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
54#include "llvm/Transforms/Scalar.h"
55#include "llvm/ADT/SmallPtrSet.h"
56#include "llvm/ADT/Statistic.h"
57#include "llvm/IR/Attributes.h"
58#include "llvm/IR/Constants.h"
59#include "llvm/IR/DataLayout.h"
60#include "llvm/IR/DerivedTypes.h"
61#include "llvm/IR/Function.h"
62#include "llvm/IR/GlobalVariable.h"
63#include "llvm/IR/Instructions.h"
64#include "llvm/IR/Intrinsics.h"
65#include "llvm/IR/Module.h"
66#include "llvm/Pass.h"
67#include "llvm/CodeGen/Passes.h"
68#include "llvm/Support/CommandLine.h"
69#include "llvm/Target/TargetLowering.h"
70#include "llvm/Target/TargetLoweringObjectFile.h"
71using namespace llvm;
72
73#define DEBUG_TYPE "global-merge"
74
75cl::opt<bool>
76EnableGlobalMerge("enable-global-merge", cl::Hidden,
77                  cl::desc("Enable global merge pass"),
78                  cl::init(true));
79
80static cl::opt<bool>
81EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
82                         cl::desc("Enable global merge pass on constants"),
83                         cl::init(false));
84
85// FIXME: this could be a transitional option, and we probably need to remove
86// it if only we are sure this optimization could always benefit all targets.
87static cl::opt<bool>
88EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
89     cl::desc("Enable global merge pass on external linkage"),
90     cl::init(false));
91
92STATISTIC(NumMerged      , "Number of globals merged");
93namespace {
94  class GlobalMerge : public FunctionPass {
95    const TargetMachine *TM;
96
97    bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
98                 Module &M, bool isConst, unsigned AddrSpace) const;
99
100    /// \brief Check if the given variable has been identified as must keep
101    /// \pre setMustKeepGlobalVariables must have been called on the Module that
102    ///      contains GV
103    bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
104      return MustKeepGlobalVariables.count(GV);
105    }
106
107    /// Collect every variables marked as "used" or used in a landing pad
108    /// instruction for this Module.
109    void setMustKeepGlobalVariables(Module &M);
110
111    /// Collect every variables marked as "used"
112    void collectUsedGlobalVariables(Module &M);
113
114    /// Keep track of the GlobalVariable that must not be merged away
115    SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
116
117  public:
118    static char ID;             // Pass identification, replacement for typeid.
119    explicit GlobalMerge(const TargetMachine *TM = nullptr)
120      : FunctionPass(ID), TM(TM) {
121      initializeGlobalMergePass(*PassRegistry::getPassRegistry());
122    }
123
124    bool doInitialization(Module &M) override;
125    bool runOnFunction(Function &F) override;
126    bool doFinalization(Module &M) override;
127
128    const char *getPassName() const override {
129      return "Merge internal globals";
130    }
131
132    void getAnalysisUsage(AnalysisUsage &AU) const override {
133      AU.setPreservesCFG();
134      FunctionPass::getAnalysisUsage(AU);
135    }
136  };
137} // end anonymous namespace
138
139char GlobalMerge::ID = 0;
140INITIALIZE_TM_PASS(GlobalMerge, "global-merge", "Merge global variables",
141                   false, false)
142
143bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
144                          Module &M, bool isConst, unsigned AddrSpace) const {
145  const TargetLowering *TLI = TM->getTargetLowering();
146  const DataLayout *DL = TLI->getDataLayout();
147
148  // FIXME: Infer the maximum possible offset depending on the actual users
149  // (these max offsets are different for the users inside Thumb or ARM
150  // functions)
151  unsigned MaxOffset = TLI->getMaximalGlobalOffset();
152
153  // FIXME: Find better heuristics
154  std::stable_sort(Globals.begin(), Globals.end(),
155                   [DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
156    Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType();
157    Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType();
158
159    return (DL->getTypeAllocSize(Ty1) < DL->getTypeAllocSize(Ty2));
160  });
161
162  Type *Int32Ty = Type::getInt32Ty(M.getContext());
163
164  assert(Globals.size() > 1);
165
166  // FIXME: This simple solution merges globals all together as maximum as
167  // possible. However, with this solution it would be hard to remove dead
168  // global symbols at link-time. An alternative solution could be checking
169  // global symbols references function by function, and make the symbols
170  // being referred in the same function merged and we would probably need
171  // to introduce heuristic algorithm to solve the merge conflict from
172  // different functions.
173  for (size_t i = 0, e = Globals.size(); i != e; ) {
174    size_t j = 0;
175    uint64_t MergedSize = 0;
176    std::vector<Type*> Tys;
177    std::vector<Constant*> Inits;
178
179    bool HasExternal = false;
180    GlobalVariable *TheFirstExternal = 0;
181    for (j = i; j != e; ++j) {
182      Type *Ty = Globals[j]->getType()->getElementType();
183      MergedSize += DL->getTypeAllocSize(Ty);
184      if (MergedSize > MaxOffset) {
185        break;
186      }
187      Tys.push_back(Ty);
188      Inits.push_back(Globals[j]->getInitializer());
189
190      if (Globals[j]->hasExternalLinkage() && !HasExternal) {
191        HasExternal = true;
192        TheFirstExternal = Globals[j];
193      }
194    }
195
196    // If merged variables doesn't have external linkage, we needn't to expose
197    // the symbol after merging.
