xref: /freebsd/contrib/llvm-project/llvm/lib/CodeGen/GlobalMerge.cpp (revision a8089ea5aee578e08acab2438e82fc9a9ae50ed8)
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/CodeGen/GlobalMerge.h"
64 #include "llvm/ADT/BitVector.h"
65 #include "llvm/ADT/DenseMap.h"
66 #include "llvm/ADT/SetVector.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 GlobalMergeImpl {
142   const TargetMachine *TM = nullptr;
143   GlobalMergeOptions Opt;
144   bool IsMachO = false;
145 
146 private:
147   bool doMerge(SmallVectorImpl<GlobalVariable *> &Globals, Module &M,
148                bool isConst, unsigned AddrSpace) const;
149 
150   /// Merge everything in \p Globals for which the corresponding bit
151   /// in \p GlobalSet is set.
152   bool doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
153                const BitVector &GlobalSet, Module &M, bool isConst,
154                unsigned AddrSpace) const;
155 
156   /// Check if the given variable has been identified as must keep
157   /// \pre setMustKeepGlobalVariables must have been called on the Module that
158   ///      contains GV
159   bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
160     return MustKeepGlobalVariables.count(GV);
161   }
162 
163   /// Collect every variables marked as "used" or used in a landing pad
164   /// instruction for this Module.
165   void setMustKeepGlobalVariables(Module &M);
166 
167   /// Collect every variables marked as "used"
168   void collectUsedGlobalVariables(Module &M, StringRef Name);
169 
170   /// Keep track of the GlobalVariable that must not be merged away
171   SmallSetVector<const GlobalVariable *, 16> MustKeepGlobalVariables;
172 
173 public:
174   GlobalMergeImpl(const TargetMachine *TM, GlobalMergeOptions Opt)
175       : TM(TM), Opt(Opt) {}
176   bool run(Module &M);
177 };
178 
179 class GlobalMerge : public FunctionPass {
180   const TargetMachine *TM = nullptr;
181   GlobalMergeOptions Opt;
182 
183 public:
184   static char ID; // Pass identification, replacement for typeid.
185 
186   explicit GlobalMerge() : FunctionPass(ID) {
187     Opt.MaxOffset = GlobalMergeMaxOffset;
188     initializeGlobalMergePass(*PassRegistry::getPassRegistry());
189   }
190 
191   explicit GlobalMerge(const TargetMachine *TM, unsigned MaximalOffset,
192                        bool OnlyOptimizeForSize, bool MergeExternalGlobals)
193       : FunctionPass(ID), TM(TM) {
194     Opt.MaxOffset = MaximalOffset;
195     Opt.SizeOnly = OnlyOptimizeForSize;
196     Opt.MergeExternal = MergeExternalGlobals;
197     initializeGlobalMergePass(*PassRegistry::getPassRegistry());
198   }
199 
200   bool doInitialization(Module &M) override {
201     GlobalMergeImpl P(TM, Opt);
202     return P.run(M);
203   }
204   bool runOnFunction(Function &F) override { return false; }
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 PreservedAnalyses GlobalMergePass::run(Module &M, ModuleAnalysisManager &) {
217   GlobalMergeImpl P(TM, Options);
218   bool Changed = P.run(M);
219   if (!Changed)
220     return PreservedAnalyses::all();
221 
222   PreservedAnalyses PA;
223   PA.preserveSet<CFGAnalyses>();
224   return PA;
225 }
226 
227 char GlobalMerge::ID = 0;
228 
229 INITIALIZE_PASS(GlobalMerge, DEBUG_TYPE, "Merge global variables", false, false)
230 
231 bool GlobalMergeImpl::doMerge(SmallVectorImpl<GlobalVariable *> &Globals,
232                               Module &M, bool isConst,
233                               unsigned AddrSpace) const {
234   auto &DL = M.getDataLayout();
235   // FIXME: Find better heuristics
236   llvm::stable_sort(
237       Globals, [&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
238         // We don't support scalable global variables.
