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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/ADT/DenseMap.h"
63 #include "llvm/ADT/SmallBitVector.h"
64 #include "llvm/ADT/SmallPtrSet.h"
65 #include "llvm/ADT/Statistic.h"
66 #include "llvm/CodeGen/Passes.h"
67 #include "llvm/IR/Attributes.h"
68 #include "llvm/IR/Constants.h"
69 #include "llvm/IR/DataLayout.h"
70 #include "llvm/IR/DerivedTypes.h"
71 #include "llvm/IR/Function.h"
72 #include "llvm/IR/GlobalVariable.h"
73 #include "llvm/IR/Instructions.h"
74 #include "llvm/IR/Intrinsics.h"
75 #include "llvm/IR/Module.h"
76 #include "llvm/Pass.h"
77 #include "llvm/Support/CommandLine.h"
78 #include "llvm/Support/Debug.h"
79 #include "llvm/Support/raw_ostream.h"
80 #include "llvm/Target/TargetLowering.h"
81 #include "llvm/Target/TargetLoweringObjectFile.h"
82 #include "llvm/Target/TargetSubtargetInfo.h"
83 #include <algorithm>
84 using namespace llvm;
85 
86 #define DEBUG_TYPE "global-merge"
87 
88 // FIXME: This is only useful as a last-resort way to disable the pass.
89 cl::opt<bool>
90 EnableGlobalMerge("enable-global-merge", cl::Hidden,
91                   cl::desc("Enable the global merge pass"),
92                   cl::init(true));
93 
94 static cl::opt<unsigned>
95 GlobalMergeMaxOffset("global-merge-max-offset", cl::Hidden,
96                      cl::desc("Set maximum offset for global merge pass"),
97                      cl::init(0));
98 
99 static cl::opt<bool> GlobalMergeGroupByUse(
100     "global-merge-group-by-use", cl::Hidden,
101     cl::desc("Improve global merge pass to look at uses"), cl::init(true));
102 
103 static cl::opt<bool> GlobalMergeIgnoreSingleUse(
104     "global-merge-ignore-single-use", cl::Hidden,
105     cl::desc("Improve global merge pass to ignore globals only used alone"),
106     cl::init(true));
107 
108 static cl::opt<bool>
109 EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
110                          cl::desc("Enable global merge pass on constants"),
111                          cl::init(false));
112 
113 // FIXME: this could be a transitional option, and we probably need to remove
114 // it if only we are sure this optimization could always benefit all targets.
115 static cl::opt<cl::boolOrDefault>
116 EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
117      cl::desc("Enable global merge pass on external linkage"));
118 
119 STATISTIC(NumMerged, "Number of globals merged");
120 namespace {
121   class GlobalMerge : public FunctionPass {
122     const TargetMachine *TM;
123     // FIXME: Infer the maximum possible offset depending on the actual users
124     // (these max offsets are different for the users inside Thumb or ARM
125     // functions), see the code that passes in the offset in the ARM backend
126     // for more information.
127     unsigned MaxOffset;
128 
129     /// Whether we should try to optimize for size only.
130     /// Currently, this applies a dead simple heuristic: only consider globals
131     /// used in minsize functions for merging.
132     /// FIXME: This could learn about optsize, and be used in the cost model.
133     bool OnlyOptimizeForSize;
134 
135     /// Whether we should merge global variables that have external linkage.
136     bool MergeExternalGlobals;
137 
138     bool IsMachO;
139 
140     bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
141                  Module &M, bool isConst, unsigned AddrSpace) const;
142     /// \brief Merge everything in \p Globals for which the corresponding bit
143     /// in \p GlobalSet is set.
144     bool doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
145                  const BitVector &GlobalSet, Module &M, bool isConst,
146                  unsigned AddrSpace) const;
147 
148     /// \brief Check if the given variable has been identified as must keep
149     /// \pre setMustKeepGlobalVariables must have been called on the Module that
150     ///      contains GV
isMustKeepGlobalVariable(const GlobalVariable * GV) const151     bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
152       return MustKeepGlobalVariables.count(GV);
153     }
154 
155     /// Collect every variables marked as "used" or used in a landing pad
156     /// instruction for this Module.
157     void setMustKeepGlobalVariables(Module &M);
158 
159     /// Collect every variables marked as "used"
160     void collectUsedGlobalVariables(Module &M);
161 
162     /// Keep track of the GlobalVariable that must not be merged away
163     SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
164 
165   public:
166     static char ID;             // Pass identification, replacement for typeid.