198    GlobalValue::LinkageTypes Linkage = HasExternal
199                                            ? GlobalValue::ExternalLinkage
200                                            : GlobalValue::InternalLinkage;
201
202    StructType *MergedTy = StructType::get(M.getContext(), Tys);
203    Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
204
205    // If merged variables have external linkage, we use symbol name of the
206    // first variable merged as the suffix of global symbol name. This would
207    // be able to avoid the link-time naming conflict for globalm symbols.
208    GlobalVariable *MergedGV = new GlobalVariable(
209        M, MergedTy, isConst, Linkage, MergedInit,
210        HasExternal ? "_MergedGlobals_" + TheFirstExternal->getName()
211                    : "_MergedGlobals",
212        nullptr, GlobalVariable::NotThreadLocal, AddrSpace);
213
214    for (size_t k = i; k < j; ++k) {
215      GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
216      std::string Name = Globals[k]->getName();
217
218      Constant *Idx[2] = {
219        ConstantInt::get(Int32Ty, 0),
220        ConstantInt::get(Int32Ty, k-i)
221      };
222      Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx);
223      Globals[k]->replaceAllUsesWith(GEP);
224      Globals[k]->eraseFromParent();
225
226      if (Linkage != GlobalValue::InternalLinkage) {
227        // Generate a new alias...
228        auto *PTy = cast<PointerType>(GEP->getType());
229        GlobalAlias::create(PTy->getElementType(), PTy->getAddressSpace(),
230                            Linkage, Name, GEP, &M);
231      }
232
233      NumMerged++;
234    }
235    i = j;
236  }
237
238  return true;
239}
240
241void GlobalMerge::collectUsedGlobalVariables(Module &M) {
242  // Extract global variables from llvm.used array
243  const GlobalVariable *GV = M.getGlobalVariable("llvm.used");
244  if (!GV || !GV->hasInitializer()) return;
245
246  // Should be an array of 'i8*'.
247  const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());
248
249  for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
250    if (const GlobalVariable *G =
251        dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
252      MustKeepGlobalVariables.insert(G);
253}
254
255void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
256  collectUsedGlobalVariables(M);
257
258  for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn;
259       ++IFn) {
260    for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
261         IBB != IEndBB; ++IBB) {
262      // Follow the invoke link to find the landing pad instruction
263      const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator());
264      if (!II) continue;
265
266      const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst();
267      // Look for globals in the clauses of the landing pad instruction
268      for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses();
269           Idx != NumClauses; ++Idx)
270        if (const GlobalVariable *GV =
271            dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
272                                     ->stripPointerCasts()))
273          MustKeepGlobalVariables.insert(GV);
274    }
275  }
276}
277
278bool GlobalMerge::doInitialization(Module &M) {
279  if (!EnableGlobalMerge)
280    return false;
281
282  DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
283                                                        BSSGlobals;
284  const TargetLowering *TLI = TM->getTargetLowering();
285  const DataLayout *DL = TLI->getDataLayout();
286  unsigned MaxOffset = TLI->getMaximalGlobalOffset();
287  bool Changed = false;
288  setMustKeepGlobalVariables(M);
289
290  // Grab all non-const globals.
291  for (Module::global_iterator I = M.global_begin(),
292         E = M.global_end(); I != E; ++I) {
293    // Merge is safe for "normal" internal or external globals only
294    if (I->isDeclaration() || I->isThreadLocal() || I->hasSection())
295      continue;
296
297    if (!(EnableGlobalMergeOnExternal && I->hasExternalLinkage()) &&
298        !I->hasInternalLinkage())
299      continue;
300
301    PointerType *PT = dyn_cast<PointerType>(I->getType());
302    assert(PT && "Global variable is not a pointer!");
303
304    unsigned AddressSpace = PT->getAddressSpace();
305
306    // Ignore fancy-aligned globals for now.
307    unsigned Alignment = DL->getPreferredAlignment(I);
308    Type *Ty = I->getType()->getElementType();
309    if (Alignment > DL->getABITypeAlignment(Ty))
310      continue;
311
312    // Ignore all 'special' globals.
313    if (I->getName().startswith("llvm.") ||
314        I->getName().startswith(".llvm."))
315      continue;
316
317    // Ignore all "required" globals:
318    if (isMustKeepGlobalVariable(I))
319      continue;
320
321    if (DL->getTypeAllocSize(Ty) < MaxOffset) {
322      if (TargetLoweringObjectFile::getKindForGlobal(I, *TM).isBSSLocal())
323        BSSGlobals[AddressSpace].push_back(I);
324      else if (I->isConstant())
325        ConstGlobals[AddressSpace].push_back(I);
326      else
327        Globals[AddressSpace].push_back(I);
328    }
329  }
330
331  for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
332       I = Globals.begin(), E = Globals.end(); I != E; ++I)
333    if (I->second.size() > 1)
334      Changed |= doMerge(I->second, M, false, I->first);
335
336  for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
337       I = BSSGlobals.begin(), E = BSSGlobals.end(); I != E; ++I)
338    if (I->second.size() > 1)
339      Changed |= doMerge(I->second, M, false, I->first);
340
341  if (EnableGlobalMergeOnConst)
342    for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
343         I = ConstGlobals.begin(), E = ConstGlobals.end(); I != E; ++I)
344      if (I->second.size() > 1)
345        Changed |= doMerge(I->second, M, true, I->first);
346
347  return Changed;
348}
349
350bool GlobalMerge::runOnFunction(Function &F) {
351  return false;
352}
353
354bool GlobalMerge::doFinalization(Module &M) {
355  MustKeepGlobalVariables.clear();
356  return false;
357}
358
359Pass *llvm::createGlobalMergePass(const TargetMachine *TM) {
360  return new GlobalMerge(TM);
361}
362