239         return DL.getTypeAllocSize(GV1->getValueType()).getFixedValue() <
240                DL.getTypeAllocSize(GV2->getValueType()).getFixedValue();
241       });
242 
243   // If we want to just blindly group all globals together, do so.
244   if (!GlobalMergeGroupByUse) {
245     BitVector AllGlobals(Globals.size());
246     AllGlobals.set();
247     return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
248   }
249 
250   // If we want to be smarter, look at all uses of each global, to try to
251   // discover all sets of globals used together, and how many times each of
252   // these sets occurred.
253   //
254   // Keep this reasonably efficient, by having an append-only list of all sets
255   // discovered so far (UsedGlobalSet), and mapping each "together-ness" unit of
256   // code (currently, a Function) to the set of globals seen so far that are
257   // used together in that unit (GlobalUsesByFunction).
258   //
259   // When we look at the Nth global, we know that any new set is either:
260   // - the singleton set {N}, containing this global only, or
261   // - the union of {N} and a previously-discovered set, containing some
262   //   combination of the previous N-1 globals.
263   // Using that knowledge, when looking at the Nth global, we can keep:
264   // - a reference to the singleton set {N} (CurGVOnlySetIdx)
265   // - a list mapping each previous set to its union with {N} (EncounteredUGS),
266   //   if it actually occurs.
267 
268   // We keep track of the sets of globals used together "close enough".
269   struct UsedGlobalSet {
270     BitVector Globals;
271     unsigned UsageCount = 1;
272 
273     UsedGlobalSet(size_t Size) : Globals(Size) {}
274   };
275 
276   // Each set is unique in UsedGlobalSets.
277   std::vector<UsedGlobalSet> UsedGlobalSets;
278 
279   // Avoid repeating the create-global-set pattern.
280   auto CreateGlobalSet = [&]() -> UsedGlobalSet & {
281     UsedGlobalSets.emplace_back(Globals.size());
282     return UsedGlobalSets.back();
283   };
284 
285   // The first set is the empty set.
286   CreateGlobalSet().UsageCount = 0;
287 
288   // We define "close enough" to be "in the same function".
289   // FIXME: Grouping uses by function is way too aggressive, so we should have
290   // a better metric for distance between uses.
291   // The obvious alternative would be to group by BasicBlock, but that's in
292   // turn too conservative..
293   // Anything in between wouldn't be trivial to compute, so just stick with
294   // per-function grouping.
295 
296   // The value type is an index into UsedGlobalSets.
297   // The default (0) conveniently points to the empty set.
298   DenseMap<Function *, size_t /*UsedGlobalSetIdx*/> GlobalUsesByFunction;
299 
300   // Now, look at each merge-eligible global in turn.
301 
302   // Keep track of the sets we already encountered to which we added the
303   // current global.
304   // Each element matches the same-index element in UsedGlobalSets.
305   // This lets us efficiently tell whether a set has already been expanded to
306   // include the current global.
307   std::vector<size_t> EncounteredUGS;
308 
309   for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
310     GlobalVariable *GV = Globals[GI];
311 
312     // Reset the encountered sets for this global...
313     std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0);
314     // ...and grow it in case we created new sets for the previous global.
315     EncounteredUGS.resize(UsedGlobalSets.size());
316 
317     // We might need to create a set that only consists of the current global.
318     // Keep track of its index into UsedGlobalSets.
319     size_t CurGVOnlySetIdx = 0;
320 
321     // For each global, look at all its Uses.
322     for (auto &U : GV->uses()) {
323       // This Use might be a ConstantExpr.  We're interested in Instruction
324       // users, so look through ConstantExpr...
325       Use *UI, *UE;
326       if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U.getUser())) {
327         if (CE->use_empty())
328           continue;
329         UI = &*CE->use_begin();
330         UE = nullptr;
331       } else if (isa<Instruction>(U.getUser())) {
332         UI = &U;
333         UE = UI->getNext();
334       } else {
335         continue;
336       }
337 
338       // ...to iterate on all the instruction users of the global.
