xref: /freebsd/contrib/llvm-project/llvm/lib/CodeGen/GlobalMerge.cpp (revision c66ec88fed842fbaad62c30d510644ceb7bd2d71)
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/SmallPtrSet.h"
66 #include "llvm/ADT/SmallVector.h"
67 #include "llvm/ADT/Statistic.h"
68 #include "llvm/ADT/StringRef.h"
69 #include "llvm/ADT/Triple.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 <algorithm>
95 #include <cassert>
96 #include <cstddef>
97 #include <cstdint>
98 #include <string>
99 #include <vector>
100 
101 using namespace llvm;
102 
103 #define DEBUG_TYPE "global-merge"
104 
105 // FIXME: This is only useful as a last-resort way to disable the pass.
106 static cl::opt<bool>
107 EnableGlobalMerge("enable-global-merge", cl::Hidden,
108                   cl::desc("Enable the global merge pass"),
109                   cl::init(true));
110 
111 static cl::opt<unsigned>
112 GlobalMergeMaxOffset("global-merge-max-offset", cl::Hidden,
113                      cl::desc("Set maximum offset for global merge pass"),
114                      cl::init(0));
115 
116 static cl::opt<bool> GlobalMergeGroupByUse(
117     "global-merge-group-by-use", cl::Hidden,
118     cl::desc("Improve global merge pass to look at uses"), cl::init(true));
119 
120 static cl::opt<bool> GlobalMergeIgnoreSingleUse(
121     "global-merge-ignore-single-use", cl::Hidden,
122     cl::desc("Improve global merge pass to ignore globals only used alone"),
123     cl::init(true));
124 
125 static cl::opt<bool>
126 EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
127                          cl::desc("Enable global merge pass on constants"),
128                          cl::init(false));
129 
130 // FIXME: this could be a transitional option, and we probably need to remove
131 // it if only we are sure this optimization could always benefit all targets.
132 static cl::opt<cl::boolOrDefault>
133 EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
134      cl::desc("Enable global merge pass on external linkage"));
135 
136 STATISTIC(NumMerged, "Number of globals merged");
137 
138 namespace {
139 
140   class GlobalMerge : public FunctionPass {
141     const TargetMachine *TM = nullptr;
142 
143     // FIXME: Infer the maximum possible offset depending on the actual users
144     // (these max offsets are different for the users inside Thumb or ARM
145     // functions), see the code that passes in the offset in the ARM backend
146     // for more information.
147     unsigned MaxOffset;
148 
149     /// Whether we should try to optimize for size only.
150     /// Currently, this applies a dead simple heuristic: only consider globals
151     /// used in minsize functions for merging.
152     /// FIXME: This could learn about optsize, and be used in the cost model.
153     bool OnlyOptimizeForSize = false;
154 
155     /// Whether we should merge global variables that have external linkage.
156     bool MergeExternalGlobals = false;
157 
158     bool IsMachO;
159 
160     bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
161                  Module &M, bool isConst, unsigned AddrSpace) const;
162 
163     /// Merge everything in \p Globals for which the corresponding bit
164     /// in \p GlobalSet is set.
165     bool doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
166                  const BitVector &GlobalSet, Module &M, bool isConst,
167                  unsigned AddrSpace) const;
168 
169     /// Check if the given variable has been identified as must keep
170     /// \pre setMustKeepGlobalVariables must have been called on the Module that
171     ///      contains GV
172     bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
173       return MustKeepGlobalVariables.count(GV);
174     }
175 
176     /// Collect every variables marked as "used" or used in a landing pad
177     /// instruction for this Module.
178     void setMustKeepGlobalVariables(Module &M);
179 
180     /// Collect every variables marked as "used"
181     void collectUsedGlobalVariables(Module &M, StringRef Name);
182 
183     /// Keep track of the GlobalVariable that must not be merged away
184     SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
185 
186   public:
187     static char ID;             // Pass identification, replacement for typeid.
