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/Attributes.h"
57 #include "llvm/Constants.h"
58 #include "llvm/DerivedTypes.h"
59 #include "llvm/Function.h"
60 #include "llvm/GlobalVariable.h"
61 #include "llvm/Instructions.h"
62 #include "llvm/Intrinsics.h"
63 #include "llvm/Module.h"
64 #include "llvm/Pass.h"
65 #include "llvm/Target/TargetData.h"
66 #include "llvm/Target/TargetLowering.h"
67 #include "llvm/Target/TargetLoweringObjectFile.h"
68 #include "llvm/ADT/Statistic.h"
69 using namespace llvm;
70
71 STATISTIC(NumMerged , "Number of globals merged");
72 namespace {
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) const;
80
81 public:
82 static char ID; // Pass identification, replacement for typeid.
GlobalMerge(const TargetLowering * tli=0)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
getPassName() const91 const char *getPassName() const {
92 return "Merge internal globals";
93 }
94
getAnalysisUsage(AnalysisUsage & AU) const95 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
96 AU.setPreservesCFG();
97 FunctionPass::getAnalysisUsage(AU);
98 }
99
100 struct GlobalCmp {
101 const TargetData *TD;
102
GlobalCmp__anon24107bfc0111::GlobalMerge::GlobalCmp103 GlobalCmp(const TargetData *td) : TD(td) { }
104
operator ()__anon24107bfc0111::GlobalMerge::GlobalCmp105 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
115 char GlobalMerge::ID = 0;
116 INITIALIZE_PASS(GlobalMerge, "global-merge",
117 "Global Merge", false, false)
118
119
doMerge(SmallVectorImpl<GlobalVariable * > & Globals,Module & M,bool isConst) const120 bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
121 Module &M, bool isConst) const {
122 const TargetData *TD = TLI->getTargetData();
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 for (size_t k = i; k < j; ++k) {
155 Constant *Idx[2] = {
156 ConstantInt::get(Int32Ty, 0),
157 ConstantInt::get(Int32Ty, k-i)
158 };
159 Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx);
160 Globals[k]->replaceAllUsesWith(GEP);
161 Globals[k]->eraseFromParent();
162 NumMerged++;
163 }
164 i = j;
165 }
166
167 return true;
168 }
169
170
doInitialization(Module & M)171 bool GlobalMerge::doInitialization(Module &M) {
172 SmallVector<GlobalVariable*, 16> Globals, ConstGlobals, BSSGlobals;
173 const TargetData *TD = TLI->getTargetData();
174 unsigned MaxOffset = TLI->getMaximalGlobalOffset();
175 bool Changed = false;
176
177 // Grab all non-const globals.
178 for (Module::global_iterator I = M.global_begin(),
179 E = M.global_end(); I != E; ++I) {
180 // Merge is safe for "normal" internal globals only
181 if (!I->hasLocalLinkage() || I->isThreadLocal() || I->hasSection())
182 continue;
183
184 // Ignore fancy-aligned globals for now.
185 unsigned Alignment = TD->getPreferredAlignment(I);
186 Type *Ty = I->getType()->getElementType();
187 if (Alignment > TD->getABITypeAlignment(Ty))
188 continue;
189
190 // Ignore all 'special' globals.
191 if (I->getName().startswith("llvm.") ||
192 I->getName().startswith(".llvm."))
193 continue;
194
195 if (TD->getTypeAllocSize(Ty) < MaxOffset) {
196 if (TargetLoweringObjectFile::getKindForGlobal(I, TLI->getTargetMachine())
197 .isBSSLocal())
198 BSSGlobals.push_back(I);
199 else if (I->isConstant())
200 ConstGlobals.push_back(I);
201 else
202 Globals.push_back(I);
203 }
204 }
205
206 if (Globals.size() > 1)
207 Changed |= doMerge(Globals, M, false);
208 if (BSSGlobals.size() > 1)
209 Changed |= doMerge(BSSGlobals, M, false);
210
211 // FIXME: This currently breaks the EH processing due to way how the
212 // typeinfo detection works. We might want to detect the TIs and ignore
213 // them in the future.
214 // if (ConstGlobals.size() > 1)
215 // Changed |= doMerge(ConstGlobals, M, true);
216
217 return Changed;
218 }
219
runOnFunction(Function & F)220 bool GlobalMerge::runOnFunction(Function &F) {
221 return false;
222 }
223
createGlobalMergePass(const TargetLowering * tli)224 Pass *llvm::createGlobalMergePass(const TargetLowering *tli) {
225 return new GlobalMerge(tli);
226 }
227