo &Incoming = MBBOutRegsInfos[pred->getNumber()];
// Incoming is null if this is a backedge from a BB
// we haven't processed yet
if (Incoming.empty())
continue;
for (unsigned Unit = 0; Unit != NumRegUnits; ++Unit) {
// Use the most recent predecessor def for each register.
LiveRegs[Unit] = std::max(LiveRegs[Unit], Incoming[Unit]);
if ((LiveRegs[Unit] != ReachingDefDefaultVal))
MBBReachingDefs[MBBNumber][Unit].push_back(LiveRegs[Unit]);
}
}
LLVM_DEBUG(dbgs() << printMBBReference(*MBB)
<< (!TraversedMBB.IsDone ? ": incomplete\n"
: ": all preds known\n"));
}
void ReachingDefAnalysis::leaveBasicBlock(
const LoopTraversal::TraversedMBBInfo &TraversedMBB) {
assert(!LiveRegs.empty() && "Must enter basic block first.");
unsigned MBBNumber = TraversedMBB.MBB->getNumber();
assert(MBBNumber < MBBOutRegsInfos.size() &&
"Unexpected basic block number.");
// Save register clearances at end of MBB - used by enterBasicBlock().
MBBOutRegsInfos[MBBNumber] = LiveRegs;
// While processing the basic block, we kept `Def` relative to the start
// of the basic block for convenience. However, future use of this information
// only cares about the clearance from the end of the block, so adjust
// everything to be relative to the end of the basic block.
for (int &OutLiveReg : MBBOutRegsInfos[MBBNumber])
OutLiveReg -= CurInstr;
LiveRegs.clear();
}
void ReachingDefAnalysis::processDefs(MachineInstr *MI) {
assert(!MI->isDebugInstr() && "Won't process debug instructions");
unsigned MBBNumber = MI->getParent()->getNumber();
assert(MBBNumber < MBBReachingDefs.size() &&
"Unexpected basic block number.");
const MCInstrDesc &MCID = MI->getDesc();
for (unsigned i = 0,
e = MI->isVariadic() ? MI->getNumOperands() : MCID.getNumDefs();
i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || !MO.getReg())
continue;
if (MO.isUse())
continue;
for (MCRegUnitIterator Unit(MO.getReg(), TRI); Unit.isValid(); ++Unit) {
// This instruction explicitly defines the current reg unit.
LLVM_DEBUG(dbgs() << printReg(MO.getReg(), TRI) << ":\t" << CurInstr
<< '\t' << *MI);
// How many instructions since this reg unit was last written?
LiveRegs[*Unit] = CurInstr;
MBBReachingDefs[MBBNumber][*Unit].push_back(CurInstr);
}
}
InstIds[MI] = CurInstr;
++CurInstr;
}
void ReachingDefAnalysis::processBasicBlock(
const LoopTraversal::TraversedMBBInfo &TraversedMBB) {
enterBasicBlock(TraversedMBB);
for (MachineInstr &MI : *TraversedMBB.MBB) {
if (!MI.isDebugInstr())
processDefs(&MI);
}
leaveBasicBlock(TraversedMBB);
}
bool ReachingDefAnalysis::runOnMachineFunction(MachineFunction &mf) {
MF = &mf;
TRI = MF->getSubtarget().getRegisterInfo();
LiveRegs.clear();
NumRegUnits = TRI->getNumRegUnits();
MBBReachingDefs.resize(mf.getNumBlockIDs());
LLVM_DEBUG(dbgs() << "********** REACHING DEFINITION ANALYSIS **********\n");
// Initialize the MBBOutRegsInfos
MBBOutRegsInfos.resize(mf.getNumBlockIDs());
// Traverse the basic blocks.
LoopTraversal Traversal;
LoopTraversal::TraversalOrder TraversedMBBOrder = Traversal.traverse(mf);
for (LoopTraversal::TraversedMBBInfo TraversedMBB : TraversedMBBOrder) {
processBasicBlock(TraversedMBB);
}
// Sorting all reaching defs found for a ceartin reg unit in a given BB.
for (MBBDefsInfo &MBBDefs : MBBReachingDefs) {
for (MBBRegUnitDefs &RegUnitDefs : MBBDefs)
llvm::sort(RegUnitDefs);
}
return false;
}
void ReachingDefAnalysis::releaseMemory() {
// Clear the internal vectors.