188 
189     explicit GlobalMerge()
190         : FunctionPass(ID), MaxOffset(GlobalMergeMaxOffset) {
191       initializeGlobalMergePass(*PassRegistry::getPassRegistry());
192     }
193 
194     explicit GlobalMerge(const TargetMachine *TM, unsigned MaximalOffset,
195                          bool OnlyOptimizeForSize, bool MergeExternalGlobals)
196         : FunctionPass(ID), TM(TM), MaxOffset(MaximalOffset),
197           OnlyOptimizeForSize(OnlyOptimizeForSize),
198           MergeExternalGlobals(MergeExternalGlobals) {
199       initializeGlobalMergePass(*PassRegistry::getPassRegistry());
200     }
201 
202     bool doInitialization(Module &M) override;
203     bool runOnFunction(Function &F) override;
204     bool doFinalization(Module &M) override;
205 
206     StringRef getPassName() const override { return "Merge internal globals"; }
207 
208     void getAnalysisUsage(AnalysisUsage &AU) const override {
209       AU.setPreservesCFG();
210       FunctionPass::getAnalysisUsage(AU);
211     }
212   };
213 
214 } // end anonymous namespace
215 
216 char GlobalMerge::ID = 0;
217 
218 INITIALIZE_PASS(GlobalMerge, DEBUG_TYPE, "Merge global variables", false, false)
219 
220 bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
221                           Module &M, bool isConst, unsigned AddrSpace) const {
222   auto &DL = M.getDataLayout();
223   // FIXME: Find better heuristics
224   llvm::stable_sort(
225       Globals, [&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
226         return DL.getTypeAllocSize(GV1->getValueType()) <
227                DL.getTypeAllocSize(GV2->getValueType());
228       });
229 
230   // If we want to just blindly group all globals together, do so.
231   if (!GlobalMergeGroupByUse) {
232     BitVector AllGlobals(Globals.size());
233     AllGlobals.set();
234     return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
235   }
236 
237   // If we want to be smarter, look at all uses of each global, to try to
238   // discover all sets of globals used together, and how many times each of
239   // these sets occurred.
240   //
241   // Keep this reasonably efficient, by having an append-only list of all sets
242   // discovered so far (UsedGlobalSet), and mapping each "together-ness" unit of
243   // code (currently, a Function) to the set of globals seen so far that are
244   // used together in that unit (GlobalUsesByFunction).
245   //
246   // When we look at the Nth global, we know that any new set is either:
247   // - the singleton set {N}, containing this global only, or
248   // - the union of {N} and a previously-discovered set, containing some
249   //   combination of the previous N-1 globals.
250   // Using that knowledge, when looking at the Nth global, we can keep:
251   // - a reference to the singleton set {N} (CurGVOnlySetIdx)
252   // - a list mapping each previous set to its union with {N} (EncounteredUGS),
253   //   if it actually occurs.
254 
255   // We keep track of the sets of globals used together "close enough".
256   struct UsedGlobalSet {
257     BitVector Globals;
258     unsigned UsageCount = 1;
259 
260     UsedGlobalSet(size_t Size) : Globals(Size) {}
261   };
262 
263   // Each set is unique in UsedGlobalSets.
264   std::vector<UsedGlobalSet> UsedGlobalSets;
265 
266   // Avoid repeating the create-global-set pattern.
267   auto CreateGlobalSet = [&]() -> UsedGlobalSet & {
268     UsedGlobalSets.emplace_back(Globals.size());
269     return UsedGlobalSets.back();
270   };
271 
272   // The first set is the empty set.
273   CreateGlobalSet().UsageCount = 0;
274 
275   // We define "close enough" to be "in the same function".
276   // FIXME: Grouping uses by function is way too aggressive, so we should have
277   // a better metric for distance between uses.
278   // The obvious alternative would be to group by BasicBlock, but that's in
279   // turn too conservative..
280   // Anything in between wouldn't be trivial to compute, so just stick with
281   // per-function grouping.
282 
283   // The value type is an index into UsedGlobalSets.
284   // The default (0) conveniently points to the empty set.
285   DenseMap<Function *, size_t /*UsedGlobalSetIdx*/> GlobalUsesByFunction;
286 
287   // Now, look at each merge-eligible global in turn.
288 
289   // Keep track of the sets we already encountered to which we added the
290   // current global.
