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#define DEBUG_TYPE "global-merge"
55#include "llvm/Transforms/Scalar.h"
56#include "llvm/ADT/SmallPtrSet.h"
57#include "llvm/ADT/Statistic.h"
58#include "llvm/IR/Attributes.h"
59#include "llvm/IR/Constants.h"
60#include "llvm/IR/DataLayout.h"
61#include "llvm/IR/DerivedTypes.h"
62#include "llvm/IR/Function.h"
63#include "llvm/IR/GlobalVariable.h"
64#include "llvm/IR/InlineAsm.h"
65#include "llvm/IR/Instructions.h"
66#include "llvm/IR/Intrinsics.h"
67#include "llvm/IR/IntrinsicInst.h"
68#include "llvm/IR/Module.h"
69#include "llvm/Pass.h"
70#include "llvm/Support/CommandLine.h"
71#include "llvm/Target/TargetLowering.h"
72#include "llvm/Target/TargetLoweringObjectFile.h"
73using namespace llvm;
74
75static cl::opt<bool>
76EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
77                  	cl::desc("Enable global merge pass on constants"),
78                  	cl::init(false));
79
80STATISTIC(NumMerged      , "Number of globals merged");
81namespace {
82  class GlobalMerge : public FunctionPass {
83    /// TLI - Keep a pointer of a TargetLowering to consult for determining
84    /// target type sizes.
85    const TargetLowering *TLI;
86
87    bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
88                 Module &M, bool isConst, unsigned AddrSpace) const;
89
90    /// \brief Check if the given variable has been identified as must keep
91    /// \pre setMustKeepGlobalVariables must have been called on the Module that
92    ///      contains GV
93    bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
94      return MustKeepGlobalVariables.count(GV);
95    }
96
97    /// Collect every variables marked as "used" or used in a landing pad
98    /// instruction for this Module.
99    void setMustKeepGlobalVariables(Module &M);
100
101    /// Collect every variables marked as "used"
102    void collectUsedGlobalVariables(Module &M);
103
104    /// Keep track of the GlobalVariable that are marked as "used"
105    SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
106
107  public:
108    static char ID;             // Pass identification, replacement for typeid.
109    explicit GlobalMerge(const TargetLowering *tli = 0)
110      : FunctionPass(ID), TLI(tli) {
111      initializeGlobalMergePass(*PassRegistry::getPassRegistry());
112    }
113
114    virtual bool doInitialization(Module &M);
115    virtual bool runOnFunction(Function &F);
116
117    const char *getPassName() const {
118      return "Merge internal globals";
119    }
120
121    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
122      AU.setPreservesCFG();
123      FunctionPass::getAnalysisUsage(AU);
124    }
125
126    struct GlobalCmp {
127      const DataLayout *TD;
128
129      GlobalCmp(const DataLayout *td) : TD(td) { }
130
131      bool operator()(const GlobalVariable *GV1, const GlobalVariable *GV2) {
132        Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType();
133        Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType();
134
135        return (TD->getTypeAllocSize(Ty1) < TD->getTypeAllocSize(Ty2));
136      }
137    };
138  };
139} // end anonymous namespace
140
141char GlobalMerge::ID = 0;
142INITIALIZE_PASS(GlobalMerge, "global-merge",
143                "Global Merge", false, false)
144
145
146bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
147                          Module &M, bool isConst, unsigned AddrSpace) const {
148  const DataLayout *TD = TLI->getDataLayout();
149
150  // FIXME: Infer the maximum possible offset depending on the actual users
151  // (these max offsets are different for the users inside Thumb or ARM
152  // functions)
153  unsigned MaxOffset = TLI->getMaximalGlobalOffset();
154
155  // FIXME: Find better heuristics
156  std::stable_sort(Globals.begin(), Globals.end(), GlobalCmp(TD));
157
158  Type *Int32Ty = Type::getInt32Ty(M.getContext());
159
160  for (size_t i = 0, e = Globals.size(); i != e; ) {
161    size_t j = 0;
162    uint64_t MergedSize = 0;
163    std::vector<Type*> Tys;
164    std::vector<Constant*> Inits;
165    for (j = i; j != e; ++j) {
166      Type *Ty = Globals[j]->getType()->getElementType();
167      MergedSize += TD->getTypeAllocSize(Ty);
168      if (MergedSize > MaxOffset) {
169        break;
170      }
171      Tys.push_back(Ty);
172      Inits.push_back(Globals[j]->getInitializer());
173    }
174
175    StructType *MergedTy = StructType::get(M.getContext(), Tys);
176    Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
177    GlobalVariable *MergedGV = new GlobalVariable(M, MergedTy, isConst,
178                                                  GlobalValue::InternalLinkage,
179                                                  MergedInit, "_MergedGlobals",
180                                                  0, GlobalVariable::NotThreadLocal,
181                                                  AddrSpace);
182    for (size_t k = i; k < j; ++k) {
183      Constant *Idx[2] = {
184        ConstantInt::get(Int32Ty, 0),
185        ConstantInt::get(Int32Ty, k-i)
186      };
187      Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx);
188      Globals[k]->replaceAllUsesWith(GEP);
189      Globals[k]->eraseFromParent();
190      NumMerged++;
191    }
192    i = j;
193  }
194
195  return true;
196}
197
198void GlobalMerge::collectUsedGlobalVariables(Module &M) {
199  // Extract global variables from llvm.used array
200  const GlobalVariable *GV = M.getGlobalVariable("llvm.used");
201  if (!GV || !GV->hasInitializer()) return;
202
203  // Should be an array of 'i8*'.
