LiveIntervalAnalysis.cpp revision b606eaca1b149ef74c578a6a11d65339c125edab 
1//===-- LiveIntervals.cpp - Live Interval Analysis ------------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by the LLVM research group and is distributed under
6// the University of Illinois Open Source License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements the LiveInterval analysis pass which is used
11// by the Linear Scan Register allocator. This pass linearizes the
12// basic blocks of the function in DFS order and uses the
13// LiveVariables pass to conservatively compute live intervals for
14// each virtual and physical register.
15//
16//===----------------------------------------------------------------------===//
17
18#define DEBUG_TYPE "liveintervals"
19#include "llvm/CodeGen/LiveIntervals.h"
20#include "llvm/Analysis/LoopInfo.h"
21#include "llvm/CodeGen/LiveVariables.h"
22#include "llvm/CodeGen/MachineFrameInfo.h"
23#include "llvm/CodeGen/MachineInstr.h"
24#include "llvm/CodeGen/Passes.h"
25#include "llvm/CodeGen/SSARegMap.h"
26#include "llvm/Target/MRegisterInfo.h"
27#include "llvm/Target/TargetInstrInfo.h"
28#include "llvm/Target/TargetMachine.h"
29#include "llvm/Support/CFG.h"
30#include "Support/CommandLine.h"
31#include "Support/Debug.h"
32#include "Support/Statistic.h"
33#include <cmath>
34#include <iostream>
35#include <limits>
36
37using namespace llvm;
38
39namespace {
40    RegisterAnalysis<LiveIntervals> X("liveintervals",
41                                      "Live Interval Analysis");
42
43    Statistic<> numIntervals("liveintervals", "Number of intervals");
44    Statistic<> numJoined   ("liveintervals", "Number of joined intervals");
45
46    cl::opt<bool>
47    join("join-liveintervals",
48         cl::desc("Join compatible live intervals"),
49         cl::init(true));
50};
51
52void LiveIntervals::getAnalysisUsage(AnalysisUsage &AU) const
53{
54    AU.addPreserved<LiveVariables>();
55    AU.addRequired<LiveVariables>();
56    AU.addPreservedID(PHIEliminationID);
57    AU.addRequiredID(PHIEliminationID);
58    AU.addRequiredID(TwoAddressInstructionPassID);
59    AU.addRequired<LoopInfo>();
60    MachineFunctionPass::getAnalysisUsage(AU);
61}
62
63void LiveIntervals::releaseMemory()
64{
65    mbbi2mbbMap_.clear();
66    mi2iMap_.clear();
67    r2iMap_.clear();
68    r2iMap_.clear();
69    r2rMap_.clear();
70    intervals_.clear();
71}
72
73
74/// runOnMachineFunction - Register allocate the whole function
75///
76bool LiveIntervals::runOnMachineFunction(MachineFunction &fn) {
77    DEBUG(std::cerr << "Machine Function\n");
78    mf_ = &fn;
79    tm_ = &fn.getTarget();
80    mri_ = tm_->getRegisterInfo();
81    lv_ = &getAnalysis<LiveVariables>();
82
83    // number MachineInstrs
84    unsigned miIndex = 0;
85    for (MachineFunction::iterator mbb = mf_->begin(), mbbEnd = mf_->end();
86         mbb != mbbEnd; ++mbb) {
87        const std::pair<MachineBasicBlock*, unsigned>& entry =
88            lv_->getMachineBasicBlockInfo(mbb);
89        bool inserted = mbbi2mbbMap_.insert(std::make_pair(entry.second,
90                                                           entry.first)).second;
91        assert(inserted && "multiple index -> MachineBasicBlock");
92
93        for (MachineBasicBlock::iterator mi = mbb->begin(), miEnd = mbb->end();
94             mi != miEnd; ++mi) {
95            inserted = mi2iMap_.insert(std::make_pair(*mi, miIndex)).second;
96            assert(inserted && "multiple MachineInstr -> index mappings");
97            ++miIndex;
98        }
99    }
100
101    computeIntervals();
102
103    // compute spill weights
104    const LoopInfo& loopInfo = getAnalysis<LoopInfo>();
105    const TargetInstrInfo& tii = tm_->getInstrInfo();
106
107    for (MachineFunction::const_iterator mbbi = mf_->begin(),
108             mbbe = mf_->end(); mbbi != mbbe; ++mbbi) {
109        const MachineBasicBlock* mbb = mbbi;
110        unsigned loopDepth = loopInfo.getLoopDepth(mbb->getBasicBlock());
111
112        for (MachineBasicBlock::const_iterator mii = mbb->begin(),
113                 mie = mbb->end(); mii != mie; ++mii) {
114            MachineInstr* mi = *mii;
115
116            for (int i = mi->getNumOperands() - 1; i >= 0; --i) {
117                MachineOperand& mop = mi->getOperand(i);
118                if (mop.isVirtualRegister()) {
119                    unsigned reg = mop.getAllocatedRegNum();