291   // Each element matches the same-index element in UsedGlobalSets.
292   // This lets us efficiently tell whether a set has already been expanded to
293   // include the current global.
294   std::vector<size_t> EncounteredUGS;
295 
296   for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
297     GlobalVariable *GV = Globals[GI];
298 
299     // Reset the encountered sets for this global...
300     std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0);
301     // ...and grow it in case we created new sets for the previous global.
302     EncounteredUGS.resize(UsedGlobalSets.size());
303 
304     // We might need to create a set that only consists of the current global.
305     // Keep track of its index into UsedGlobalSets.
306     size_t CurGVOnlySetIdx = 0;
307 
308     // For each global, look at all its Uses.
309     for (auto &U : GV->uses()) {
310       // This Use might be a ConstantExpr.  We're interested in Instruction
311       // users, so look through ConstantExpr...
312       Use *UI, *UE;
313       if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U.getUser())) {
314         if (CE->use_empty())
315           continue;
316         UI = &*CE->use_begin();
317         UE = nullptr;
318       } else if (isa<Instruction>(U.getUser())) {
319         UI = &U;
320         UE = UI->getNext();
321       } else {
322         continue;
323       }
324 
325       // ...to iterate on all the instruction users of the global.
326       // Note that we iterate on Uses and not on Users to be able to getNext().
327       for (; UI != UE; UI = UI->getNext()) {
328         Instruction *I = dyn_cast<Instruction>(UI->getUser());
329         if (!I)
330           continue;
331 
332         Function *ParentFn = I->getParent()->getParent();
333 
334         // If we're only optimizing for size, ignore non-minsize functions.
335         if (OnlyOptimizeForSize && !ParentFn->hasMinSize())
336           continue;
337 
338         size_t UGSIdx = GlobalUsesByFunction[ParentFn];
339 
340         // If this is the first global the basic block uses, map it to the set
341         // consisting of this global only.
342         if (!UGSIdx) {
343           // If that set doesn't exist yet, create it.
344           if (!CurGVOnlySetIdx) {
345             CurGVOnlySetIdx = UsedGlobalSets.size();
346             CreateGlobalSet().Globals.set(GI);
347           } else {
348             ++UsedGlobalSets[CurGVOnlySetIdx].UsageCount;
349           }
350 
351           GlobalUsesByFunction[ParentFn] = CurGVOnlySetIdx;
352           continue;
353         }
354 
355         // If we already encountered this BB, just increment the counter.
356         if (UsedGlobalSets[UGSIdx].Globals.test(GI)) {
357           ++UsedGlobalSets[UGSIdx].UsageCount;
358           continue;
359         }
360 
361         // If not, the previous set wasn't actually used in this function.
362         --UsedGlobalSets[UGSIdx].UsageCount;
363 
364         // If we already expanded the previous set to include this global, just
365         // reuse that expanded set.
366         if (size_t ExpandedIdx = EncounteredUGS[UGSIdx]) {
367           ++UsedGlobalSets[ExpandedIdx].UsageCount;
368           GlobalUsesByFunction[ParentFn] = ExpandedIdx;
369           continue;
370         }
371 
372         // If not, create a new set consisting of the union of the previous set
373         // and this global.  Mark it as encountered, so we can reuse it later.
374         GlobalUsesByFunction[ParentFn] = EncounteredUGS[UGSIdx] =
375             UsedGlobalSets.size();
376 
377         UsedGlobalSet &NewUGS = CreateGlobalSet();
378         NewUGS.Globals.set(GI);
379         NewUGS.Globals |= UsedGlobalSets[UGSIdx].Globals;
380       }
381     }
382   }
383 
384   // Now we found a bunch of sets of globals used together.  We accumulated
385   // the number of times we encountered the sets (i.e., the number of blocks
386   // that use that exact set of globals).
387   //
388   // Multiply that by the size of the set to give us a crude profitability
389   // metric.
390   llvm::stable_sort(UsedGlobalSets,
391                     [](const UsedGlobalSet &UGS1, const UsedGlobalSet &UGS2) {
392                       return UGS1.Globals.count() * UGS1.UsageCount <
393                              UGS2.Globals.count() * UGS2.UsageCount;
394                     });
395 
396   // We can choose to merge all globals together, but ignore globals never used
397   // with another global.  This catches the obviously non-profitable cases of
398   // having a single global, but is aggressive enough for any other case.