204  const ConstantArray *InitList = dyn_cast<ConstantArray>(GV->getInitializer());
205  if (InitList == 0) return;
206
207  for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
208    if (const GlobalVariable *G =
209        dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
210      MustKeepGlobalVariables.insert(G);
211}
212
213void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
214  // If we already processed a Module, UsedGlobalVariables may have been
215  // populated. Reset the information for this module.
216  MustKeepGlobalVariables.clear();
217  collectUsedGlobalVariables(M);
218
219  for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn;
220       ++IFn) {
221    for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
222         IBB != IEndBB; ++IBB) {
223      // Follow the inwoke link to find the landing pad instruction
224      const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator());
225      if (!II) continue;
226
227      const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst();
228      // Look for globals in the clauses of the landing pad instruction
229      for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses();
230           Idx != NumClauses; ++Idx)
231        if (const GlobalVariable *GV =
232            dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
233                                     ->stripPointerCasts()))
234          MustKeepGlobalVariables.insert(GV);
235    }
236  }
237}
238
239bool GlobalMerge::doInitialization(Module &M) {
240  DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
241                                                        BSSGlobals;
242  const DataLayout *TD = TLI->getDataLayout();
243  unsigned MaxOffset = TLI->getMaximalGlobalOffset();
244  bool Changed = false;
245  setMustKeepGlobalVariables(M);
246
247  // Grab all non-const globals.
248  for (Module::global_iterator I = M.global_begin(),
249         E = M.global_end(); I != E; ++I) {
250    // Merge is safe for "normal" internal globals only
251    if (!I->hasLocalLinkage() || I->isThreadLocal() || I->hasSection())
252      continue;
253
254    PointerType *PT = dyn_cast<PointerType>(I->getType());
255    assert(PT && "Global variable is not a pointer!");
256
257    unsigned AddressSpace = PT->getAddressSpace();
258
259    // Ignore fancy-aligned globals for now.
260    unsigned Alignment = TD->getPreferredAlignment(I);
261    Type *Ty = I->getType()->getElementType();
262    if (Alignment > TD->getABITypeAlignment(Ty))
263      continue;
264
265    // Ignore all 'special' globals.
266    if (I->getName().startswith("llvm.") ||
267        I->getName().startswith(".llvm."))
268      continue;
269
270    // Ignore all "required" globals:
271    // - the ones used for EH
272    // - the ones marked with "used" attribute
273    if (isMustKeepGlobalVariable(I))
274      continue;
275
276    if (TD->getTypeAllocSize(Ty) < MaxOffset) {
277      if (TargetLoweringObjectFile::getKindForGlobal(I, TLI->getTargetMachine())
278          .isBSSLocal())
279        BSSGlobals[AddressSpace].push_back(I);
280      else if (I->isConstant())
281        ConstGlobals[AddressSpace].push_back(I);
282      else
283        Globals[AddressSpace].push_back(I);
284    }
285  }
286
287  for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
288       I = Globals.begin(), E = Globals.end(); I != E; ++I)
289    if (I->second.size() > 1)
290      Changed |= doMerge(I->second, M, false, I->first);
291
292  for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
293       I = BSSGlobals.begin(), E = BSSGlobals.end(); I != E; ++I)
294    if (I->second.size() > 1)
295      Changed |= doMerge(I->second, M, false, I->first);
296
297  if (EnableGlobalMergeOnConst)
298    for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
299         I = ConstGlobals.begin(), E = ConstGlobals.end(); I != E; ++I)
300      if (I->second.size() > 1)
301        Changed |= doMerge(I->second, M, true, I->first);
302
303  return Changed;
304}
305
306bool GlobalMerge::runOnFunction(Function &F) {
307  return false;
308}
309
310Pass *llvm::createGlobalMergePass(const TargetLowering *tli) {
311  return new GlobalMerge(tli);
312}
313