1 //===-- GlobalStatus.cpp - Compute status info for globals -----------------==//
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
10 #include "llvm/ADT/SmallPtrSet.h"
11 #include "llvm/IR/BasicBlock.h"
12 #include "llvm/IR/CallSite.h"
13 #include "llvm/IR/GlobalVariable.h"
14 #include "llvm/IR/IntrinsicInst.h"
15 #include "llvm/Transforms/Utils/GlobalStatus.h"
16
17 using namespace llvm;
18
19 /// Return the stronger of the two ordering. If the two orderings are acquire
20 /// and release, then return AcquireRelease.
21 ///
strongerOrdering(AtomicOrdering X,AtomicOrdering Y)22 static AtomicOrdering strongerOrdering(AtomicOrdering X, AtomicOrdering Y) {
23 if (X == Acquire && Y == Release)
24 return AcquireRelease;
25 if (Y == Acquire && X == Release)
26 return AcquireRelease;
27 return (AtomicOrdering)std::max(X, Y);
28 }
29
30 /// It is safe to destroy a constant iff it is only used by constants itself.
31 /// Note that constants cannot be cyclic, so this test is pretty easy to
32 /// implement recursively.
33 ///
isSafeToDestroyConstant(const Constant * C)34 bool llvm::isSafeToDestroyConstant(const Constant *C) {
35 if (isa<GlobalValue>(C))
36 return false;
37
38 if (isa<ConstantInt>(C) || isa<ConstantFP>(C))
39 return false;
40
41 for (const User *U : C->users())
42 if (const Constant *CU = dyn_cast<Constant>(U)) {
43 if (!isSafeToDestroyConstant(CU))
44 return false;
45 } else
46 return false;
47 return true;
48 }
49
analyzeGlobalAux(const Value * V,GlobalStatus & GS,SmallPtrSetImpl<const PHINode * > & PhiUsers)50 static bool analyzeGlobalAux(const Value *V, GlobalStatus &GS,
51 SmallPtrSetImpl<const PHINode *> &PhiUsers) {
52 for (const Use &U : V->uses()) {
53 const User *UR = U.getUser();
54 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(UR)) {
55 GS.HasNonInstructionUser = true;
56
57 // If the result of the constantexpr isn't pointer type, then we won't
58 // know to expect it in various places. Just reject early.
59 if (!isa<PointerType>(CE->getType()))
60 return true;
61
62 if (analyzeGlobalAux(CE, GS, PhiUsers))
63 return true;
64 } else if (const Instruction *I = dyn_cast<Instruction>(UR)) {
65 if (!GS.HasMultipleAccessingFunctions) {
66 const Function *F = I->getParent()->getParent();
67 if (!GS.AccessingFunction)
68 GS.AccessingFunction = F;
69 else if (GS.AccessingFunction != F)
70 GS.HasMultipleAccessingFunctions = true;
71 }
72 if (const LoadInst *LI = dyn_cast<LoadInst>(I)) {
73 GS.IsLoaded = true;
74 // Don't hack on volatile loads.
75 if (LI->isVolatile())
76 return true;
77 GS.Ordering = strongerOrdering(GS.Ordering, LI->getOrdering());
78 } else if (const StoreInst *SI = dyn_cast<StoreInst>(I)) {
79 // Don't allow a store OF the address, only stores TO the address.
80 if (SI->getOperand(0) == V)
81 return true;
82
83 // Don't hack on volatile stores.
84 if (SI->isVolatile())
85 return true;
86
87 GS.Ordering = strongerOrdering(GS.Ordering, SI->getOrdering());
88
89 // If this is a direct store to the global (i.e., the global is a scalar
90 // value, not an aggregate), keep more specific information about
91 // stores.
92 if (GS.StoredType != GlobalStatus::Stored) {
93 if (const GlobalVariable *GV =
94 dyn_cast<GlobalVariable>(SI->getOperand(1))) {
95 Value *StoredVal = SI->getOperand(0);
96
97 if (Constant *C = dyn_cast<Constant>(StoredVal)) {
98 if (C->isThreadDependent()) {
99 // The stored value changes between threads; don't track it.