339       // Note that we iterate on Uses and not on Users to be able to getNext().
340       for (; UI != UE; UI = UI->getNext()) {
341         Instruction *I = dyn_cast<Instruction>(UI->getUser());
342         if (!I)
343           continue;
344 
345         Function *ParentFn = I->getParent()->getParent();
346 
347         // If we're only optimizing for size, ignore non-minsize functions.
348         if (Opt.SizeOnly && !ParentFn->hasMinSize())
349           continue;
350 
351         size_t UGSIdx = GlobalUsesByFunction[ParentFn];
352 
353         // If this is the first global the basic block uses, map it to the set
354         // consisting of this global only.
355         if (!UGSIdx) {
356           // If that set doesn't exist yet, create it.
357           if (!CurGVOnlySetIdx) {
358             CurGVOnlySetIdx = UsedGlobalSets.size();
359             CreateGlobalSet().Globals.set(GI);
360           } else {
361             ++UsedGlobalSets[CurGVOnlySetIdx].UsageCount;
362           }
363 
364           GlobalUsesByFunction[ParentFn] = CurGVOnlySetIdx;
365           continue;
366         }
367 
368         // If we already encountered this BB, just increment the counter.
369         if (UsedGlobalSets[UGSIdx].Globals.test(GI)) {
370           ++UsedGlobalSets[UGSIdx].UsageCount;
371           continue;
372         }
373 
374         // If not, the previous set wasn't actually used in this function.
375         --UsedGlobalSets[UGSIdx].UsageCount;
376 
377         // If we already expanded the previous set to include this global, just
378         // reuse that expanded set.
379         if (size_t ExpandedIdx = EncounteredUGS[UGSIdx]) {
380           ++UsedGlobalSets[ExpandedIdx].UsageCount;
381           GlobalUsesByFunction[ParentFn] = ExpandedIdx;
382           continue;
383         }
384 
385         // If not, create a new set consisting of the union of the previous set
386         // and this global.  Mark it as encountered, so we can reuse it later.
387         GlobalUsesByFunction[ParentFn] = EncounteredUGS[UGSIdx] =
388             UsedGlobalSets.size();
389 
390         UsedGlobalSet &NewUGS = CreateGlobalSet();
391         NewUGS.Globals.set(GI);
392         NewUGS.Globals |= UsedGlobalSets[UGSIdx].Globals;
393       }
394     }
395   }
396 
397   // Now we found a bunch of sets of globals used together.  We accumulated
398   // the number of times we encountered the sets (i.e., the number of blocks
399   // that use that exact set of globals).
400   //
401   // Multiply that by the size of the set to give us a crude profitability
402   // metric.
403   llvm::stable_sort(UsedGlobalSets,
404                     [](const UsedGlobalSet &UGS1, const UsedGlobalSet &UGS2) {
405                       return UGS1.Globals.count() * UGS1.UsageCount <
406                              UGS2.Globals.count() * UGS2.UsageCount;
407                     });
408 
409   // We can choose to merge all globals together, but ignore globals never used
410   // with another global.  This catches the obviously non-profitable cases of
411   // having a single global, but is aggressive enough for any other case.
412   if (GlobalMergeIgnoreSingleUse) {
413     BitVector AllGlobals(Globals.size());
414     for (const UsedGlobalSet &UGS : llvm::reverse(UsedGlobalSets)) {
415       if (UGS.UsageCount == 0)
416         continue;
417       if (UGS.Globals.count() > 1)
418         AllGlobals |= UGS.Globals;
419     }
420     return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
421   }
422 
423   // Starting from the sets with the best (=biggest) profitability, find a
424   // good combination.
425   // The ideal (and expensive) solution can only be found by trying all
426   // combinations, looking for the one with the best profitability.