GlobalMerge()167     explicit GlobalMerge()
168         : FunctionPass(ID), TM(nullptr), MaxOffset(GlobalMergeMaxOffset),
169           OnlyOptimizeForSize(false), MergeExternalGlobals(false) {
170       initializeGlobalMergePass(*PassRegistry::getPassRegistry());
171     }
172 
GlobalMerge(const TargetMachine * TM,unsigned MaximalOffset,bool OnlyOptimizeForSize,bool MergeExternalGlobals)173     explicit GlobalMerge(const TargetMachine *TM, unsigned MaximalOffset,
174                          bool OnlyOptimizeForSize, bool MergeExternalGlobals)
175         : FunctionPass(ID), TM(TM), MaxOffset(MaximalOffset),
176           OnlyOptimizeForSize(OnlyOptimizeForSize),
177           MergeExternalGlobals(MergeExternalGlobals) {
178       initializeGlobalMergePass(*PassRegistry::getPassRegistry());
179     }
180 
181     bool doInitialization(Module &M) override;
182     bool runOnFunction(Function &F) override;
183     bool doFinalization(Module &M) override;
184 
getPassName() const185     const char *getPassName() const override {
186       return "Merge internal globals";
187     }
188 
getAnalysisUsage(AnalysisUsage & AU) const189     void getAnalysisUsage(AnalysisUsage &AU) const override {
190       AU.setPreservesCFG();
191       FunctionPass::getAnalysisUsage(AU);
192     }
193   };
194 } // end anonymous namespace
195 
196 char GlobalMerge::ID = 0;
197 INITIALIZE_PASS_BEGIN(GlobalMerge, "global-merge", "Merge global variables",
198                       false, false)
199 INITIALIZE_PASS_END(GlobalMerge, "global-merge", "Merge global variables",
200                     false, false)
201 
doMerge(SmallVectorImpl<GlobalVariable * > & Globals,Module & M,bool isConst,unsigned AddrSpace) const202 bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
203                           Module &M, bool isConst, unsigned AddrSpace) const {
204   auto &DL = M.getDataLayout();
205   // FIXME: Find better heuristics
206   std::stable_sort(Globals.begin(), Globals.end(),
207                    [&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
208                      return DL.getTypeAllocSize(GV1->getValueType()) <
209                             DL.getTypeAllocSize(GV2->getValueType());
210                    });
211 
212   // If we want to just blindly group all globals together, do so.
213   if (!GlobalMergeGroupByUse) {
214     BitVector AllGlobals(Globals.size());
215     AllGlobals.set();
216     return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
217   }
218 
219   // If we want to be smarter, look at all uses of each global, to try to
220   // discover all sets of globals used together, and how many times each of
221   // these sets occurred.
222   //
223   // Keep this reasonably efficient, by having an append-only list of all sets
224   // discovered so far (UsedGlobalSet), and mapping each "together-ness" unit of
225   // code (currently, a Function) to the set of globals seen so far that are
226   // used together in that unit (GlobalUsesByFunction).
227   //
228   // When we look at the Nth global, we now that any new set is either:
229   // - the singleton set {N}, containing this global only, or
230   // - the union of {N} and a previously-discovered set, containing some
231   //   combination of the previous N-1 globals.
232   // Using that knowledge, when looking at the Nth global, we can keep:
233   // - a reference to the singleton set {N} (CurGVOnlySetIdx)
234   // - a list mapping each previous set to its union with {N} (EncounteredUGS),
235   //   if it actually occurs.
236 
237   // We keep track of the sets of globals used together "close enough".
238   struct UsedGlobalSet {
239     UsedGlobalSet(size_t Size) : Globals(Size), UsageCount(1) {}
240     BitVector Globals;
241     unsigned UsageCount;
242   };
243 
244   // Each set is unique in UsedGlobalSets.
245   std::vector<UsedGlobalSet> UsedGlobalSets;
246 
247   // Avoid repeating the create-global-set pattern.
248   auto CreateGlobalSet = [&]() -> UsedGlobalSet & {
249     UsedGlobalSets.emplace_back(Globals.size());
250     return UsedGlobalSets.back();
251   };
252 
253   // The first set is the empty set.
254   CreateGlobalSet().UsageCount = 0;
255 
256   // We define "close enough" to be "in the same function".
257   // FIXME: Grouping uses by function is way too aggressive, so we should have
258   // a better metric for distance between uses.
259   // The obvious alternative would be to group by BasicBlock, but that's in
260   // turn too conservative..