399   if (GlobalMergeIgnoreSingleUse) {
400     BitVector AllGlobals(Globals.size());
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 (UGS.Globals.count() > 1)
406         AllGlobals |= UGS.Globals;
407     }
408     return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
409   }
410 
411   // Starting from the sets with the best (=biggest) profitability, find a
412   // good combination.
413   // The ideal (and expensive) solution can only be found by trying all
414   // combinations, looking for the one with the best profitability.
415   // Don't be smart about it, and just pick the first compatible combination,
416   // starting with the sets with the best profitability.
417   BitVector PickedGlobals(Globals.size());
418   bool Changed = false;
419 
420   for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
421     const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
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       }
597     }
598   }
599 }
600 
601 bool GlobalMerge::doInitialization(Module &M) {
602   if (!EnableGlobalMerge)
603     return false;
604 
605   IsMachO = Triple(M.getTargetTriple()).isOSBinFormatMachO();
606 
607   auto &DL = M.getDataLayout();
608   DenseMap<std::pair<unsigned, StringRef>, SmallVector<GlobalVariable *, 16>>
609       Globals, ConstGlobals, BSSGlobals;
610   bool Changed = false;
611   setMustKeepGlobalVariables(M);
612 
613   // Grab all non-const globals.
614   for (auto &GV : M.globals()) {
615     // Merge is safe for "normal" internal or external globals only
616     if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasImplicitSection())
617       continue;
618 
619     // It's not safe to merge globals that may be preempted
620     if (TM && !TM->shouldAssumeDSOLocal(M, &GV))
621       continue;
622 
623     if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
624         !GV.hasInternalLinkage())
625       continue;
626 
627     PointerType *PT = dyn_cast<PointerType>(GV.getType());
628     assert(PT && "Global variable is not a pointer!");
629 
630     unsigned AddressSpace = PT->getAddressSpace();
631     StringRef Section = GV.getSection();
632 
633     // Ignore all 'special' globals.
634     if (GV.getName().startswith("llvm.") ||
635         GV.getName().startswith(".llvm."))
636       continue;
637 
638     // Ignore all "required" globals:
639     if (isMustKeepGlobalVariable(&GV))
640       continue;
641 
642     Type *Ty = GV.getValueType();
643     if (DL.getTypeAllocSize(Ty) < MaxOffset) {
644       if (TM &&
645           TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSS())
646         BSSGlobals[{AddressSpace, Section}].push_back(&GV);
647       else if (GV.isConstant())
648         ConstGlobals[{AddressSpace, Section}].push_back(&GV);
649       else
650         Globals[{AddressSpace, Section}].push_back(&GV);
651     }
652   }
653 
654   for (auto &P : Globals)
655     if (P.second.size() > 1)
656       Changed |= doMerge(P.second, M, false, P.first.first);
657 
658   for (auto &P : BSSGlobals)
659     if (P.second.size() > 1)
660       Changed |= doMerge(P.second, M, false, P.first.first);
661 
662   if (EnableGlobalMergeOnConst)
663     for (auto &P : ConstGlobals)
664       if (P.second.size() > 1)
665         Changed |= doMerge(P.second, M, true, P.first.first);
666 
667   return Changed;
668 }
669 
670 bool GlobalMerge::runOnFunction(Function &F) {
671   return false;
672 }
673 
674 bool GlobalMerge::doFinalization(Module &M) {
675   MustKeepGlobalVariables.clear();
676   return false;
677 }
678 
679 Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset,
680                                   bool OnlyOptimizeForSize,
681                                   bool MergeExternalByDefault) {
682   bool MergeExternal = (EnableGlobalMergeOnExternal == cl::BOU_UNSET) ?
683     MergeExternalByDefault : (EnableGlobalMergeOnExternal == cl::BOU_TRUE);
684   return new GlobalMerge(TM, Offset, OnlyOptimizeForSize, MergeExternal);
685 }
686