100 return true;
101 }
102 }
103
104 if (StoredVal == GV->getInitializer()) {
105 if (GS.StoredType < GlobalStatus::InitializerStored)
106 GS.StoredType = GlobalStatus::InitializerStored;
107 } else if (isa<LoadInst>(StoredVal) &&
108 cast<LoadInst>(StoredVal)->getOperand(0) == GV) {
109 if (GS.StoredType < GlobalStatus::InitializerStored)
110 GS.StoredType = GlobalStatus::InitializerStored;
111 } else if (GS.StoredType < GlobalStatus::StoredOnce) {
112 GS.StoredType = GlobalStatus::StoredOnce;
113 GS.StoredOnceValue = StoredVal;
114 } else if (GS.StoredType == GlobalStatus::StoredOnce &&
115 GS.StoredOnceValue == StoredVal) {
116 // noop.
117 } else {
118 GS.StoredType = GlobalStatus::Stored;
119 }
120 } else {
121 GS.StoredType = GlobalStatus::Stored;
122 }
123 }
124 } else if (isa<BitCastInst>(I)) {
125 if (analyzeGlobalAux(I, GS, PhiUsers))
126 return true;
127 } else if (isa<GetElementPtrInst>(I)) {
128 if (analyzeGlobalAux(I, GS, PhiUsers))
129 return true;
130 } else if (isa<SelectInst>(I)) {
131 if (analyzeGlobalAux(I, GS, PhiUsers))
132 return true;
133 } else if (const PHINode *PN = dyn_cast<PHINode>(I)) {
134 // PHI nodes we can check just like select or GEP instructions, but we
135 // have to be careful about infinite recursion.
136 if (PhiUsers.insert(PN).second) // Not already visited.
137 if (analyzeGlobalAux(I, GS, PhiUsers))
138 return true;
139 } else if (isa<CmpInst>(I)) {
140 GS.IsCompared = true;
141 } else if (const MemTransferInst *MTI = dyn_cast<MemTransferInst>(I)) {
142 if (MTI->isVolatile())
143 return true;
144 if (MTI->getArgOperand(0) == V)
145 GS.StoredType = GlobalStatus::Stored;
146 if (MTI->getArgOperand(1) == V)
147 GS.IsLoaded = true;
148 } else if (const MemSetInst *MSI = dyn_cast<MemSetInst>(I)) {
149 assert(MSI->getArgOperand(0) == V && "Memset only takes one pointer!");
150 if (MSI->isVolatile())
151 return true;
152 GS.StoredType = GlobalStatus::Stored;
153 } else if (auto C = ImmutableCallSite(I)) {
154 if (!C.isCallee(&U))
155 return true;
156 GS.IsLoaded = true;
157 } else {
158 return true; // Any other non-load instruction might take address!
159 }
160 } else if (const Constant *C = dyn_cast<Constant>(UR)) {
161 GS.HasNonInstructionUser = true;
162 // We might have a dead and dangling constant hanging off of here.
163 if (!isSafeToDestroyConstant(C))
164 return true;
165 } else {
166 GS.HasNonInstructionUser = true;
167 // Otherwise must be some other user.
168 return true;
169 }
170 }
171
172 return false;
173 }
174
analyzeGlobal(const Value * V,GlobalStatus & GS)175 bool GlobalStatus::analyzeGlobal(const Value *V, GlobalStatus &GS) {
176 SmallPtrSet<const PHINode *, 16> PhiUsers;
177 return analyzeGlobalAux(V, GS, PhiUsers);
178 }
179
GlobalStatus()180 GlobalStatus::GlobalStatus()
181 : IsCompared(false), IsLoaded(false), StoredType(NotStored),
182 StoredOnceValue(nullptr), AccessingFunction(nullptr),
183 HasMultipleAccessingFunctions(false), HasNonInstructionUser(false),
184 Ordering(NotAtomic) {}
185