GlobalMerge.cpp revision e97165901ee51216b22b808041b10febbb4afa5e 
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/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/Target/TargetLowering.h"
68#include "llvm/Target/TargetLoweringObjectFile.h"
69using namespace llvm;
70
71STATISTIC(NumMerged      , "Number of globals merged");
72namespace {
73  class GlobalMerge : public FunctionPass {
74    /// TLI - Keep a pointer of a TargetLowering to consult for determining
75    /// target type sizes.
76    const TargetLowering *TLI;
77
78    bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
79                 Module &M, bool isConst, unsigned AddrSpace) const;
80
81  public:
82    static char ID;             // Pass identification, replacement for typeid.
83    explicit GlobalMerge(const TargetLowering *tli = 0)
84      : FunctionPass(ID), TLI(tli) {
85      initializeGlobalMergePass(*PassRegistry::getPassRegistry());
86    }
87
88    virtual bool doInitialization(Module &M);
89    virtual bool runOnFunction(Function &F);
90
91    const char *getPassName() const {
92      return "Merge internal globals";
93    }
94
95    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
96      AU.setPreservesCFG();
97      FunctionPass::getAnalysisUsage(AU);
98    }
99
100    struct GlobalCmp {
101      const DataLayout *TD;
102
103      GlobalCmp(const DataLayout *td) : TD(td) { }
104
105      bool operator()(const GlobalVariable *GV1, const GlobalVariable *GV2) {
106        Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType();
107        Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType();
108
109        return (TD->getTypeAllocSize(Ty1) < TD->getTypeAllocSize(Ty2));
110      }
111    };
112  };
113} // end anonymous namespace
114
115char GlobalMerge::ID = 0;
116INITIALIZE_PASS(GlobalMerge, "global-merge",
117                "Global Merge", false, false)
118
119
120bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
121                          Module &M, bool isConst, unsigned AddrSpace) const {
122  const DataLayout *TD = TLI->getDataLayout();
123
124  // FIXME: Infer the maximum possible offset depending on the actual users
125  // (these max offsets are different for the users inside Thumb or ARM
126  // functions)
127  unsigned MaxOffset = TLI->getMaximalGlobalOffset();
128
129  // FIXME: Find better heuristics
130  std::stable_sort(Globals.begin(), Globals.end(), GlobalCmp(TD));
131
132  Type *Int32Ty = Type::getInt32Ty(M.getContext());
133
134  for (size_t i = 0, e = Globals.size(); i != e; ) {
135    size_t j = 0;
136    uint64_t MergedSize = 0;
137    std::vector<Type*> Tys;
138    std::vector<Constant*> Inits;
139    for (j = i; j != e; ++j) {
140      Type *Ty = Globals[j]->getType()->getElementType();
141      MergedSize += TD->getTypeAllocSize(Ty);
142      if (MergedSize > MaxOffset) {
143        break;
144      }
145      Tys.push_back(Ty);
146      Inits.push_back(Globals[j]->getInitializer());
147    }
148
149    StructType *MergedTy = StructType::get(M.getContext(), Tys);
150    Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
151    GlobalVariable *MergedGV = new GlobalVariable(M, MergedTy, isConst,
152                                                  GlobalValue::InternalLinkage,
153                                                  MergedInit, "_MergedGlobals",
154                                                  0, GlobalVariable::NotThreadLocal,
155                                                  AddrSpace);
156    for (size_t k = i; k < j; ++k) {
157      Constant *Idx[2] = {
158        ConstantInt::get(Int32Ty, 0),
159        ConstantInt::get(Int32Ty, k-i)
160      };
161      Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx);
162      Globals[k]->replaceAllUsesWith(GEP);
163      Globals[k]->eraseFromParent();
164      NumMerged++;
165    }
166    i = j;
167  }
168
169  return true;
170}
171
172
173bool GlobalMerge::doInitialization(Module &M) {
174  DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
175                                                        BSSGlobals;
176  const DataLayout *TD = TLI->getDataLayout();
177  unsigned MaxOffset = TLI->getMaximalGlobalOffset();
178  bool Changed = false;
179
180  // Grab all non-const globals.
181  for (Module::global_iterator I = M.global_begin(),
182         E = M.global_end(); I != E; ++I) {
183    // Merge is safe for "normal" internal globals only
184    if (!I->hasLocalLinkage() || I->isThreadLocal() || I->hasSection())
185      continue;
186
187    PointerType *PT = dyn_cast<PointerType>(I->getType());
188    assert(PT && "Global variable is not a pointer!");
189
190    unsigned AddressSpace = PT->getAddressSpace();
191
192    // Ignore fancy-aligned globals for now.
193    unsigned Alignment = TD->getPreferredAlignment(I);
194    Type *Ty = I->getType()->getElementType();
195    if (Alignment > TD->getABITypeAlignment(Ty))
196      continue;
197
198    // Ignore all 'special' globals.
199    if (I->getName().startswith("llvm.") ||
200        I->getName().startswith(".llvm."))
201      continue;
202
203    if (TD->getTypeAllocSize(Ty) < MaxOffset) {
204      if (TargetLoweringObjectFile::getKindForGlobal(I, TLI->getTargetMachine())
205          .isBSSLocal())
206        BSSGlobals[AddressSpace].push_back(I);
207      else if (I->isConstant())
208        ConstGlobals[AddressSpace].push_back(I);
209      else
210        Globals[AddressSpace].push_back(I);
211    }
212  }
213
214  for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
215       I = Globals.begin(), E = Globals.end(); I != E; ++I)
216    if (I->second.size() > 1)
217      Changed |= doMerge(I->second, M, false, I->first);
218
219  for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
220       I = BSSGlobals.begin(), E = BSSGlobals.end(); I != E; ++I)
221    if (I->second.size() > 1)
222      Changed |= doMerge(I->second, M, false, I->first);
223
224  // FIXME: This currently breaks the EH processing due to way how the
225  // typeinfo detection works. We might want to detect the TIs and ignore
226  // them in the future.
227  // if (ConstGlobals.size() > 1)
228  //  Changed |= doMerge(ConstGlobals, M, true);
229
230  return Changed;
231}
232
233bool GlobalMerge::runOnFunction(Function &F) {
234  return false;
235}
236
237Pass *llvm::createGlobalMergePass(const TargetLowering *tli) {
238  return new GlobalMerge(tli);
239}
240