xref: /freebsd/contrib/llvm-project/llvm/lib/CodeGen/GlobalMerge.cpp (revision 069ac18495ad8fde2748bc94b0f80a50250bb01d)
1 //===- GlobalMerge.cpp - Internal globals merging -------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
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 
63 #include "llvm/ADT/BitVector.h"
64 #include "llvm/ADT/DenseMap.h"
65 #include "llvm/ADT/SetVector.h"
66 #include "llvm/ADT/SmallPtrSet.h"
67 #include "llvm/ADT/SmallVector.h"
68 #include "llvm/ADT/Statistic.h"
69 #include "llvm/ADT/StringRef.h"
70 #include "llvm/ADT/Twine.h"
71 #include "llvm/CodeGen/Passes.h"
72 #include "llvm/IR/BasicBlock.h"
73 #include "llvm/IR/Constants.h"
74 #include "llvm/IR/DataLayout.h"
75 #include "llvm/IR/DerivedTypes.h"
76 #include "llvm/IR/Function.h"
77 #include "llvm/IR/GlobalAlias.h"
78 #include "llvm/IR/GlobalValue.h"
79 #include "llvm/IR/GlobalVariable.h"
80 #include "llvm/IR/Instruction.h"
81 #include "llvm/IR/Module.h"
82 #include "llvm/IR/Type.h"
83 #include "llvm/IR/Use.h"
84 #include "llvm/IR/User.h"
85 #include "llvm/InitializePasses.h"
86 #include "llvm/MC/SectionKind.h"
87 #include "llvm/Pass.h"
88 #include "llvm/Support/Casting.h"
89 #include "llvm/Support/CommandLine.h"
90 #include "llvm/Support/Debug.h"
91 #include "llvm/Support/raw_ostream.h"
92 #include "llvm/Target/TargetLoweringObjectFile.h"
93 #include "llvm/Target/TargetMachine.h"
94 #include "llvm/TargetParser/Triple.h"
95 #include <algorithm>
96 #include <cassert>
97 #include <cstddef>
98 #include <cstdint>
99 #include <string>
100 #include <vector>
101 
102 using namespace llvm;
103 
104 #define DEBUG_TYPE "global-merge"
105 
106 // FIXME: This is only useful as a last-resort way to disable the pass.
107 static cl::opt<bool>
108 EnableGlobalMerge("enable-global-merge", cl::Hidden,
109                   cl::desc("Enable the global merge pass"),
110                   cl::init(true));
111 
112 static cl::opt<unsigned>
113 GlobalMergeMaxOffset("global-merge-max-offset", cl::Hidden,
114                      cl::desc("Set maximum offset for global merge pass"),
115                      cl::init(0));
116 
117 static cl::opt<bool> GlobalMergeGroupByUse(
118     "global-merge-group-by-use", cl::Hidden,
119     cl::desc("Improve global merge pass to look at uses"), cl::init(true));
120 
121 static cl::opt<bool> GlobalMergeIgnoreSingleUse(
122     "global-merge-ignore-single-use", cl::Hidden,
123     cl::desc("Improve global merge pass to ignore globals only used alone"),
124     cl::init(true));
125 
126 static cl::opt<bool>
127 EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
128                          cl::desc("Enable global merge pass on constants"),
129                          cl::init(false));
130 
131 // FIXME: this could be a transitional option, and we probably need to remove
132 // it if only we are sure this optimization could always benefit all targets.
133 static cl::opt<cl::boolOrDefault>
134 EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
135      cl::desc("Enable global merge pass on external linkage"));
136 
137 STATISTIC(NumMerged, "Number of globals merged");
138 
139 namespace {
140 
141   class GlobalMerge : public FunctionPass {
142     const TargetMachine *TM = nullptr;
143 
144     // FIXME: Infer the maximum possible offset depending on the actual users
145     // (these max offsets are different for the users inside Thumb or ARM
146     // functions), see the code that passes in the offset in the ARM backend
147     // for more information.
148     unsigned MaxOffset;
149 
150     /// Whether we should try to optimize for size only.
151     /// Currently, this applies a dead simple heuristic: only consider globals
152     /// used in minsize functions for merging.
153     /// FIXME: This could learn about optsize, and be used in the cost model.
154     bool OnlyOptimizeForSize = false;
155 
156     /// Whether we should merge global variables that have external linkage.