120                    Reg2IntervalMap::iterator r2iit = r2iMap_.find(reg);
121                    assert(r2iit != r2iMap_.end());
122                    r2iit->second->weight += pow(10.0F, loopDepth);
123                }
124            }
125        }
126    }
127
128    // join intervals if requested
129    if (join) joinIntervals();
130
131    numIntervals += intervals_.size();
132
133    intervals_.sort(StartPointComp());
134    DEBUG(std::copy(intervals_.begin(), intervals_.end(),
135                    std::ostream_iterator<Interval>(std::cerr, "\n")));
136    return true;
137}
138
139void LiveIntervals::printRegName(unsigned reg) const
140{
141    if (MRegisterInfo::isPhysicalRegister(reg))
142        std::cerr << mri_->getName(reg);
143    else
144        std::cerr << '%' << reg;
145}
146
147void LiveIntervals::handleVirtualRegisterDef(MachineBasicBlock* mbb,
148                                             MachineBasicBlock::iterator mi,
149                                             unsigned reg)
150{
151    DEBUG(std::cerr << "\t\tregister: ";printRegName(reg); std::cerr << '\n');
152
153    unsigned instrIndex = getInstructionIndex(*mi);
154
155    LiveVariables::VarInfo& vi = lv_->getVarInfo(reg);
156
157    Interval* interval = 0;
158    Reg2IntervalMap::iterator r2iit = r2iMap_.lower_bound(reg);
159    if (r2iit == r2iMap_.end() || r2iit->first != reg) {
160        // add new interval
161        intervals_.push_back(Interval(reg));
162        // update interval index for this register
163        r2iMap_.insert(r2iit, std::make_pair(reg, --intervals_.end()));
164        interval = &intervals_.back();
165    }
166    else {
167        interval = &*r2iit->second;
168    }
169
170    // iterate over all of the blocks that the variable is completely
171    // live in, adding them to the live interval
172    for (unsigned i = 0, e = vi.AliveBlocks.size(); i != e; ++i) {
173        if (vi.AliveBlocks[i]) {
174            MachineBasicBlock* mbb = lv_->getIndexMachineBasicBlock(i);
175            if (!mbb->empty()) {
176                interval->addRange(getInstructionIndex(mbb->front()),
177                                   getInstructionIndex(mbb->back()) + 1);
178            }
179        }
180    }
181
182    bool killedInDefiningBasicBlock = false;
183    for (int i = 0, e = vi.Kills.size(); i != e; ++i) {
184        MachineBasicBlock* killerBlock = vi.Kills[i].first;
185        MachineInstr* killerInstr = vi.Kills[i].second;
186        unsigned start = (mbb == killerBlock ?
187                          instrIndex :
188                          getInstructionIndex(killerBlock->front()));
189        unsigned end = getInstructionIndex(killerInstr) + 1;
190        // we do not want to add invalid ranges. these can happen when
191        // a variable has its latest use and is redefined later on in
192        // the same basic block (common with variables introduced by
193        // PHI elimination)
194        if (start < end) {
195            killedInDefiningBasicBlock |= mbb == killerBlock;
196            interval->addRange(start, end);
197        }
198    }
199
200    if (!killedInDefiningBasicBlock) {
201        unsigned end = getInstructionIndex(mbb->back()) + 1;
202        interval->addRange(instrIndex, end);
203    }
204}
205
206void LiveIntervals::handlePhysicalRegisterDef(MachineBasicBlock* mbb,
207                                              MachineBasicBlock::iterator mi,
208                                              unsigned reg)
209{
210    typedef LiveVariables::killed_iterator KillIter;
211
212    DEBUG(std::cerr << "\t\tregister: "; printRegName(reg));
213
214    MachineBasicBlock::iterator e = mbb->end();
215    unsigned start = getInstructionIndex(*mi);
216    unsigned end = start + 1;
217
218    // a variable can be dead by the instruction defining it
219    for (KillIter ki = lv_->dead_begin(*mi), ke = lv_->dead_end(*mi);
220         ki != ke; ++ki) {
221        if (reg == ki->second) {
222            DEBUG(std::cerr << " dead\n");
223            goto exit;
224        }
225    }
226
227    // a variable can only be killed by subsequent instructions
228    do {
229        ++mi;
230        ++end;
231        for (KillIter ki = lv_->killed_begin(*mi), ke = lv_->killed_end(*mi);
232             ki != ke; ++ki) {
233            if (reg == ki->second) {