261   // Anything in between wouldn't be trivial to compute, so just stick with
262   // per-function grouping.
263 
264   // The value type is an index into UsedGlobalSets.
265   // The default (0) conveniently points to the empty set.
266   DenseMap<Function *, size_t /*UsedGlobalSetIdx*/> GlobalUsesByFunction;
267 
268   // Now, look at each merge-eligible global in turn.
269 
270   // Keep track of the sets we already encountered to which we added the
271   // current global.
272   // Each element matches the same-index element in UsedGlobalSets.
273   // This lets us efficiently tell whether a set has already been expanded to
274   // include the current global.
275   std::vector<size_t> EncounteredUGS;
276 
277   for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
278     GlobalVariable *GV = Globals[GI];
279 
280     // Reset the encountered sets for this global...
281     std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0);
282     // ...and grow it in case we created new sets for the previous global.
283     EncounteredUGS.resize(UsedGlobalSets.size());
284 
285     // We might need to create a set that only consists of the current global.
286     // Keep track of its index into UsedGlobalSets.
287     size_t CurGVOnlySetIdx = 0;
288 
289     // For each global, look at all its Uses.
290     for (auto &U : GV->uses()) {
291       // This Use might be a ConstantExpr.  We're interested in Instruction
292       // users, so look through ConstantExpr...
293       Use *UI, *UE;
294       if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U.getUser())) {
295         if (CE->use_empty())
296           continue;
297         UI = &*CE->use_begin();
298         UE = nullptr;
299       } else if (isa<Instruction>(U.getUser())) {
300         UI = &U;
301         UE = UI->getNext();
302       } else {
303         continue;
304       }
305 
306       // ...to iterate on all the instruction users of the global.
307       // Note that we iterate on Uses and not on Users to be able to getNext().
308       for (; UI != UE; UI = UI->getNext()) {
309         Instruction *I = dyn_cast<Instruction>(UI->getUser());
310         if (!I)
311           continue;
312 
313         Function *ParentFn = I->getParent()->getParent();
314 
315         // If we're only optimizing for size, ignore non-minsize functions.
316         if (OnlyOptimizeForSize && !ParentFn->optForMinSize())
317           continue;
318 
319         size_t UGSIdx = GlobalUsesByFunction[ParentFn];
320 
321         // If this is the first global the basic block uses, map it to the set
322         // consisting of this global only.
323         if (!UGSIdx) {
324           // If that set doesn't exist yet, create it.
325           if (!CurGVOnlySetIdx) {
326             CurGVOnlySetIdx = UsedGlobalSets.size();
327             CreateGlobalSet().Globals.set(GI);
328           } else {
329             ++UsedGlobalSets[CurGVOnlySetIdx].UsageCount;
330           }
331 
332           GlobalUsesByFunction[ParentFn] = CurGVOnlySetIdx;
333           continue;
334         }
335 
336         // If we already encountered this BB, just increment the counter.
337         if (UsedGlobalSets[UGSIdx].Globals.test(GI)) {
338           ++UsedGlobalSets[UGSIdx].UsageCount;
339           continue;
340         }
341 
342         // If not, the previous set wasn't actually used in this function.
343         --UsedGlobalSets[UGSIdx].UsageCount;
344 
345         // If we already expanded the previous set to include this global, just
346         // reuse that expanded set.
347         if (size_t ExpandedIdx = EncounteredUGS[UGSIdx]) {
348           ++UsedGlobalSets[ExpandedIdx].UsageCount;
349           GlobalUsesByFunction[ParentFn] = ExpandedIdx;
350           continue;
351         }
352 
353         // If not, create a new set consisting of the union of the previous set
354         // and this global.  Mark it as encountered, so we can reuse it later.
355         GlobalUsesByFunction[ParentFn] = EncounteredUGS[UGSIdx] =
356             UsedGlobalSets.size();
357 
358         UsedGlobalSet &NewUGS = CreateGlobalSet();
359         NewUGS.Globals.set(GI);
360         NewUGS.Globals |= UsedGlobalSets[UGSIdx].Globals;
361       }
362     }
363   }
364 
365   // Now we found a bunch of sets of globals used together.  We accumulated
366   // the number of times we encountered the sets (i.e., the number of blocks
367   // that use that exact set of globals).
368   //
369   // Multiply that by the size of the set to give us a crude profitability
370   // metric.
