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