MBBOutRegsInfos.clear();
MBBReachingDefs.clear();
InstIds.clear();
}
int ReachingDefAnalysis::getReachingDef(MachineInstr *MI, int PhysReg) const {
assert(InstIds.count(MI) && "Unexpected machine instuction.");
int InstId = InstIds.lookup(MI);
int DefRes = ReachingDefDefaultVal;
unsigned MBBNumber = MI->getParent()->getNumber();
assert(MBBNumber < MBBReachingDefs.size() &&
"Unexpected basic block number.");
int LatestDef = ReachingDefDefaultVal;
for (MCRegUnitIterator Unit(PhysReg, TRI); Unit.isValid(); ++Unit) {
for (int Def : MBBReachingDefs[MBBNumber][*Unit]) {
if (Def >= InstId)
break;
DefRes = Def;
}
LatestDef = std::max(LatestDef, DefRes);
}
return LatestDef;
}
MachineInstr* ReachingDefAnalysis::getReachingMIDef(MachineInstr *MI,
int PhysReg) const {
return getInstFromId(MI->getParent(), getReachingDef(MI, PhysReg));
}
bool ReachingDefAnalysis::hasSameReachingDef(MachineInstr *A, MachineInstr *B,
int PhysReg) const {
MachineBasicBlock *ParentA = A->getParent();
MachineBasicBlock *ParentB = B->getParent();
if (ParentA != ParentB)
return false;
return getReachingDef(A, PhysReg) == getReachingDef(B, PhysReg);
}
MachineInstr *ReachingDefAnalysis::getInstFromId(MachineBasicBlock *MBB,
int InstId) const {
assert(static_cast<size_t>(MBB->getNumber()) < MBBReachingDefs.size() &&
"Unexpected basic block number.");
assert(InstId < static_cast<int>(MBB->size()) &&
"Unexpected instruction id.");
if (InstId < 0)
return nullptr;
for (auto &MI : *MBB) {
auto F = InstIds.find(&MI);
if (F != InstIds.end() && F->second == InstId)
return &MI;
}
return nullptr;
}
int
ReachingDefAnalysis::getClearance(MachineInstr *MI, MCPhysReg PhysReg) const {
assert(InstIds.count(MI) && "Unexpected machine instuction.");
return InstIds.lookup(MI) - getReachingDef(MI, PhysReg);
}
bool
ReachingDefAnalysis::hasLocalDefBefore(MachineInstr *MI, int PhysReg) const {
return getReachingDef(MI, PhysReg) >= 0;
}
void ReachingDefAnalysis::getReachingLocalUses(MachineInstr *Def, int PhysReg,
InstSet &Uses) const {
MachineBasicBlock *MBB = Def->getParent();
MachineBasicBlock::iterator MI = MachineBasicBlock::iterator(Def);
while (++MI != MBB->end()) {
if (MI->isDebugInstr())
continue;
// If/when we find a new reaching def, we know that there's no more uses
// of 'Def'.
if (getReachingMIDef(&*MI, PhysReg) != Def)
return;
for (auto &MO : MI->operands()) {
if (!MO.isReg() || !MO.isUse() || MO.getReg() != PhysReg)
continue;
Uses.insert(&*MI);
if (MO.isKill())
return;
}
}
}
bool
ReachingDefAnalysis::getLiveInUses(MachineBasicBlock *MBB, int PhysReg,
InstSet &Uses) const {
for (auto &MI : *MBB) {
if (MI.isDebugInstr())
continue;
for (auto &MO : MI.operands()) {
if (!MO.isReg() || !MO.isUse() || MO.getReg() != PhysReg)
continue;
if (getReachingDef(&MI, PhysReg) >= 0)
return false;
Uses.insert(&MI);
}
}
return isReachingDefLiveOut(&MBB->back(), PhysReg);
}
void
ReachingDefAnalysis::getGlobalUses(MachineInstr *MI, int PhysReg,
InstSet &Uses) const {
MachineBasicBlock *MBB = MI->getParent();
// Collect the uses that each def touches within the block.
