1//===-- GlobalMerge.cpp - Internal globals merging  -----------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9// This pass merges globals with internal linkage into one. This way all the
10// globals which were merged into a biggest one can be addressed using offsets
11// from the same base pointer (no need for separate base pointer for each of the
12// global). Such a transformation can significantly reduce the register pressure
13// when many globals are involved.
14//
15// For example, consider the code which touches several global variables at
16// once:
17//
18// static int foo[N], bar[N], baz[N];
19//
20// for (i = 0; i < N; ++i) {
21//    foo[i] = bar[i] * baz[i];
22// }
23//
24//  On ARM the addresses of 3 arrays should be kept in the registers, thus
25//  this code has quite large register pressure (loop body):
26//
27//  ldr     r1, [r5], #4
28//  ldr     r2, [r6], #4
29//  mul     r1, r2, r1
30//  str     r1, [r0], #4
31//
32//  Pass converts the code to something like:
33//
34//  static struct {
35//    int foo[N];
36//    int bar[N];
37//    int baz[N];
38//  } merged;
39//
40//  for (i = 0; i < N; ++i) {
41//    merged.foo[i] = merged.bar[i] * merged.baz[i];
42//  }
43//
44//  and in ARM code this becomes:
45//
46//  ldr     r0, [r5, #40]
47//  ldr     r1, [r5, #80]
48//  mul     r0, r1, r0
49//  str     r0, [r5], #4
50//
51//  note that we saved 2 registers here almostly "for free".
52//
53// However, merging globals can have tradeoffs:
54// - it confuses debuggers, tools, and users
55// - it makes linker optimizations less useful (order files, LOHs, ...)
56// - it forces usage of indexed addressing (which isn't necessarily "free")
57// - it can increase register pressure when the uses are disparate enough.
58//
59// We use heuristics to discover the best global grouping we can (cf cl::opts).
60// ===---------------------------------------------------------------------===//
61
62#include "llvm/Transforms/Scalar.h"
63#include "llvm/ADT/DenseMap.h"
64#include "llvm/ADT/SmallBitVector.h"
65#include "llvm/ADT/SmallPtrSet.h"
66#include "llvm/ADT/Statistic.h"
67#include "llvm/CodeGen/Passes.h"
68#include "llvm/IR/Attributes.h"
69#include "llvm/IR/Constants.h"
70#include "llvm/IR/DataLayout.h"
71#include "llvm/IR/DerivedTypes.h"
72#include "llvm/IR/Function.h"
73#include "llvm/IR/GlobalVariable.h"
74#include "llvm/IR/Instructions.h"
75#include "llvm/IR/Intrinsics.h"
76#include "llvm/IR/Module.h"
77#include "llvm/Pass.h"
78#include "llvm/Support/CommandLine.h"
79#include "llvm/Support/Debug.h"
80#include "llvm/Support/raw_ostream.h"
81#include "llvm/Target/TargetLowering.h"
82#include "llvm/Target/TargetLoweringObjectFile.h"
83#include "llvm/Target/TargetSubtargetInfo.h"
84#include <algorithm>
85using namespace llvm;
86
87#define DEBUG_TYPE "global-merge"
88
89// FIXME: This is only useful as a last-resort way to disable the pass.
90cl::opt<bool>
91EnableGlobalMerge("enable-global-merge", cl::Hidden,
92                  cl::desc("Enable the global merge pass"),
93                  cl::init(true));
94
95static cl::opt<bool> GlobalMergeGroupByUse(
96    "global-merge-group-by-use", cl::Hidden,
97    cl::desc("Improve global merge pass to look at uses"), cl::init(true));
98
99static cl::opt<bool> GlobalMergeIgnoreSingleUse(
100    "global-merge-ignore-single-use", cl::Hidden,
101    cl::desc("Improve global merge pass to ignore globals only used alone"),
102    cl::init(true));
103
104static cl::opt<bool>
105EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
106                         cl::desc("Enable global merge pass on constants"),
107                         cl::init(false));
108
109// FIXME: this could be a transitional option, and we probably need to remove
110// it if only we are sure this optimization could always benefit all targets.
111static cl::opt<cl::boolOrDefault>
112EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
113     cl::desc("Enable global merge pass on external linkage"));
114
115STATISTIC(NumMerged, "Number of globals merged");
116namespace {
117  class GlobalMerge : public FunctionPass {
118    const TargetMachine *TM;
119    // FIXME: Infer the maximum possible offset depending on the actual users
120    // (these max offsets are different for the users inside Thumb or ARM
121    // functions), see the code that passes in the offset in the ARM backend
122    // for more information.