371   std::sort(UsedGlobalSets.begin(), UsedGlobalSets.end(),
372             [](const UsedGlobalSet &UGS1, const UsedGlobalSet &UGS2) {
373               return UGS1.Globals.count() * UGS1.UsageCount <
374                      UGS2.Globals.count() * UGS2.UsageCount;
375             });
376 
377   // We can choose to merge all globals together, but ignore globals never used
378   // with another global.  This catches the obviously non-profitable cases of
379   // having a single global, but is aggressive enough for any other case.
380   if (GlobalMergeIgnoreSingleUse) {
381     BitVector AllGlobals(Globals.size());
382     for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
383       const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
384       if (UGS.UsageCount == 0)
385         continue;
386       if (UGS.Globals.count() > 1)
387         AllGlobals |= UGS.Globals;
388     }
389     return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
390   }
391 
392   // Starting from the sets with the best (=biggest) profitability, find a
393   // good combination.
394   // The ideal (and expensive) solution can only be found by trying all
395   // combinations, looking for the one with the best profitability.
396   // Don't be smart about it, and just pick the first compatible combination,
397   // starting with the sets with the best profitability.
398   BitVector PickedGlobals(Globals.size());
399   bool Changed = false;
400 
401   for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
402     const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
403     if (UGS.UsageCount == 0)
404       continue;
405     if (PickedGlobals.anyCommon(UGS.Globals))
406       continue;
407     PickedGlobals |= UGS.Globals;
408     // If the set only contains one global, there's no point in merging.
409     // Ignore the global for inclusion in other sets though, so keep it in
410     // PickedGlobals.
411     if (UGS.Globals.count() < 2)
412       continue;
413     Changed |= doMerge(Globals, UGS.Globals, M, isConst, AddrSpace);
414   }
415 
416   return Changed;
417 }
418 
doMerge(const SmallVectorImpl<GlobalVariable * > & Globals,const BitVector & GlobalSet,Module & M,bool isConst,unsigned AddrSpace) const419 bool GlobalMerge::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
420                           const BitVector &GlobalSet, Module &M, bool isConst,
421                           unsigned AddrSpace) const {
422   assert(Globals.size() > 1);
423 
424   Type *Int32Ty = Type::getInt32Ty(M.getContext());
425   auto &DL = M.getDataLayout();
426 
427   DEBUG(dbgs() << " Trying to merge set, starts with #"
428                << GlobalSet.find_first() << "\n");
429 
430   ssize_t i = GlobalSet.find_first();
431   while (i != -1) {
432     ssize_t j = 0;
433     uint64_t MergedSize = 0;
434     std::vector<Type*> Tys;
435     std::vector<Constant*> Inits;
436 
437     for (j = i; j != -1; j = GlobalSet.find_next(j)) {
438       Type *Ty = Globals[j]->getValueType();
439       MergedSize += DL.getTypeAllocSize(Ty);
440       if (MergedSize > MaxOffset) {
441         break;
442       }
443       Tys.push_back(Ty);
444       Inits.push_back(Globals[j]->getInitializer());
445     }
446 
447     StructType *MergedTy = StructType::get(M.getContext(), Tys);
448     Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
449 
450     GlobalVariable *MergedGV = new GlobalVariable(
451         M, MergedTy, isConst, GlobalValue::PrivateLinkage, MergedInit,
452         "_MergedGlobals", nullptr, GlobalVariable::NotThreadLocal, AddrSpace);
453 
454     for (ssize_t k = i, idx = 0; k != j; k = GlobalSet.find_next(k), ++idx) {
455       GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
456       std::string Name = Globals[k]->getName();
457 
458       Constant *Idx[2] = {
459         ConstantInt::get(Int32Ty, 0),
460         ConstantInt::get(Int32Ty, idx),
461       };
462       Constant *GEP =
463           ConstantExpr::getInBoundsGetElementPtr(MergedTy, MergedGV, Idx);
464       Globals[k]->replaceAllUsesWith(GEP);
465       Globals[k]->eraseFromParent();
466 
467       // When the linkage is not internal we must emit an alias for the original
468       // variable name as it may be accessed from another object. On non-Mach-O
469       // we can also emit an alias for internal linkage as it's safe to do so.
470       // It's not safe on Mach-O as the alias (and thus the portion of the
471       // MergedGlobals variable) may be dead stripped at link time.