getReachingLocalUses(MI, PhysReg, Uses);
// Handle live-out values.
if (auto *LiveOut = getLocalLiveOutMIDef(MI->getParent(), PhysReg)) {
if (LiveOut != MI)
return;
SmallVector<MachineBasicBlock*, 4> ToVisit;
ToVisit.insert(ToVisit.begin(), MBB->successors().begin(),
MBB->successors().end());
SmallPtrSet<MachineBasicBlock*, 4>Visited;
while (!ToVisit.empty()) {
MachineBasicBlock *MBB = ToVisit.back();
ToVisit.pop_back();
if (Visited.count(MBB) || !MBB->isLiveIn(PhysReg))
continue;
if (getLiveInUses(MBB, PhysReg, Uses))
ToVisit.insert(ToVisit.end(), MBB->successors().begin(),
MBB->successors().end());
Visited.insert(MBB);
}
}
}
bool ReachingDefAnalysis::isRegUsedAfter(MachineInstr *MI, int PhysReg) const {
MachineBasicBlock *MBB = MI->getParent();
LivePhysRegs LiveRegs(*TRI);
LiveRegs.addLiveOuts(*MBB);
// Yes if the register is live out of the basic block.
if (LiveRegs.contains(PhysReg))
return true;
// Walk backwards through the block to see if the register is live at some
// point.
for (auto Last = MBB->rbegin(), End = MBB->rend(); Last != End; ++Last) {
LiveRegs.stepBackward(*Last);
if (LiveRegs.contains(PhysReg))
return InstIds.lookup(&*Last) > InstIds.lookup(MI);
}
return false;
}
bool ReachingDefAnalysis::isRegDefinedAfter(MachineInstr *MI,
int PhysReg) const {
MachineBasicBlock *MBB = MI->getParent();
if (getReachingDef(MI, PhysReg) != getReachingDef(&MBB->back(), PhysReg))
return true;
if (auto *Def = getLocalLiveOutMIDef(MBB, PhysReg))
return Def == getReachingMIDef(MI, PhysReg);
return false;
}
bool
ReachingDefAnalysis::isReachingDefLiveOut(MachineInstr *MI, int PhysReg) const {
MachineBasicBlock *MBB = MI->getParent();
LivePhysRegs LiveRegs(*TRI);
LiveRegs.addLiveOuts(*MBB);
if (!LiveRegs.contains(PhysReg))
return false;
MachineInstr *Last = &MBB->back();
int Def = getReachingDef(MI, PhysReg);
if (getReachingDef(Last, PhysReg) != Def)
return false;
// Finally check that the last instruction doesn't redefine the register.
for (auto &MO : Last->operands())
if (MO.isReg() && MO.isDef() && MO.getReg() == PhysReg)
return false;
return true;
}
MachineInstr* ReachingDefAnalysis::getLocalLiveOutMIDef(MachineBasicBlock *MBB,
int PhysReg) const {
LivePhysRegs LiveRegs(*TRI);
LiveRegs.addLiveOuts(*MBB);
if (!LiveRegs.contains(PhysReg))
return nullptr;
MachineInstr *Last = &MBB->back();
int Def = getReachingDef(Last, PhysReg);
for (auto &MO : Last->operands())
if (MO.isReg() && MO.isDef() && MO.getReg() == PhysReg)
return Last;
return Def < 0 ? nullptr : getInstFromId(MBB, Def);
}
static bool mayHaveSideEffects(MachineInstr &MI) {
return MI.mayLoadOrStore() || MI.mayRaiseFPException() ||
MI.hasUnmodeledSideEffects() || MI.isTerminator() ||
MI.isCall() || MI.isBarrier() || MI.isBranch() || MI.isReturn();
}
// Can we safely move 'From' to just before 'To'? To satisfy this, 'From' must
// not define a register that is used by any instructions, after and including,
// 'To'. These instructions also must not redefine any of Froms operands.