234                DEBUG(std::cerr << " killed\n");
235                goto exit;
236            }
237        }
238    } while (mi != e);
239
240exit:
241    assert(start < end && "did not find end of interval?");
242
243    Reg2IntervalMap::iterator r2iit = r2iMap_.lower_bound(reg);
244    if (r2iit != r2iMap_.end() && r2iit->first == reg) {
245        r2iit->second->addRange(start, end);
246    }
247    else {
248        intervals_.push_back(Interval(reg));
249        // update interval index for this register
250        r2iMap_.insert(r2iit, std::make_pair(reg, --intervals_.end()));
251        intervals_.back().addRange(start, end);
252    }
253}
254
255void LiveIntervals::handleRegisterDef(MachineBasicBlock* mbb,
256                                      MachineBasicBlock::iterator mi,
257                                      unsigned reg)
258{
259    if (MRegisterInfo::isPhysicalRegister(reg)) {
260        if (lv_->getAllocatablePhysicalRegisters()[reg]) {
261            handlePhysicalRegisterDef(mbb, mi, reg);
262            for (const unsigned* as = mri_->getAliasSet(reg); *as; ++as)
263                handlePhysicalRegisterDef(mbb, mi, *as);
264        }
265    }
266    else {
267        handleVirtualRegisterDef(mbb, mi, reg);
268    }
269}
270
271unsigned LiveIntervals::getInstructionIndex(MachineInstr* instr) const
272{
273    assert(mi2iMap_.find(instr) != mi2iMap_.end() &&
274           "instruction not assigned a number");
275    return mi2iMap_.find(instr)->second;
276}
277
278/// computeIntervals - computes the live intervals for virtual
279/// registers. for some ordering of the machine instructions [1,N] a
280/// live interval is an interval [i, j) where 1 <= i <= j < N for
281/// which a variable is live
282void LiveIntervals::computeIntervals()
283{
284    DEBUG(std::cerr << "computing live intervals:\n");
285
286    for (MbbIndex2MbbMap::iterator
287             it = mbbi2mbbMap_.begin(), itEnd = mbbi2mbbMap_.end();
288         it != itEnd; ++it) {
289        MachineBasicBlock* mbb = it->second;
290        DEBUG(std::cerr << "machine basic block: "
291              << mbb->getBasicBlock()->getName() << "\n");
292
293        for (MachineBasicBlock::iterator mi = mbb->begin(), miEnd = mbb->end();
294             mi != miEnd; ++mi) {
295            MachineInstr* instr = *mi;
296            const TargetInstrDescriptor& tid =
297                tm_->getInstrInfo().get(instr->getOpcode());
298            DEBUG(std::cerr << "\t[" << getInstructionIndex(instr) << "] ";
299                  instr->print(std::cerr, *tm_););
300
301            // handle implicit defs
302            for (const unsigned* id = tid.ImplicitDefs; *id; ++id)
303                handleRegisterDef(mbb, mi, *id);
304
305            // handle explicit defs
306            for (int i = instr->getNumOperands() - 1; i >= 0; --i) {
307                MachineOperand& mop = instr->getOperand(i);
308                // handle register defs - build intervals
309                if (mop.isRegister() && mop.isDef())
310                    handleRegisterDef(mbb, mi, mop.getAllocatedRegNum());
311            }
312        }
313    }
314}
315
316unsigned LiveIntervals::rep(unsigned reg)
317{
318    Reg2RegMap::iterator it = r2rMap_.find(reg);
319    if (it != r2rMap_.end())
320        return it->second = rep(it->second);
321    return reg;
322}
323
324void LiveIntervals::joinIntervals()
325{
326    DEBUG(std::cerr << "joining compatible intervals:\n");
327
328    const TargetInstrInfo& tii = tm_->getInstrInfo();
329
330    for (MachineFunction::const_iterator mbbi = mf_->begin(),
331             mbbe = mf_->end(); mbbi != mbbe; ++mbbi) {
332        const MachineBasicBlock* mbb = mbbi;
333        DEBUG(std::cerr << "machine basic block: "
334              << mbb->getBasicBlock()->getName() << "\n");
335
336        for (MachineBasicBlock::const_iterator mii = mbb->begin(),
337                 mie = mbb->end(); mii != mie; ++mii) {
338            MachineInstr* mi = *mii;
339            const TargetInstrDescriptor& tid =
340                tm_->getInstrInfo().get(mi->getOpcode());
341            DEBUG(std::cerr << "\t\tinstruction["
342                  << getInstructionIndex(mi) << "]: ";
343                  mi->print(std::cerr, *tm_););
344
345            // we only join virtual registers with allocatable
346            // physical registers since we do not have liveness information
