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1 //===-- ARMGlobalMerge.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 "arm-global-merge"
55 #include "ARM.h"
56 #include "llvm/CodeGen/Passes.h"
57 #include "llvm/Attributes.h"
58 #include "llvm/Constants.h"
59 #include "llvm/DerivedTypes.h"
60 #include "llvm/Function.h"
61 #include "llvm/GlobalVariable.h"
62 #include "llvm/Instructions.h"
63 #include "llvm/Intrinsics.h"
64 #include "llvm/Module.h"
65 #include "llvm/Pass.h"
66 #include "llvm/Target/TargetData.h"
67 #include "llvm/Target/TargetLowering.h"
68 #include "llvm/Target/TargetLoweringObjectFile.h"
69 using namespace llvm;
70 
71 namespace {
72   class ARMGlobalMerge : public FunctionPass {
73     /// TLI - Keep a pointer of a TargetLowering to consult for determining
74     /// target type sizes.
75     const TargetLowering *TLI;
76 
77     bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
78                  Module &M, bool isConst) const;
79 
80   public:
81     static char ID;             // Pass identification, replacement for typeid.
ARMGlobalMerge(const TargetLowering * tli)82     explicit ARMGlobalMerge(const TargetLowering *tli)
83       : FunctionPass(ID), TLI(tli) {}
84 
85     virtual bool doInitialization(Module &M);
86     virtual bool runOnFunction(Function &F);
87 
getPassName() const88     const char *getPassName() const {
89       return "Merge internal globals";
90     }
91 
getAnalysisUsage(AnalysisUsage & AU) const92     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
93       AU.setPreservesCFG();
94       FunctionPass::getAnalysisUsage(AU);
95     }
96 
97     struct GlobalCmp {
98       const TargetData *TD;
99 
GlobalCmp__anon7a3d2e4d0111::ARMGlobalMerge::GlobalCmp100       GlobalCmp(const TargetData *td) : TD(td) { }
101 
operator ()__anon7a3d2e4d0111::ARMGlobalMerge::GlobalCmp102       bool operator()(const GlobalVariable *GV1, const GlobalVariable *GV2) {
103         Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType();
104         Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType();
105 
106         return (TD->getTypeAllocSize(Ty1) < TD->getTypeAllocSize(Ty2));
107       }
108     };
109   };
110 } // end anonymous namespace
111 
112 char ARMGlobalMerge::ID = 0;
113 
doMerge(SmallVectorImpl<GlobalVariable * > & Globals,Module & M,bool isConst) const114 bool ARMGlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
115                              Module &M, bool isConst) const {
116   const TargetData *TD = TLI->getTargetData();
117 
118   // FIXME: Infer the maximum possible offset depending on the actual users
119   // (these max offsets are different for the users inside Thumb or ARM
120   // functions)
121   unsigned MaxOffset = TLI->getMaximalGlobalOffset();
122 
123   // FIXME: Find better heuristics
124   std::stable_sort(Globals.begin(), Globals.end(), GlobalCmp(TD));
125 
126   Type *Int32Ty = Type::getInt32Ty(M.getContext());
127 
128   for (size_t i = 0, e = Globals.size(); i != e; ) {
129     size_t j = 0;
130     uint64_t MergedSize = 0;
131     std::vector<Type*> Tys;
132     std::vector<Constant*> Inits;
133     for (j = i; j != e; ++j) {
134       Type *Ty = Globals[j]->getType()->getElementType();
135       MergedSize += TD->getTypeAllocSize(Ty);
136       if (MergedSize > MaxOffset) {
137         break;
138       }
139       Tys.push_back(Ty);
140       Inits.push_back(Globals[j]->getInitializer());
141     }
142 
143     StructType *MergedTy = StructType::get(M.getContext(), Tys);
144     Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
145     GlobalVariable *MergedGV = new GlobalVariable(M, MergedTy, isConst,
146                                                   GlobalValue::InternalLinkage,
147                                                   MergedInit, "_MergedGlobals");
148     for (size_t k = i; k < j; ++k) {
149       Constant *Idx[2] = {
150         ConstantInt::get(Int32Ty, 0),
151         ConstantInt::get(Int32Ty, k-i)
152       };
153       Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx);
154       Globals[k]->replaceAllUsesWith(GEP);
155       Globals[k]->eraseFromParent();
156     }
157     i = j;
158   }
159 
160   return true;
161 }
162 
163 
doInitialization(Module & M)164 bool ARMGlobalMerge::doInitialization(Module &M) {
165   SmallVector<GlobalVariable*, 16> Globals, ConstGlobals, BSSGlobals;
166   const TargetData *TD = TLI->getTargetData();
167   unsigned MaxOffset = TLI->getMaximalGlobalOffset();
168   bool Changed = false;
169 
170   // Grab all non-const globals.
171   for (Module::global_iterator I = M.global_begin(),
172          E = M.global_end(); I != E; ++I) {
173     // Merge is safe for "normal" internal globals only
174     if (!I->hasLocalLinkage() || I->isThreadLocal() || I->hasSection())
175       continue;
176 
177     // Ignore fancy-aligned globals for now.
178     unsigned Alignment = I->getAlignment();
179     Type *Ty = I->getType()->getElementType();
180     if (Alignment > TD->getABITypeAlignment(Ty))
181       continue;
182 
183     // Ignore all 'special' globals.
184     if (I->getName().startswith("llvm.") ||
185         I->getName().startswith(".llvm."))
186       continue;
187 
188     if (TD->getTypeAllocSize(Ty) < MaxOffset) {
189       const TargetLoweringObjectFile &TLOF = TLI->getObjFileLowering();
190       if (TLOF.getKindForGlobal(I, TLI->getTargetMachine()).isBSSLocal())
191         BSSGlobals.push_back(I);
192       else if (I->isConstant())
193         ConstGlobals.push_back(I);
194       else
195         Globals.push_back(I);
196     }
197   }
198 
199   if (Globals.size() > 1)
200     Changed |= doMerge(Globals, M, false);
201   if (BSSGlobals.size() > 1)
202     Changed |= doMerge(BSSGlobals, M, false);
203 
204   // FIXME: This currently breaks the EH processing due to way how the
205   // typeinfo detection works. We might want to detect the TIs and ignore
206   // them in the future.
207   // if (ConstGlobals.size() > 1)
208   //  Changed |= doMerge(ConstGlobals, M, true);
209 
210   return Changed;
211 }
212 
runOnFunction(Function & F)213 bool ARMGlobalMerge::runOnFunction(Function &F) {
214   return false;
215 }
216 
createARMGlobalMergePass(const TargetLowering * tli)217 FunctionPass *llvm::createARMGlobalMergePass(const TargetLowering *tli) {
218   return new ARMGlobalMerge(tli);
219 }
220