427   // Don't be smart about it, and just pick the first compatible combination,
428   // starting with the sets with the best profitability.
429   BitVector PickedGlobals(Globals.size());
430   bool Changed = false;
431 
432   for (const UsedGlobalSet &UGS : llvm::reverse(UsedGlobalSets)) {
433     if (UGS.UsageCount == 0)
434       continue;
435     if (PickedGlobals.anyCommon(UGS.Globals))
436       continue;
437     PickedGlobals |= UGS.Globals;
438     // If the set only contains one global, there's no point in merging.
439     // Ignore the global for inclusion in other sets though, so keep it in
440     // PickedGlobals.
441     if (UGS.Globals.count() < 2)
442       continue;
443     Changed |= doMerge(Globals, UGS.Globals, M, isConst, AddrSpace);
444   }
445 
446   return Changed;
447 }
448 
449 bool GlobalMergeImpl::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
450                               const BitVector &GlobalSet, Module &M,
451                               bool isConst, unsigned AddrSpace) const {
452   assert(Globals.size() > 1);
453 
454   Type *Int32Ty = Type::getInt32Ty(M.getContext());
455   Type *Int8Ty = Type::getInt8Ty(M.getContext());
456   auto &DL = M.getDataLayout();
457 
458   LLVM_DEBUG(dbgs() << " Trying to merge set, starts with #"
459                     << GlobalSet.find_first() << "\n");
460 
461   bool Changed = false;
462   ssize_t i = GlobalSet.find_first();
463   while (i != -1) {
464     ssize_t j = 0;
465     uint64_t MergedSize = 0;
466     std::vector<Type*> Tys;
467     std::vector<Constant*> Inits;
468     std::vector<unsigned> StructIdxs;
469 
470     bool HasExternal = false;
471     StringRef FirstExternalName;
472     Align MaxAlign;
473     unsigned CurIdx = 0;
474     for (j = i; j != -1; j = GlobalSet.find_next(j)) {
475       Type *Ty = Globals[j]->getValueType();
476 
477       // Make sure we use the same alignment AsmPrinter would use.
478       Align Alignment = DL.getPreferredAlign(Globals[j]);
479       unsigned Padding = alignTo(MergedSize, Alignment) - MergedSize;
480       MergedSize += Padding;
481       MergedSize += DL.getTypeAllocSize(Ty);
482       if (MergedSize > Opt.MaxOffset) {
483         break;
484       }
485       if (Padding) {
486         Tys.push_back(ArrayType::get(Int8Ty, Padding));
487         Inits.push_back(ConstantAggregateZero::get(Tys.back()));
488         ++CurIdx;
489       }
490       Tys.push_back(Ty);
491       Inits.push_back(Globals[j]->getInitializer());
492       StructIdxs.push_back(CurIdx++);
493 
494       MaxAlign = std::max(MaxAlign, Alignment);
495 
496       if (Globals[j]->hasExternalLinkage() && !HasExternal) {
497         HasExternal = true;
498         FirstExternalName = Globals[j]->getName();
499       }
500     }
501 
502     // Exit early if there is only one global to merge.
503     if (Tys.size() < 2) {
504       i = j;
505       continue;
506     }
507 
508     // If merged variables doesn't have external linkage, we needn't to expose
509     // the symbol after merging.
510     GlobalValue::LinkageTypes Linkage = HasExternal
511                                             ? GlobalValue::ExternalLinkage
512                                             : GlobalValue::InternalLinkage;
513     // Use a packed struct so we can control alignment.
514     StructType *MergedTy = StructType::get(M.getContext(), Tys, true);
515     Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
516 
517     // On Darwin external linkage needs to be preserved, otherwise
518     // dsymutil cannot preserve the debug info for the merged
519     // variables.  If they have external linkage, use the symbol name
520     // of the first variable merged as the suffix of global symbol
521     // name.  This avoids a link-time naming conflict for the
522     // _MergedGlobals symbols.