157     bool MergeExternalGlobals = false;
158 
159     bool IsMachO = false;
160 
161     bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
162                  Module &M, bool isConst, unsigned AddrSpace) const;
163 
164     /// Merge everything in \p Globals for which the corresponding bit
165     /// in \p GlobalSet is set.
166     bool doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
167                  const BitVector &GlobalSet, Module &M, bool isConst,
168                  unsigned AddrSpace) const;
169 
170     /// Check if the given variable has been identified as must keep
171     /// \pre setMustKeepGlobalVariables must have been called on the Module that
172     ///      contains GV
173     bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
174       return MustKeepGlobalVariables.count(GV);
175     }
176 
177     /// Collect every variables marked as "used" or used in a landing pad
178     /// instruction for this Module.
179     void setMustKeepGlobalVariables(Module &M);
180 
181     /// Collect every variables marked as "used"
182     void collectUsedGlobalVariables(Module &M, StringRef Name);
183 
184     /// Keep track of the GlobalVariable that must not be merged away
185     SmallSetVector<const GlobalVariable *, 16> MustKeepGlobalVariables;
186 
187   public:
188     static char ID;             // Pass identification, replacement for typeid.
189 
190     explicit GlobalMerge()
191         : FunctionPass(ID), MaxOffset(GlobalMergeMaxOffset) {
192       initializeGlobalMergePass(*PassRegistry::getPassRegistry());
193     }
194 
195     explicit GlobalMerge(const TargetMachine *TM, unsigned MaximalOffset,
196                          bool OnlyOptimizeForSize, bool MergeExternalGlobals)
197         : FunctionPass(ID), TM(TM), MaxOffset(MaximalOffset),
198           OnlyOptimizeForSize(OnlyOptimizeForSize),
199           MergeExternalGlobals(MergeExternalGlobals) {
200       initializeGlobalMergePass(*PassRegistry::getPassRegistry());
201     }
202 
203     bool doInitialization(Module &M) override;
204     bool runOnFunction(Function &F) override;
205     bool doFinalization(Module &M) override;
206 
207     StringRef getPassName() const override { return "Merge internal globals"; }
208 
209     void getAnalysisUsage(AnalysisUsage &AU) const override {
210       AU.setPreservesCFG();
211       FunctionPass::getAnalysisUsage(AU);
212     }
213   };
214 
215 } // end anonymous namespace
216 
217 char GlobalMerge::ID = 0;
218 
219 INITIALIZE_PASS(GlobalMerge, DEBUG_TYPE, "Merge global variables", false, false)
220 
221 bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
222                           Module &M, bool isConst, unsigned AddrSpace) const {
223   auto &DL = M.getDataLayout();
224   // FIXME: Find better heuristics
225   llvm::stable_sort(
226       Globals, [&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
227         // We don't support scalable global variables.
228         return DL.getTypeAllocSize(GV1->getValueType()).getFixedValue() <
229                DL.getTypeAllocSize(GV2->getValueType()).getFixedValue();
230       });
231 
232   // If we want to just blindly group all globals together, do so.
233   if (!GlobalMergeGroupByUse) {
234     BitVector AllGlobals(Globals.size());
235     AllGlobals.set();
236     return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
237   }
238 
239   // If we want to be smarter, look at all uses of each global, to try to
240   // discover all sets of globals used together, and how many times each of
241   // these sets occurred.
242   //
243   // Keep this reasonably efficient, by having an append-only list of all sets
244   // discovered so far (UsedGlobalSet), and mapping each "together-ness" unit of
245   // code (currently, a Function) to the set of globals seen so far that are
246   // used together in that unit (GlobalUsesByFunction).
247   //
248   // When we look at the Nth global, we know that any new set is either:
249   // - the singleton set {N}, containing this global only, or
250   // - the union of {N} and a previously-discovered set, containing some
251   //   combination of the previous N-1 globals.
252   // Using that knowledge, when looking at the Nth global, we can keep:
253   // - a reference to the singleton set {N} (CurGVOnlySetIdx)
254   // - a list mapping each previous set to its union with {N} (EncounteredUGS),
255   //   if it actually occurs.
256 
257   // We keep track of the sets of globals used together "close enough".
258   struct UsedGlobalSet {
259     BitVector Globals;
260     unsigned UsageCount = 1;
261 
262     UsedGlobalSet(size_t Size) : Globals(Size) {}
263   };
264 
265   // Each set is unique in UsedGlobalSets.
266   std::vector<UsedGlobalSet> UsedGlobalSets;
267 
268   // Avoid repeating the create-global-set pattern.