123    unsigned MaxOffset;
124
125    /// Whether we should try to optimize for size only.
126    /// Currently, this applies a dead simple heuristic: only consider globals
127    /// used in minsize functions for merging.
128    /// FIXME: This could learn about optsize, and be used in the cost model.
129    bool OnlyOptimizeForSize;
130
131    /// Whether we should merge global variables that have external linkage.
132    bool MergeExternalGlobals;
133
134    bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
135                 Module &M, bool isConst, unsigned AddrSpace) const;
136    /// \brief Merge everything in \p Globals for which the corresponding bit
137    /// in \p GlobalSet is set.
138    bool doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
139                 const BitVector &GlobalSet, Module &M, bool isConst,
140                 unsigned AddrSpace) const;
141
142    /// \brief Check if the given variable has been identified as must keep
143    /// \pre setMustKeepGlobalVariables must have been called on the Module that
144    ///      contains GV
145    bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
146      return MustKeepGlobalVariables.count(GV);
147    }
148
149    /// Collect every variables marked as "used" or used in a landing pad
150    /// instruction for this Module.
151    void setMustKeepGlobalVariables(Module &M);
152
153    /// Collect every variables marked as "used"
154    void collectUsedGlobalVariables(Module &M);
155
156    /// Keep track of the GlobalVariable that must not be merged away
157    SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
158
159  public:
160    static char ID;             // Pass identification, replacement for typeid.
161    explicit GlobalMerge(const TargetMachine *TM = nullptr,
162                         unsigned MaximalOffset = 0,
163                         bool OnlyOptimizeForSize = false,
164                         bool MergeExternalGlobals = false)
165        : FunctionPass(ID), TM(TM), MaxOffset(MaximalOffset),
166          OnlyOptimizeForSize(OnlyOptimizeForSize),
167          MergeExternalGlobals(MergeExternalGlobals) {
168      initializeGlobalMergePass(*PassRegistry::getPassRegistry());
169    }
170
171    bool doInitialization(Module &M) override;
172    bool runOnFunction(Function &F) override;
173    bool doFinalization(Module &M) override;
174
175    const char *getPassName() const override {
176      return "Merge internal globals";
177    }
178
179    void getAnalysisUsage(AnalysisUsage &AU) const override {
180      AU.setPreservesCFG();
181      FunctionPass::getAnalysisUsage(AU);
182    }
183  };
184} // end anonymous namespace
185
186char GlobalMerge::ID = 0;
187INITIALIZE_PASS_BEGIN(GlobalMerge, "global-merge", "Merge global variables",
188                      false, false)
189INITIALIZE_PASS_END(GlobalMerge, "global-merge", "Merge global variables",
190                    false, false)
191
192bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
193                          Module &M, bool isConst, unsigned AddrSpace) const {
194  auto &DL = M.getDataLayout();
195  // FIXME: Find better heuristics
196  std::stable_sort(Globals.begin(), Globals.end(),
197                   [&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
198                     return DL.getTypeAllocSize(GV1->getValueType()) <
199                            DL.getTypeAllocSize(GV2->getValueType());
200                   });
201
202  // If we want to just blindly group all globals together, do so.
203  if (!GlobalMergeGroupByUse) {
204    BitVector AllGlobals(Globals.size());
205    AllGlobals.set();
206    return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
207  }
208
209  // If we want to be smarter, look at all uses of each global, to try to
210  // discover all sets of globals used together, and how many times each of
211  // these sets occurred.
212  //
213  // Keep this reasonably efficient, by having an append-only list of all sets
214  // discovered so far (UsedGlobalSet), and mapping each "together-ness" unit of
215  // code (currently, a Function) to the set of globals seen so far that are
216  // used together in that unit (GlobalUsesByFunction).
217  //
218  // When we look at the Nth global, we now that any new set is either:
219  // - the singleton set {N}, containing this global only, or
220  // - the union of {N} and a previously-discovered set, containing some
221  //   combination of the previous N-1 globals.
222  // Using that knowledge, when looking at the Nth global, we can keep:
223  // - a reference to the singleton set {N} (CurGVOnlySetIdx)
224  // - a list mapping each previous set to its union with {N} (EncounteredUGS),
225  //   if it actually occurs.
226
227  // We keep track of the sets of globals used together "close enough".