523     Twine MergedName =
524         (IsMachO && HasExternal)
525             ? "_MergedGlobals_" + FirstExternalName
526             : "_MergedGlobals";
527     auto MergedLinkage = IsMachO ? Linkage : GlobalValue::PrivateLinkage;
528     auto *MergedGV = new GlobalVariable(
529         M, MergedTy, isConst, MergedLinkage, MergedInit, MergedName, nullptr,
530         GlobalVariable::NotThreadLocal, AddrSpace);
531 
532     MergedGV->setAlignment(MaxAlign);
533     MergedGV->setSection(Globals[i]->getSection());
534 
535     const StructLayout *MergedLayout = DL.getStructLayout(MergedTy);
536     for (ssize_t k = i, idx = 0; k != j; k = GlobalSet.find_next(k), ++idx) {
537       GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
538       std::string Name(Globals[k]->getName());
539       GlobalValue::VisibilityTypes Visibility = Globals[k]->getVisibility();
540       GlobalValue::DLLStorageClassTypes DLLStorage =
541           Globals[k]->getDLLStorageClass();
542 
543       // Copy metadata while adjusting any debug info metadata by the original
544       // global's offset within the merged global.
545       MergedGV->copyMetadata(Globals[k],
546                              MergedLayout->getElementOffset(StructIdxs[idx]));
547 
548       Constant *Idx[2] = {
549           ConstantInt::get(Int32Ty, 0),
550           ConstantInt::get(Int32Ty, StructIdxs[idx]),
551       };
552       Constant *GEP =
553           ConstantExpr::getInBoundsGetElementPtr(MergedTy, MergedGV, Idx);
554       Globals[k]->replaceAllUsesWith(GEP);
555       Globals[k]->eraseFromParent();
556 
557       // When the linkage is not internal we must emit an alias for the original
558       // variable name as it may be accessed from another object. On non-Mach-O
559       // we can also emit an alias for internal linkage as it's safe to do so.
560       // It's not safe on Mach-O as the alias (and thus the portion of the
561       // MergedGlobals variable) may be dead stripped at link time.
562       if (Linkage != GlobalValue::InternalLinkage || !IsMachO) {
563         GlobalAlias *GA = GlobalAlias::create(Tys[StructIdxs[idx]], AddrSpace,
564                                               Linkage, Name, GEP, &M);
565         GA->setVisibility(Visibility);
566         GA->setDLLStorageClass(DLLStorage);
567       }
568 
569       NumMerged++;
570     }
571     Changed = true;
572     i = j;
573   }
574 
575   return Changed;
576 }
577 
578 void GlobalMergeImpl::collectUsedGlobalVariables(Module &M, StringRef Name) {
579   // Extract global variables from llvm.used array
580   const GlobalVariable *GV = M.getGlobalVariable(Name);
581   if (!GV || !GV->hasInitializer()) return;
582 
583   // Should be an array of 'i8*'.
584   const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());
585 
586   for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
587     if (const GlobalVariable *G =
588         dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
589       MustKeepGlobalVariables.insert(G);
590 }
591 
592 void GlobalMergeImpl::setMustKeepGlobalVariables(Module &M) {
593   collectUsedGlobalVariables(M, "llvm.used");
594   collectUsedGlobalVariables(M, "llvm.compiler.used");
595 
596   for (Function &F : M) {
597     for (BasicBlock &BB : F) {
598       Instruction *Pad = BB.getFirstNonPHI();
599       if (!Pad->isEHPad())
600         continue;
601 
602       // Keep globals used by landingpads and catchpads.