472       if (Linkage != GlobalValue::InternalLinkage || !IsMachO) {
473         GlobalAlias::create(Tys[idx], AddrSpace, Linkage, Name, GEP, &M);
474       }
475 
476       NumMerged++;
477     }
478     i = j;
479   }
480 
481   return true;
482 }
483 
collectUsedGlobalVariables(Module & M)484 void GlobalMerge::collectUsedGlobalVariables(Module &M) {
485   // Extract global variables from llvm.used array
486   const GlobalVariable *GV = M.getGlobalVariable("llvm.used");
487   if (!GV || !GV->hasInitializer()) return;
488 
489   // Should be an array of 'i8*'.
490   const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());
491 
492   for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
493     if (const GlobalVariable *G =
494         dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
495       MustKeepGlobalVariables.insert(G);
496 }
497 
setMustKeepGlobalVariables(Module & M)498 void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
499   collectUsedGlobalVariables(M);
500 
501   for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn;
502        ++IFn) {
503     for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
504          IBB != IEndBB; ++IBB) {
505       // Follow the invoke link to find the landing pad instruction
506       const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator());
507       if (!II) continue;
508 
509       const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst();
510       // Look for globals in the clauses of the landing pad instruction
511       for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses();
512            Idx != NumClauses; ++Idx)
513         if (const GlobalVariable *GV =
514             dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
515                                      ->stripPointerCasts()))
516           MustKeepGlobalVariables.insert(GV);
517     }
518   }
519 }
520 
doInitialization(Module & M)521 bool GlobalMerge::doInitialization(Module &M) {
522   if (!EnableGlobalMerge)
523     return false;
524 
525   IsMachO = Triple(M.getTargetTriple()).isOSBinFormatMachO();
526 
527   auto &DL = M.getDataLayout();
528   DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
529                                                         BSSGlobals;
530   bool Changed = false;
531   setMustKeepGlobalVariables(M);
532 
533   // Grab all non-const globals.
534   for (auto &GV : M.globals()) {
535     // Merge is safe for "normal" internal or external globals only
536     if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasSection())
537       continue;
538 
539     if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
540         !GV.hasInternalLinkage())
541       continue;
542 
543     PointerType *PT = dyn_cast<PointerType>(GV.getType());
544     assert(PT && "Global variable is not a pointer!");
545 
546     unsigned AddressSpace = PT->getAddressSpace();
547 
548     // Ignore fancy-aligned globals for now.
549     unsigned Alignment = DL.getPreferredAlignment(&GV);
550     Type *Ty = GV.getValueType();
551     if (Alignment > DL.getABITypeAlignment(Ty))
552       continue;
553 
554     // Ignore all 'special' globals.
555     if (GV.getName().startswith("llvm.") ||
556         GV.getName().startswith(".llvm."))
557       continue;
558 
559     // Ignore all "required" globals:
560     if (isMustKeepGlobalVariable(&GV))
561       continue;
562 
563     if (DL.getTypeAllocSize(Ty) < MaxOffset) {
564       if (TM &&
565           TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSSLocal())
566         BSSGlobals[AddressSpace].push_back(&GV);
567       else if (GV.isConstant())
568         ConstGlobals[AddressSpace].push_back(&GV);
569       else
570         Globals[AddressSpace].push_back(&GV);
571     }
572   }
573 
574   for (auto &P : Globals)
575     if (P.second.size() > 1)
576       Changed |= doMerge(P.second, M, false, P.first);
577 
578   for (auto &P : BSSGlobals)
579     if (P.second.size() > 1)
580       Changed |= doMerge(P.second, M, false, P.first);
581 
582   if (EnableGlobalMergeOnConst)
583     for (auto &P : ConstGlobals)
584       if (P.second.size() > 1)
585         Changed |= doMerge(P.second, M, true, P.first);
586 
587   return Changed;
588 }
589 
runOnFunction(Function & F)590 bool GlobalMerge::runOnFunction(Function &F) {
591   return false;
592 }
593 
doFinalization(Module & M)594 bool GlobalMerge::doFinalization(Module &M) {
595   MustKeepGlobalVariables.clear();
596   return false;
597 }
598 
createGlobalMergePass(const TargetMachine * TM,unsigned Offset,bool OnlyOptimizeForSize,bool MergeExternalByDefault)599 Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset,
600                                   bool OnlyOptimizeForSize,
601                                   bool MergeExternalByDefault) {
602   bool MergeExternal = (EnableGlobalMergeOnExternal == cl::BOU_UNSET) ?
603     MergeExternalByDefault : (EnableGlobalMergeOnExternal == cl::BOU_TRUE);
604   return new GlobalMerge(TM, Offset, OnlyOptimizeForSize, MergeExternal);
605 }
606