228  struct UsedGlobalSet {
229    UsedGlobalSet(size_t Size) : Globals(Size), UsageCount(1) {}
230    BitVector Globals;
231    unsigned UsageCount;
232  };
233
234  // Each set is unique in UsedGlobalSets.
235  std::vector<UsedGlobalSet> UsedGlobalSets;
236
237  // Avoid repeating the create-global-set pattern.
238  auto CreateGlobalSet = [&]() -> UsedGlobalSet & {
239    UsedGlobalSets.emplace_back(Globals.size());
240    return UsedGlobalSets.back();
241  };
242
243  // The first set is the empty set.
244  CreateGlobalSet().UsageCount = 0;
245
246  // We define "close enough" to be "in the same function".
247  // FIXME: Grouping uses by function is way too aggressive, so we should have
248  // a better metric for distance between uses.
249  // The obvious alternative would be to group by BasicBlock, but that's in
250  // turn too conservative..
251  // Anything in between wouldn't be trivial to compute, so just stick with
252  // per-function grouping.
253
254  // The value type is an index into UsedGlobalSets.
255  // The default (0) conveniently points to the empty set.
256  DenseMap<Function *, size_t /*UsedGlobalSetIdx*/> GlobalUsesByFunction;
257
258  // Now, look at each merge-eligible global in turn.
259
260  // Keep track of the sets we already encountered to which we added the
261  // current global.
262  // Each element matches the same-index element in UsedGlobalSets.
263  // This lets us efficiently tell whether a set has already been expanded to
264  // include the current global.
265  std::vector<size_t> EncounteredUGS;
266
267  for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
268    GlobalVariable *GV = Globals[GI];
269
270    // Reset the encountered sets for this global...
271    std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0);
272    // ...and grow it in case we created new sets for the previous global.
273    EncounteredUGS.resize(UsedGlobalSets.size());
274
275    // We might need to create a set that only consists of the current global.
276    // Keep track of its index into UsedGlobalSets.
277    size_t CurGVOnlySetIdx = 0;
278
279    // For each global, look at all its Uses.
280    for (auto &U : GV->uses()) {
281      // This Use might be a ConstantExpr.  We're interested in Instruction
282      // users, so look through ConstantExpr...
283      Use *UI, *UE;
284      if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U.getUser())) {
285        if (CE->use_empty())
286          continue;
287        UI = &*CE->use_begin();
288        UE = nullptr;
289      } else if (isa<Instruction>(U.getUser())) {
290        UI = &U;
291        UE = UI->getNext();
292      } else {
293        continue;
294      }
295
296      // ...to iterate on all the instruction users of the global.
297      // Note that we iterate on Uses and not on Users to be able to getNext().
298      for (; UI != UE; UI = UI->getNext()) {
299        Instruction *I = dyn_cast<Instruction>(UI->getUser());
300        if (!I)
301          continue;
302
303        Function *ParentFn = I->getParent()->getParent();
304
305        // If we're only optimizing for size, ignore non-minsize functions.
306        if (OnlyOptimizeForSize && !ParentFn->optForMinSize())
307          continue;
308
309        size_t UGSIdx = GlobalUsesByFunction[ParentFn];
310
311        // If this is the first global the basic block uses, map it to the set
312        // consisting of this global only.
313        if (!UGSIdx) {
314          // If that set doesn't exist yet, create it.
315          if (!CurGVOnlySetIdx) {
316            CurGVOnlySetIdx = UsedGlobalSets.size();
317            CreateGlobalSet().Globals.set(GI);
318          } else {
319            ++UsedGlobalSets[CurGVOnlySetIdx].UsageCount;
320          }
321
322          GlobalUsesByFunction[ParentFn] = CurGVOnlySetIdx;
323          continue;
324        }
325
326        // If we already encountered this BB, just increment the counter.
327        if (UsedGlobalSets[UGSIdx].Globals.test(GI)) {
328          ++UsedGlobalSets[UGSIdx].UsageCount;
329          continue;
330        }
331
332        // If not, the previous set wasn't actually used in this function.
333        --UsedGlobalSets[UGSIdx].UsageCount;
334
335        // If we already expanded the previous set to include this global, just
336        // reuse that expanded set.
337        if (size_t ExpandedIdx = EncounteredUGS[UGSIdx]) {
338          ++UsedGlobalSets[ExpandedIdx].UsageCount;
339          GlobalUsesByFunction[ParentFn] = ExpandedIdx;
340          continue;
341        }
342
343        // If not, create a new set consisting of the union of the previous set
344        // and this global.  Mark it as encountered, so we can reuse it later.
