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   if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
53     if (GV->isExternallyInitialized())
54       GS.StoredType = GlobalStatus::StoredOnce;
55 
56   for (const Use &U : V->uses()) {
57     const User *UR = U.getUser();
58     if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(UR)) {
59       GS.HasNonInstructionUser = true;
60 
61       // If the result of the constantexpr isn't pointer type, then we won't
62       // know to expect it in various places.  Just reject early.
63       if (!isa<PointerType>(CE->getType()))
64         return true;
65 
66       if (analyzeGlobalAux(CE, GS, PhiUsers))
67         return true;
68     } else if (const Instruction *I = dyn_cast<Instruction>(UR)) {
69       if (!GS.HasMultipleAccessingFunctions) {
70         const Function *F = I->getParent()->getParent();
71         if (!GS.AccessingFunction)
72           GS.AccessingFunction = F;
73         else if (GS.AccessingFunction != F)
74           GS.HasMultipleAccessingFunctions = true;
75       }
76       if (const LoadInst *LI = dyn_cast<LoadInst>(I)) {
77         GS.IsLoaded = true;
78         // Don't hack on volatile loads.
79         if (LI->isVolatile())
80           return true;
81         GS.Ordering = strongerOrdering(GS.Ordering, LI->getOrdering());
82       } else if (const StoreInst *SI = dyn_cast<StoreInst>(I)) {
83         // Don't allow a store OF the address, only stores TO the address.
84         if (SI->getOperand(0) == V)
85           return true;
86 
87         // Don't hack on volatile stores.
88         if (SI->isVolatile())
89           return true;
90 
91         GS.Ordering = strongerOrdering(GS.Ordering, SI->getOrdering());
92 
93         // If this is a direct store to the global (i.e., the global is a scalar
94         // value, not an aggregate), keep more specific information about
95         // stores.
96         if (GS.StoredType != GlobalStatus::Stored) {
97           if (const GlobalVariable *GV =
98                   dyn_cast<GlobalVariable>(SI->getOperand(1))) {
99             Value *StoredVal = SI->getOperand(0);
100 
101             if (Constant *C = dyn_cast<Constant>(StoredVal)) {
102               if (C->isThreadDependent()) {
103                 // The stored value changes between threads; don't track it.
104                 return true;
105               }
106             }
107 
108             if (StoredVal == GV->getInitializer()) {
109               if (GS.StoredType < GlobalStatus::InitializerStored)
110                 GS.StoredType = GlobalStatus::InitializerStored;
111             } else if (isa<LoadInst>(StoredVal) &&
112                        cast<LoadInst>(StoredVal)->getOperand(0) == GV) {
113               if (GS.StoredType < GlobalStatus::InitializerStored)
114                 GS.StoredType = GlobalStatus::InitializerStored;
115             } else if (GS.StoredType < GlobalStatus::StoredOnce) {
116               GS.StoredType = GlobalStatus::StoredOnce;
117               GS.StoredOnceValue = StoredVal;
118             } else if (GS.StoredType == GlobalStatus::StoredOnce &&
119                        GS.StoredOnceValue == StoredVal) {
120               // noop.
121             } else {
122               GS.StoredType = GlobalStatus::Stored;
123             }
124           } else {
125             GS.StoredType = GlobalStatus::Stored;
126           }
127         }
128       } else if (isa<BitCastInst>(I)) {
129         if (analyzeGlobalAux(I, GS, PhiUsers))
130           return true;
131       } else if (isa<GetElementPtrInst>(I)) {
132         if (analyzeGlobalAux(I, GS, PhiUsers))
133           return true;
134       } else if (isa<SelectInst>(I)) {
135         if (analyzeGlobalAux(I, GS, PhiUsers))
136           return true;
137       } else if (const PHINode *PN = dyn_cast<PHINode>(I)) {
138         // PHI nodes we can check just like select or GEP instructions, but we
139         // have to be careful about infinite recursion.
140         if (PhiUsers.insert(PN).second) // Not already visited.
141           if (analyzeGlobalAux(I, GS, PhiUsers))
142             return true;
143       } else if (isa<CmpInst>(I)) {
144         GS.IsCompared = true;
145       } else if (const MemTransferInst *MTI = dyn_cast<MemTransferInst>(I)) {
146         if (MTI->isVolatile())
147           return true;
148         if (MTI->getArgOperand(0) == V)
149           GS.StoredType = GlobalStatus::Stored;
150         if (MTI->getArgOperand(1) == V)
151           GS.IsLoaded = true;
152       } else if (const MemSetInst *MSI = dyn_cast<MemSetInst>(I)) {
153         assert(MSI->getArgOperand(0) == V && "Memset only takes one pointer!");
154         if (MSI->isVolatile())
155           return true;
156         GS.StoredType = GlobalStatus::Stored;
157       } else if (auto C = ImmutableCallSite(I)) {
158         if (!C.isCallee(&U))
159           return true;
160         GS.IsLoaded = true;
161       } else {
162         return true; // Any other non-load instruction might take address!
163       }
164     } else if (const Constant *C = dyn_cast<Constant>(UR)) {
165       GS.HasNonInstructionUser = true;
166       // We might have a dead and dangling constant hanging off of here.
167       if (!isSafeToDestroyConstant(C))
168         return true;
169     } else {
170       GS.HasNonInstructionUser = true;
171       // Otherwise must be some other user.
172       return true;
173     }
174   }
175 
176   return false;
177 }
178 
analyzeGlobal(const Value * V,GlobalStatus & GS)179 bool GlobalStatus::analyzeGlobal(const Value *V, GlobalStatus &GS) {
180   SmallPtrSet<const PHINode *, 16> PhiUsers;
181   return analyzeGlobalAux(V, GS, PhiUsers);
182 }
183 
GlobalStatus()184 GlobalStatus::GlobalStatus()
185     : IsCompared(false), IsLoaded(false), StoredType(NotStored),
186       StoredOnceValue(nullptr), AccessingFunction(nullptr),
187       HasMultipleAccessingFunctions(false), HasNonInstructionUser(false),
188       Ordering(NotAtomic) {}
189