603       for (const Use &U : Pad->operands()) {
604         if (const GlobalVariable *GV =
605                 dyn_cast<GlobalVariable>(U->stripPointerCasts()))
606           MustKeepGlobalVariables.insert(GV);
607         else if (const ConstantArray *CA = dyn_cast<ConstantArray>(U->stripPointerCasts())) {
608           for (const Use &Elt : CA->operands()) {
609             if (const GlobalVariable *GV =
610                     dyn_cast<GlobalVariable>(Elt->stripPointerCasts()))
611               MustKeepGlobalVariables.insert(GV);
612           }
613         }
614       }
615     }
616   }
617 }
618 
619 bool GlobalMergeImpl::run(Module &M) {
620   if (!EnableGlobalMerge)
621     return false;
622 
623   IsMachO = Triple(M.getTargetTriple()).isOSBinFormatMachO();
624 
625   auto &DL = M.getDataLayout();
626   DenseMap<std::pair<unsigned, StringRef>, SmallVector<GlobalVariable *, 16>>
627       Globals, ConstGlobals, BSSGlobals;
628   bool Changed = false;
629   setMustKeepGlobalVariables(M);
630 
631   LLVM_DEBUG({
632       dbgs() << "Number of GV that must be kept:  " <<
633                 MustKeepGlobalVariables.size() << "\n";
634       for (const GlobalVariable *KeptGV : MustKeepGlobalVariables)
635         dbgs() << "Kept: " << *KeptGV << "\n";
636   });
637   // Grab all non-const globals.
638   for (auto &GV : M.globals()) {
639     // Merge is safe for "normal" internal or external globals only
640     if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasImplicitSection())
641       continue;
642 
643     // It's not safe to merge globals that may be preempted
644     if (TM && !TM->shouldAssumeDSOLocal(M, &GV))
645       continue;
646 
647     if (!(Opt.MergeExternal && GV.hasExternalLinkage()) &&
648         !GV.hasInternalLinkage())
649       continue;
650 
651     PointerType *PT = dyn_cast<PointerType>(GV.getType());
652     assert(PT && "Global variable is not a pointer!");
653 
654     unsigned AddressSpace = PT->getAddressSpace();
655     StringRef Section = GV.getSection();
656 
657     // Ignore all 'special' globals.
658     if (GV.getName().starts_with("llvm.") || GV.getName().starts_with(".llvm."))
659       continue;
660 
661     // Ignore all "required" globals:
662     if (isMustKeepGlobalVariable(&GV))
663       continue;
664 
665     // Don't merge tagged globals, as each global should have its own unique
666     // memory tag at runtime. TODO(hctim): This can be relaxed: constant globals
667     // with compatible alignment and the same contents may be merged as long as
668     // the globals occupy the same number of tag granules (i.e. `size_a / 16 ==
669     // size_b / 16`).
670     if (GV.isTagged())
671       continue;
672 
673     Type *Ty = GV.getValueType();
674     if (DL.getTypeAllocSize(Ty) < Opt.MaxOffset) {
675       if (TM &&
676           TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSS())
677         BSSGlobals[{AddressSpace, Section}].push_back(&GV);
678       else if (GV.isConstant())
679         ConstGlobals[{AddressSpace, Section}].push_back(&GV);
680       else
681         Globals[{AddressSpace, Section}].push_back(&GV);
682     }
683   }
684 
685   for (auto &P : Globals)
686     if (P.second.size() > 1)
687       Changed |= doMerge(P.second, M, false, P.first.first);
688 
689   for (auto &P : BSSGlobals)
690     if (P.second.size() > 1)
691       Changed |= doMerge(P.second, M, false, P.first.first);
692 
693   if (EnableGlobalMergeOnConst)
694     for (auto &P : ConstGlobals)
695       if (P.second.size() > 1)
696         Changed |= doMerge(P.second, M, true, P.first.first);
697 
698   return Changed;
699 }
700 
701 Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset,
702                                   bool OnlyOptimizeForSize,
703                                   bool MergeExternalByDefault) {
704   bool MergeExternal = (EnableGlobalMergeOnExternal == cl::BOU_UNSET) ?
705     MergeExternalByDefault : (EnableGlobalMergeOnExternal == cl::BOU_TRUE);
706   return new GlobalMerge(TM, Offset, OnlyOptimizeForSize, MergeExternal);
707 }
708