269   auto CreateGlobalSet = [&]() -> UsedGlobalSet & {
270     UsedGlobalSets.emplace_back(Globals.size());
271     return UsedGlobalSets.back();
272   };
273 
274   // The first set is the empty set.
275   CreateGlobalSet().UsageCount = 0;
276 
277   // We define "close enough" to be "in the same function".
278   // FIXME: Grouping uses by function is way too aggressive, so we should have
279   // a better metric for distance between uses.
280   // The obvious alternative would be to group by BasicBlock, but that's in
281   // turn too conservative..
282   // Anything in between wouldn't be trivial to compute, so just stick with
283   // per-function grouping.
284 
285   // The value type is an index into UsedGlobalSets.
286   // The default (0) conveniently points to the empty set.
287   DenseMap<Function *, size_t /*UsedGlobalSetIdx*/> GlobalUsesByFunction;
288 
289   // Now, look at each merge-eligible global in turn.
290 
291   // Keep track of the sets we already encountered to which we added the
292   // current global.
293   // Each element matches the same-index element in UsedGlobalSets.
294   // This lets us efficiently tell whether a set has already been expanded to
295   // include the current global.
296   std::vector<size_t> EncounteredUGS;
297 
298   for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
299     GlobalVariable *GV = Globals[GI];
300 
301     // Reset the encountered sets for this global...
302     std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0);
303     // ...and grow it in case we created new sets for the previous global.
304     EncounteredUGS.resize(UsedGlobalSets.size());
305 
306     // We might need to create a set that only consists of the current global.
307     // Keep track of its index into UsedGlobalSets.
308     size_t CurGVOnlySetIdx = 0;
309 
310     // For each global, look at all its Uses.
311     for (auto &U : GV->uses()) {
312       // This Use might be a ConstantExpr.  We're interested in Instruction
313       // users, so look through ConstantExpr...
314       Use *UI, *UE;
315       if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U.getUser())) {
316         if (CE->use_empty())
317           continue;
318         UI = &*CE->use_begin();
319         UE = nullptr;
320       } else if (isa<Instruction>(U.getUser())) {
321         UI = &U;
322         UE = UI->getNext();
323       } else {
324         continue;
325       }
326 
327       // ...to iterate on all the instruction users of the global.
328       // Note that we iterate on Uses and not on Users to be able to getNext().
329       for (; UI != UE; UI = UI->getNext()) {
330         Instruction *I = dyn_cast<Instruction>(UI->getUser());
331         if (!I)
332           continue;
333 
334         Function *ParentFn = I->getParent()->getParent();
335 
336         // If we're only optimizing for size, ignore non-minsize functions.
337         if (OnlyOptimizeForSize && !ParentFn->hasMinSize())
338           continue;
339 
340         size_t UGSIdx = GlobalUsesByFunction[ParentFn];
341 
342         // If this is the first global the basic block uses, map it to the set
343         // consisting of this global only.
344         if (!UGSIdx) {
345           // If that set doesn't exist yet, create it.
346           if (!CurGVOnlySetIdx) {
347             CurGVOnlySetIdx = UsedGlobalSets.size();
348             CreateGlobalSet().Globals.set(GI);
349           } else {
350             ++UsedGlobalSets[CurGVOnlySetIdx].UsageCount;
351           }
352 
353           GlobalUsesByFunction[ParentFn] = CurGVOnlySetIdx;
354           continue;
355         }
356 
357         // If we already encountered this BB, just increment the counter.
358         if (UsedGlobalSets[UGSIdx].Globals.test(GI)) {
359           ++UsedGlobalSets[UGSIdx].UsageCount;
360           continue;
361         }
362 
363         // If not, the previous set wasn't actually used in this function.
364         --UsedGlobalSets[UGSIdx].UsageCount;
365 
366         // If we already expanded the previous set to include this global, just
367         // reuse that expanded set.
368         if (size_t ExpandedIdx = EncounteredUGS[UGSIdx]) {
369           ++UsedGlobalSets[ExpandedIdx].UsageCount;
370           GlobalUsesByFunction[ParentFn] = ExpandedIdx;
371           continue;
372         }
373 
374         // If not, create a new set consisting of the union of the previous set
375         // and this global.  Mark it as encountered, so we can reuse it later.