347            // on not allocatable physical registers
348            unsigned regA, regB;
349            if (tii.isMoveInstr(*mi, regA, regB) &&
350                (MRegisterInfo::isVirtualRegister(regA) ||
351                 lv_->getAllocatablePhysicalRegisters()[regA]) &&
352                (MRegisterInfo::isVirtualRegister(regB) ||
353                 lv_->getAllocatablePhysicalRegisters()[regB])) {
354
355                // get representative registers
356                regA = rep(regA);
357                regB = rep(regB);
358
359                // if they are already joined we continue
360                if (regA == regB)
361                    continue;
362
363                Reg2IntervalMap::iterator r2iA = r2iMap_.find(regA);
364                assert(r2iA != r2iMap_.end());
365                Reg2IntervalMap::iterator r2iB = r2iMap_.find(regB);
366                assert(r2iB != r2iMap_.end());
367
368                Intervals::iterator intA = r2iA->second;
369                Intervals::iterator intB = r2iB->second;
370
371                // both A and B are virtual registers
372                if (MRegisterInfo::isVirtualRegister(intA->reg) &&
373                    MRegisterInfo::isVirtualRegister(intB->reg)) {
374
375                    const TargetRegisterClass *rcA, *rcB;
376                    rcA = mf_->getSSARegMap()->getRegClass(intA->reg);
377                    rcB = mf_->getSSARegMap()->getRegClass(intB->reg);
378                    assert(rcA == rcB && "registers must be of the same class");
379
380                    // if their intervals do not overlap we join them
381                    if (!intB->overlaps(*intA)) {
382                        intA->join(*intB);
383                        r2iB->second = r2iA->second;
384                        r2rMap_.insert(std::make_pair(intB->reg, intA->reg));
385                        intervals_.erase(intB);
386                        ++numJoined;
387                    }
388                }
389                else if (MRegisterInfo::isPhysicalRegister(intA->reg) ^
390                         MRegisterInfo::isPhysicalRegister(intB->reg)) {
391                    if (MRegisterInfo::isPhysicalRegister(intB->reg)) {
392                        std::swap(regA, regB);
393                        std::swap(intA, intB);
394                        std::swap(r2iA, r2iB);
395                    }
396
397                    assert(MRegisterInfo::isPhysicalRegister(intA->reg) &&
398                           MRegisterInfo::isVirtualRegister(intB->reg) &&
399                           "A must be physical and B must be virtual");
400
401                    if (!intA->overlaps(*intB) &&
402                         !overlapsAliases(*intA, *intB)) {
403                        intA->join(*intB);
404                        r2iB->second = r2iA->second;
405                        r2rMap_.insert(std::make_pair(intB->reg, intA->reg));
406                        intervals_.erase(intB);
407                        ++numJoined;
408                    }
409                }
410            }
411        }
412    }
413}
414
415bool LiveIntervals::overlapsAliases(const Interval& lhs,
416                                    const Interval& rhs) const
417{
418    assert(MRegisterInfo::isPhysicalRegister(lhs.reg) &&
419           "first interval must describe a physical register");
420
421    for (const unsigned* as = mri_->getAliasSet(lhs.reg); *as; ++as) {
422        Reg2IntervalMap::const_iterator r2i = r2iMap_.find(*as);
423        assert(r2i != r2iMap_.end() && "alias does not have interval?");
424        if (rhs.overlaps(*r2i->second))
425            return true;
426    }
427
428    return false;
429}
430
431LiveIntervals::Interval::Interval(unsigned r)
432    : reg(r),
433      weight((MRegisterInfo::isPhysicalRegister(r) ?
434              std::numeric_limits<float>::max() : 0.0F))
435{
436
437}
438
439// This example is provided becaues liveAt() is non-obvious:
440//
441// this = [1,2), liveAt(1) will return false. The idea is that the
442// variable is defined in 1 and not live after definition. So it was
443// dead to begin with (defined but never used).
444//
445// this = [1,3), liveAt(2) will return false. The variable is used at
446// 2 but 2 is the last use so the variable's allocated register is
447// available for reuse.