345        GlobalUsesByFunction[ParentFn] = EncounteredUGS[UGSIdx] =
346            UsedGlobalSets.size();
347
348        UsedGlobalSet &NewUGS = CreateGlobalSet();
349        NewUGS.Globals.set(GI);
350        NewUGS.Globals |= UsedGlobalSets[UGSIdx].Globals;
351      }
352    }
353  }
354
355  // Now we found a bunch of sets of globals used together.  We accumulated
356  // the number of times we encountered the sets (i.e., the number of blocks
357  // that use that exact set of globals).
358  //
359  // Multiply that by the size of the set to give us a crude profitability
360  // metric.
361  std::sort(UsedGlobalSets.begin(), UsedGlobalSets.end(),
362            [](const UsedGlobalSet &UGS1, const UsedGlobalSet &UGS2) {
363              return UGS1.Globals.count() * UGS1.UsageCount <
364                     UGS2.Globals.count() * UGS2.UsageCount;
365            });
366
367  // We can choose to merge all globals together, but ignore globals never used
368  // with another global.  This catches the obviously non-profitable cases of
369  // having a single global, but is aggressive enough for any other case.
370  if (GlobalMergeIgnoreSingleUse) {
371    BitVector AllGlobals(Globals.size());
372    for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
373      const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
374      if (UGS.UsageCount == 0)
375        continue;
376      if (UGS.Globals.count() > 1)
377        AllGlobals |= UGS.Globals;
378    }
379    return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
380  }
381
382  // Starting from the sets with the best (=biggest) profitability, find a
383  // good combination.
384  // The ideal (and expensive) solution can only be found by trying all
385  // combinations, looking for the one with the best profitability.
386  // Don't be smart about it, and just pick the first compatible combination,
387  // starting with the sets with the best profitability.
388  BitVector PickedGlobals(Globals.size());
389  bool Changed = false;
390
391  for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
392    const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
393    if (UGS.UsageCount == 0)
394      continue;
395    if (PickedGlobals.anyCommon(UGS.Globals))
396      continue;
397    PickedGlobals |= UGS.Globals;
398    // If the set only contains one global, there's no point in merging.
399    // Ignore the global for inclusion in other sets though, so keep it in
400    // PickedGlobals.
401    if (UGS.Globals.count() < 2)
402      continue;
403    Changed |= doMerge(Globals, UGS.Globals, M, isConst, AddrSpace);
404  }
405
406  return Changed;
407}
408
409bool GlobalMerge::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
410                          const BitVector &GlobalSet, Module &M, bool isConst,
411                          unsigned AddrSpace) const {
412  assert(Globals.size() > 1);
413
414  Type *Int32Ty = Type::getInt32Ty(M.getContext());
415  auto &DL = M.getDataLayout();
416
417  DEBUG(dbgs() << " Trying to merge set, starts with #"
418               << GlobalSet.find_first() << "\n");
419
420  ssize_t i = GlobalSet.find_first();
421  while (i != -1) {
422    ssize_t j = 0;
423    uint64_t MergedSize = 0;
424    std::vector<Type*> Tys;
425    std::vector<Constant*> Inits;
426
427    for (j = i; j != -1; j = GlobalSet.find_next(j)) {
428      Type *Ty = Globals[j]->getValueType();
429      MergedSize += DL.getTypeAllocSize(Ty);
430      if (MergedSize > MaxOffset) {
431        break;
432      }
433      Tys.push_back(Ty);
434      Inits.push_back(Globals[j]->getInitializer());
435    }
436
437    StructType *MergedTy = StructType::get(M.getContext(), Tys);
438    Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
439
440    GlobalVariable *MergedGV = new GlobalVariable(
441        M, MergedTy, isConst, GlobalValue::PrivateLinkage, MergedInit,
442        "_MergedGlobals", nullptr, GlobalVariable::NotThreadLocal, AddrSpace);
443
444    for (ssize_t k = i, idx = 0; k != j; k = GlobalSet.find_next(k), ++idx) {
445      GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
446      std::string Name = Globals[k]->getName();
447
448      Constant *Idx[2] = {
449        ConstantInt::get(Int32Ty, 0),
450        ConstantInt::get(Int32Ty, idx),
451      };
452      Constant *GEP =
453          ConstantExpr::getInBoundsGetElementPtr(MergedTy, MergedGV, Idx);
454      Globals[k]->replaceAllUsesWith(GEP);
455      Globals[k]->eraseFromParent();
456
457      // When the linkage is not internal we must emit an alias for the original
458      // variable name as it may be accessed from another object. On non-Mach-O
459      // we can also emit an alias for internal linkage as it's safe to do so.