376         GlobalUsesByFunction[ParentFn] = EncounteredUGS[UGSIdx] =
377             UsedGlobalSets.size();
378 
379         UsedGlobalSet &NewUGS = CreateGlobalSet();
380         NewUGS.Globals.set(GI);
381         NewUGS.Globals |= UsedGlobalSets[UGSIdx].Globals;
382       }
383     }
384   }
385 
386   // Now we found a bunch of sets of globals used together.  We accumulated
387   // the number of times we encountered the sets (i.e., the number of blocks
388   // that use that exact set of globals).
389   //
390   // Multiply that by the size of the set to give us a crude profitability
391   // metric.
392   llvm::stable_sort(UsedGlobalSets,
393                     [](const UsedGlobalSet &UGS1, const UsedGlobalSet &UGS2) {
394                       return UGS1.Globals.count() * UGS1.UsageCount <
395                              UGS2.Globals.count() * UGS2.UsageCount;
396                     });
397 
398   // We can choose to merge all globals together, but ignore globals never used
399   // with another global.  This catches the obviously non-profitable cases of
400   // having a single global, but is aggressive enough for any other case.
401   if (GlobalMergeIgnoreSingleUse) {
402     BitVector AllGlobals(Globals.size());
403     for (const UsedGlobalSet &UGS : llvm::reverse(UsedGlobalSets)) {
404       if (UGS.UsageCount == 0)
405         continue;
406       if (UGS.Globals.count() > 1)
407         AllGlobals |= UGS.Globals;
408     }
409     return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
410   }
411 
412   // Starting from the sets with the best (=biggest) profitability, find a
413   // good combination.
414   // The ideal (and expensive) solution can only be found by trying all
415   // combinations, looking for the one with the best profitability.
416   // Don't be smart about it, and just pick the first compatible combination,
417   // starting with the sets with the best profitability.
418   BitVector PickedGlobals(Globals.size());
419   bool Changed = false;
420 
421   for (const UsedGlobalSet &UGS : llvm::reverse(UsedGlobalSets)) {
422     if (UGS.UsageCount == 0)
423       continue;
424     if (PickedGlobals.anyCommon(UGS.Globals))
425       continue;
426     PickedGlobals |= UGS.Globals;
427     // If the set only contains one global, there's no point in merging.
428     // Ignore the global for inclusion in other sets though, so keep it in
429     // PickedGlobals.
430     if (UGS.Globals.count() < 2)
431       continue;
432     Changed |= doMerge(Globals, UGS.Globals, M, isConst, AddrSpace);
433   }
434 
435   return Changed;
436 }
437 
438 bool GlobalMerge::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
439                           const BitVector &GlobalSet, Module &M, bool isConst,
440                           unsigned AddrSpace) const {
441   assert(Globals.size() > 1);
442 
443   Type *Int32Ty = Type::getInt32Ty(M.getContext());
444   Type *Int8Ty = Type::getInt8Ty(M.getContext());
445   auto &DL = M.getDataLayout();
446 
447   LLVM_DEBUG(dbgs() << " Trying to merge set, starts with #"
448                     << GlobalSet.find_first() << "\n");
449 
450   bool Changed = false;
451   ssize_t i = GlobalSet.find_first();
452   while (i != -1) {
453     ssize_t j = 0;
454     uint64_t MergedSize = 0;
455     std::vector<Type*> Tys;
456     std::vector<Constant*> Inits;
457     std::vector<unsigned> StructIdxs;
458 
459     bool HasExternal = false;
460     StringRef FirstExternalName;
461     Align MaxAlign;
462     unsigned CurIdx = 0;
463     for (j = i; j != -1; j = GlobalSet.find_next(j)) {
464       Type *Ty = Globals[j]->getValueType();
465 
466       // Make sure we use the same alignment AsmPrinter would use.
467       Align Alignment = DL.getPreferredAlign(Globals[j]);
468       unsigned Padding = alignTo(MergedSize, Alignment) - MergedSize;
469       MergedSize += Padding;
470       MergedSize += DL.getTypeAllocSize(Ty);
471       if (MergedSize > MaxOffset) {
472         break;
473       }
474       if (Padding) {
475         Tys.push_back(ArrayType::get(Int8Ty, Padding));
476         Inits.push_back(ConstantAggregateZero::get(Tys.back()));
477         ++CurIdx;
478       }
479       Tys.push_back(Ty);
480       Inits.push_back(Globals[j]->getInitializer());
481       StructIdxs.push_back(CurIdx++);
482 
483       MaxAlign = std::max(MaxAlign, Alignment);
484 
485       if (Globals[j]->hasExternalLinkage() && !HasExternal) {
486         HasExternal = true;
487         FirstExternalName = Globals[j]->getName();
488       }
489     }
490 
491     // Exit early if there is only one global to merge.