448bool LiveIntervals::Interval::liveAt(unsigned index) const
449{
450    Range dummy(index, index+1);
451    Ranges::const_iterator r = std::upper_bound(ranges.begin(),
452                                                ranges.end(),
453                                                dummy);
454    if (r == ranges.begin())
455        return false;
456
457    --r;
458    return index >= r->first && index < (r->second - 1);
459}
460
461// This example is provided because overlaps() is non-obvious:
462//
463// 0: A = ...
464// 1: B = ...
465// 2: C = A + B ;; last use of A
466//
467// The live intervals should look like:
468//
469// A = [0, 3)
470// B = [1, x)
471// C = [2, y)
472//
473// A->overlaps(C) should return false since we want to be able to join
474// A and C.
475bool LiveIntervals::Interval::overlaps(const Interval& other) const
476{
477    Ranges::const_iterator i = ranges.begin();
478    Ranges::const_iterator ie = ranges.end();
479    Ranges::const_iterator j = other.ranges.begin();
480    Ranges::const_iterator je = other.ranges.end();
481    if (i->first < j->first) {
482        i = std::upper_bound(i, ie, *j);
483        if (i != ranges.begin()) --i;
484    }
485    else if (j->first < i->first) {
486        j = std::upper_bound(j, je, *i);
487        if (j != other.ranges.begin()) --j;
488    }
489
490    while (i != ie && j != je) {
491        if (i->first == j->first) {
492            return true;
493        }
494        else {
495            if (i->first > j->first) {
496                swap(i, j);
497                swap(ie, je);
498            }
499            assert(i->first < j->first);
500
501            if ((i->second - 1) > j->first) {
502                return true;
503            }
504            else {
505                ++i;
506            }
507        }
508    }
509
510    return false;
511}
512
513void LiveIntervals::Interval::addRange(unsigned start, unsigned end)
514{
515    assert(start < end && "Invalid range to add!");
516    DEBUG(std::cerr << "\t\t\tadding range: [" << start <<','<< end << ") -> ");
517    //assert(start < end && "invalid range?");
518    Range range = std::make_pair(start, end);
519    Ranges::iterator it =
520        ranges.insert(std::upper_bound(ranges.begin(), ranges.end(), range),
521                      range);
522
523    it = mergeRangesForward(it);
524    it = mergeRangesBackward(it);
525    DEBUG(std::cerr << "\t\t\t\tafter merging: " << *this << '\n');
526}
527
528void LiveIntervals::Interval::join(const LiveIntervals::Interval& other)
529{
530    DEBUG(std::cerr << "\t\t\t\tjoining intervals: "
531          << other << " and " << *this << '\n');
532    Ranges::iterator cur = ranges.begin();
533
534    for (Ranges::const_iterator i = other.ranges.begin(),
535             e = other.ranges.end(); i != e; ++i) {
536        cur = ranges.insert(std::upper_bound(cur, ranges.end(), *i), *i);
537        cur = mergeRangesForward(cur);
538        cur = mergeRangesBackward(cur);
539    }
540    if (MRegisterInfo::isVirtualRegister(reg))
541        weight += other.weight;
542
543    DEBUG(std::cerr << "\t\t\t\tafter merging: " << *this << '\n');
544}
545
546LiveIntervals::Interval::Ranges::iterator
547LiveIntervals::Interval::mergeRangesForward(Ranges::iterator it)
548{
549    for (Ranges::iterator next = it + 1;
550         next != ranges.end() && it->second >= next->first; ) {
551        it->second = std::max(it->second, next->second);
552        next = ranges.erase(next);
553    }
554    return it;
555}
556
557LiveIntervals::Interval::Ranges::iterator
558LiveIntervals::Interval::mergeRangesBackward(Ranges::iterator it)
559{
560    while (it != ranges.begin()) {
561        Ranges::iterator prev = it - 1;
562        if (it->first > prev->second) break;
563
564        it->first = std::min(it->first, prev->first);
565        it->second = std::max(it->second, prev->second);
566        it = ranges.erase(prev);
567    }
568
569    return it;
570}
571
572std::ostream& llvm::operator<<(std::ostream& os,
573                               const LiveIntervals::Interval& li)
574{
575    os << "%reg" << li.reg << ',' << li.weight << " = ";
576    for (LiveIntervals::Interval::Ranges::const_iterator
577             i = li.ranges.begin(), e = li.ranges.end(); i != e; ++i) {
578        os << "[" << i->first << "," << i->second << ")";
579    }
580    return os;
581}
582