460      // It's not safe on Mach-O as the alias (and thus the portion of the
461      // MergedGlobals variable) may be dead stripped at link time.
462      if (Linkage != GlobalValue::InternalLinkage ||
463          !TM->getTargetTriple().isOSBinFormatMachO()) {
464        GlobalAlias::create(Tys[idx], AddrSpace, Linkage, Name, GEP, &M);
465      }
466
467      NumMerged++;
468    }
469    i = j;
470  }
471
472  return true;
473}
474
475void GlobalMerge::collectUsedGlobalVariables(Module &M) {
476  // Extract global variables from llvm.used array
477  const GlobalVariable *GV = M.getGlobalVariable("llvm.used");
478  if (!GV || !GV->hasInitializer()) return;
479
480  // Should be an array of 'i8*'.
481  const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());
482
483  for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
484    if (const GlobalVariable *G =
485        dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
486      MustKeepGlobalVariables.insert(G);
487}
488
489void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
490  collectUsedGlobalVariables(M);
491
492  for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn;
493       ++IFn) {
494    for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
495         IBB != IEndBB; ++IBB) {
496      // Follow the invoke link to find the landing pad instruction
497      const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator());
498      if (!II) continue;
499
500      const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst();
501      // Look for globals in the clauses of the landing pad instruction
502      for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses();
503           Idx != NumClauses; ++Idx)
504        if (const GlobalVariable *GV =
505            dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
506                                     ->stripPointerCasts()))
507          MustKeepGlobalVariables.insert(GV);
508    }
509  }
510}
511
512bool GlobalMerge::doInitialization(Module &M) {
513  if (!EnableGlobalMerge)
514    return false;
515
516  auto &DL = M.getDataLayout();
517  DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
518                                                        BSSGlobals;
519  bool Changed = false;
520  setMustKeepGlobalVariables(M);
521
522  // Grab all non-const globals.
523  for (auto &GV : M.globals()) {
524    // Merge is safe for "normal" internal or external globals only
525    if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasSection())
526      continue;
527
528    if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
529        !GV.hasInternalLinkage())
530      continue;
531
532    PointerType *PT = dyn_cast<PointerType>(GV.getType());
533    assert(PT && "Global variable is not a pointer!");
534
535    unsigned AddressSpace = PT->getAddressSpace();
536
537    // Ignore fancy-aligned globals for now.
538    unsigned Alignment = DL.getPreferredAlignment(&GV);
539    Type *Ty = GV.getValueType();
540    if (Alignment > DL.getABITypeAlignment(Ty))
541      continue;
542
543    // Ignore all 'special' globals.
544    if (GV.getName().startswith("llvm.") ||
545        GV.getName().startswith(".llvm."))
546      continue;
547
548    // Ignore all "required" globals:
549    if (isMustKeepGlobalVariable(&GV))
550      continue;
551
552    if (DL.getTypeAllocSize(Ty) < MaxOffset) {
553      if (TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSSLocal())
554        BSSGlobals[AddressSpace].push_back(&GV);
555      else if (GV.isConstant())
556        ConstGlobals[AddressSpace].push_back(&GV);
557      else
558        Globals[AddressSpace].push_back(&GV);
559    }
560  }
561
562  for (auto &P : Globals)
563    if (P.second.size() > 1)
564      Changed |= doMerge(P.second, M, false, P.first);
565
566  for (auto &P : BSSGlobals)
567    if (P.second.size() > 1)
568      Changed |= doMerge(P.second, M, false, P.first);
569
570  if (EnableGlobalMergeOnConst)
571    for (auto &P : ConstGlobals)
572      if (P.second.size() > 1)
573        Changed |= doMerge(P.second, M, true, P.first);
574
575  return Changed;
576}
577
578bool GlobalMerge::runOnFunction(Function &F) {
579  return false;
580}
581
582bool GlobalMerge::doFinalization(Module &M) {
583  MustKeepGlobalVariables.clear();
584  return false;
585}
586
587Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset,
588                                  bool OnlyOptimizeForSize,
589                                  bool MergeExternalByDefault) {
590  bool MergeExternal = (EnableGlobalMergeOnExternal == cl::BOU_UNSET) ?
591    MergeExternalByDefault : (EnableGlobalMergeOnExternal == cl::BOU_TRUE);
592  return new GlobalMerge(TM, Offset, OnlyOptimizeForSize, MergeExternal);
593}
594