492     if (Tys.size() < 2) {
493       i = j;
494       continue;
495     }
496 
497     // If merged variables doesn't have external linkage, we needn't to expose
498     // the symbol after merging.
499     GlobalValue::LinkageTypes Linkage = HasExternal
500                                             ? GlobalValue::ExternalLinkage
501                                             : GlobalValue::InternalLinkage;
502     // Use a packed struct so we can control alignment.
503     StructType *MergedTy = StructType::get(M.getContext(), Tys, true);
504     Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
505 
506     // On Darwin external linkage needs to be preserved, otherwise
507     // dsymutil cannot preserve the debug info for the merged
508     // variables.  If they have external linkage, use the symbol name
509     // of the first variable merged as the suffix of global symbol
510     // name.  This avoids a link-time naming conflict for the
511     // _MergedGlobals symbols.
512     Twine MergedName =
513         (IsMachO && HasExternal)
514             ? "_MergedGlobals_" + FirstExternalName
515             : "_MergedGlobals";
516     auto MergedLinkage = IsMachO ? Linkage : GlobalValue::PrivateLinkage;
517     auto *MergedGV = new GlobalVariable(
518         M, MergedTy, isConst, MergedLinkage, MergedInit, MergedName, nullptr,
519         GlobalVariable::NotThreadLocal, AddrSpace);
520 
521     MergedGV->setAlignment(MaxAlign);
522     MergedGV->setSection(Globals[i]->getSection());
523 
524     const StructLayout *MergedLayout = DL.getStructLayout(MergedTy);
525     for (ssize_t k = i, idx = 0; k != j; k = GlobalSet.find_next(k), ++idx) {
526       GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
527       std::string Name(Globals[k]->getName());
528       GlobalValue::VisibilityTypes Visibility = Globals[k]->getVisibility();
529       GlobalValue::DLLStorageClassTypes DLLStorage =
530           Globals[k]->getDLLStorageClass();
531 
532       // Copy metadata while adjusting any debug info metadata by the original
533       // global's offset within the merged global.
534       MergedGV->copyMetadata(Globals[k],
535                              MergedLayout->getElementOffset(StructIdxs[idx]));
536 
537       Constant *Idx[2] = {
538           ConstantInt::get(Int32Ty, 0),
539           ConstantInt::get(Int32Ty, StructIdxs[idx]),
540       };
541       Constant *GEP =
542           ConstantExpr::getInBoundsGetElementPtr(MergedTy, MergedGV, Idx);
543       Globals[k]->replaceAllUsesWith(GEP);
544       Globals[k]->eraseFromParent();
545 
546       // When the linkage is not internal we must emit an alias for the original
547       // variable name as it may be accessed from another object. On non-Mach-O
548       // we can also emit an alias for internal linkage as it's safe to do so.
549       // It's not safe on Mach-O as the alias (and thus the portion of the
550       // MergedGlobals variable) may be dead stripped at link time.
551       if (Linkage != GlobalValue::InternalLinkage || !IsMachO) {
552         GlobalAlias *GA = GlobalAlias::create(Tys[StructIdxs[idx]], AddrSpace,
553                                               Linkage, Name, GEP, &M);
554         GA->setVisibility(Visibility);
555         GA->setDLLStorageClass(DLLStorage);
556       }
557 
558       NumMerged++;
559     }
560     Changed = true;
561     i = j;
562   }
563 
564   return Changed;
565 }
566 
567 void GlobalMerge::collectUsedGlobalVariables(Module &M, StringRef Name) {
568   // Extract global variables from llvm.used array
569   const GlobalVariable *GV = M.getGlobalVariable(Name);
570   if (!GV || !GV->hasInitializer()) return;
571 
572   // Should be an array of 'i8*'.
573   const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());
574 
575   for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
576     if (const GlobalVariable *G =
577         dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
578       MustKeepGlobalVariables.insert(G);
579 }
580 
581 void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
582   collectUsedGlobalVariables(M, "llvm.used");
583   collectUsedGlobalVariables(M, "llvm.compiler.used");
584 
585   for (Function &F : M) {
586     for (BasicBlock &BB : F) {
587       Instruction *Pad = BB.getFirstNonPHI();
588       if (!Pad->isEHPad())
589         continue;
590 
591       // Keep globals used by landingpads and catchpads.