template<typename Iterator>
bool ReachingDefAnalysis::isSafeToMove(MachineInstr *From,
MachineInstr *To) const {
if (From->getParent() != To->getParent())
return false;
SmallSet<int, 2> Defs;
// First check that From would compute the same value if moved.
for (auto &MO : From->operands()) {
if (!MO.isReg() || MO.isUndef() || !MO.getReg())
continue;
if (MO.isDef())
Defs.insert(MO.getReg());
else if (!hasSameReachingDef(From, To, MO.getReg()))
return false;
}
// Now walk checking that the rest of the instructions will compute the same
// value and that we're not overwriting anything. Don't move the instruction
// past any memory, control-flow or other ambigious instructions.
for (auto I = ++Iterator(From), E = Iterator(To); I != E; ++I) {
if (mayHaveSideEffects(*I))
return false;
for (auto &MO : I->operands())
if (MO.isReg() && MO.getReg() && Defs.count(MO.getReg()))
return false;
}
return true;
}
bool ReachingDefAnalysis::isSafeToMoveForwards(MachineInstr *From,
MachineInstr *To) const {
return isSafeToMove<MachineBasicBlock::reverse_iterator>(From, To);
}
bool ReachingDefAnalysis::isSafeToMoveBackwards(MachineInstr *From,
MachineInstr *To) const {
return isSafeToMove<MachineBasicBlock::iterator>(From, To);
}
bool ReachingDefAnalysis::isSafeToRemove(MachineInstr *MI,
InstSet &ToRemove) const {
SmallPtrSet<MachineInstr*, 1> Ignore;
SmallPtrSet<MachineInstr*, 2> Visited;
return isSafeToRemove(MI, Visited, ToRemove, Ignore);
}
bool
ReachingDefAnalysis::isSafeToRemove(MachineInstr *MI, InstSet &ToRemove,
InstSet &Ignore) const {
SmallPtrSet<MachineInstr*, 2> Visited;
return isSafeToRemove(MI, Visited, ToRemove, Ignore);
}
bool
ReachingDefAnalysis::isSafeToRemove(MachineInstr *MI, InstSet &Visited,
InstSet &ToRemove, InstSet &Ignore) const {
if (Visited.count(MI) || Ignore.count(MI))
return true;
else if (mayHaveSideEffects(*MI)) {
// Unless told to ignore the instruction, don't remove anything which has
// side effects.
return false;
}
Visited.insert(MI);
for (auto &MO : MI->operands()) {
if (!MO.isReg() || MO.isUse() || MO.getReg() == 0)
continue;
SmallPtrSet<MachineInstr*, 4> Uses;
getGlobalUses(MI, MO.getReg(), Uses);
for (auto I : Uses) {
if (Ignore.count(I) || ToRemove.count(I))
continue;
if (!isSafeToRemove(I, Visited, ToRemove, Ignore))
return false;
}
}
ToRemove.insert(MI);
return true;
}
bool ReachingDefAnalysis::isSafeToDefRegAt(MachineInstr *MI,
int PhysReg) const {
SmallPtrSet<MachineInstr*, 1> Ignore;
return isSafeToDefRegAt(MI, PhysReg, Ignore);
}
bool ReachingDefAnalysis::isSafeToDefRegAt(MachineInstr *MI, int PhysReg,
InstSet &Ignore) const {
// Check for any uses of the register after MI.
if (isRegUsedAfter(MI, PhysReg)) {
if (auto *Def = getReachingMIDef(MI, PhysReg)) {
SmallPtrSet<MachineInstr*, 2> Uses;
getReachingLocalUses(Def, PhysReg, Uses);
for (auto *Use : Uses)
if (!Ignore.count(Use))
return false;
} else
return false;
}
MachineBasicBlock *MBB = MI->getParent();
// Check for any defs after MI.
if (isRegDefinedAfter(MI, PhysReg)) {
auto I = MachineBasicBlock::iterator(MI);
for (auto E = MBB->end(); I != E; ++I) {
if (Ignore.count(&*I))
continue;
for (auto &MO : I->operands())
if (MO.isReg() && MO.isDef() && MO.getReg() == PhysReg)
return false;
}
}
return true;
}
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