592       for (const Use &U : Pad->operands()) {
593         if (const GlobalVariable *GV =
594                 dyn_cast<GlobalVariable>(U->stripPointerCasts()))
595           MustKeepGlobalVariables.insert(GV);
596         else if (const ConstantArray *CA = dyn_cast<ConstantArray>(U->stripPointerCasts())) {
597           for (const Use &Elt : CA->operands()) {
598             if (const GlobalVariable *GV =
599                     dyn_cast<GlobalVariable>(Elt->stripPointerCasts()))
600               MustKeepGlobalVariables.insert(GV);
601           }
602         }
603       }
604     }
605   }
606 }
607 
608 bool GlobalMerge::doInitialization(Module &M) {
609   if (!EnableGlobalMerge)
610     return false;
611 
612   IsMachO = Triple(M.getTargetTriple()).isOSBinFormatMachO();
613 
614   auto &DL = M.getDataLayout();
615   DenseMap<std::pair<unsigned, StringRef>, SmallVector<GlobalVariable *, 16>>
616       Globals, ConstGlobals, BSSGlobals;
617   bool Changed = false;
618   setMustKeepGlobalVariables(M);
619 
620   LLVM_DEBUG({
621       dbgs() << "Number of GV that must be kept:  " <<
622                 MustKeepGlobalVariables.size() << "\n";
623       for (const GlobalVariable *KeptGV : MustKeepGlobalVariables)
624         dbgs() << "Kept: " << *KeptGV << "\n";
625   });
626   // Grab all non-const globals.
627   for (auto &GV : M.globals()) {
628     // Merge is safe for "normal" internal or external globals only
629     if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasImplicitSection())
630       continue;
631 
632     // It's not safe to merge globals that may be preempted
633     if (TM && !TM->shouldAssumeDSOLocal(M, &GV))
634       continue;
635 
636     if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
637         !GV.hasInternalLinkage())
638       continue;
639 
640     PointerType *PT = dyn_cast<PointerType>(GV.getType());
641     assert(PT && "Global variable is not a pointer!");
642 
643     unsigned AddressSpace = PT->getAddressSpace();
644     StringRef Section = GV.getSection();
645 
646     // Ignore all 'special' globals.
647     if (GV.getName().startswith("llvm.") ||
648         GV.getName().startswith(".llvm."))
649       continue;
650 
651     // Ignore all "required" globals:
652     if (isMustKeepGlobalVariable(&GV))
653       continue;
654 
655     // Don't merge tagged globals, as each global should have its own unique
656     // memory tag at runtime. TODO(hctim): This can be relaxed: constant globals
657     // with compatible alignment and the same contents may be merged as long as
658     // the globals occupy the same number of tag granules (i.e. `size_a / 16 ==
659     // size_b / 16`).
660     if (GV.isTagged())
661       continue;
662 
663     Type *Ty = GV.getValueType();
664     if (DL.getTypeAllocSize(Ty) < MaxOffset) {
665       if (TM &&
666           TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSS())
667         BSSGlobals[{AddressSpace, Section}].push_back(&GV);
668       else if (GV.isConstant())
669         ConstGlobals[{AddressSpace, Section}].push_back(&GV);
670       else
671         Globals[{AddressSpace, Section}].push_back(&GV);
672     }
673   }
674 
675   for (auto &P : Globals)
676     if (P.second.size() > 1)
677       Changed |= doMerge(P.second, M, false, P.first.first);
678 
679   for (auto &P : BSSGlobals)
680     if (P.second.size() > 1)
681       Changed |= doMerge(P.second, M, false, P.first.first);
682 
683   if (EnableGlobalMergeOnConst)
684     for (auto &P : ConstGlobals)
685       if (P.second.size() > 1)
686         Changed |= doMerge(P.second, M, true, P.first.first);
687 
688   return Changed;
689 }
690 
691 bool GlobalMerge::runOnFunction(Function &F) {
692   return false;
693 }
694 
695 bool GlobalMerge::doFinalization(Module &M) {
696   MustKeepGlobalVariables.clear();
697   return false;
698 }
699 
700 Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset,
701                                   bool OnlyOptimizeForSize,
702                                   bool MergeExternalByDefault) {
703   bool MergeExternal = (EnableGlobalMergeOnExternal == cl::BOU_UNSET) ?
704     MergeExternalByDefault : (EnableGlobalMergeOnExternal == cl::BOU_TRUE);
705   return new GlobalMerge(TM, Offset, OnlyOptimizeForSize, MergeExternal);
706 }
707