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/Transforms/Utils/GlobalStatus.h"
11 #include "llvm/ADT/SmallPtrSet.h"
12 #include "llvm/IR/BasicBlock.h"
13 #include "llvm/IR/CallSite.h"
14 #include "llvm/IR/Constant.h"
15 #include "llvm/IR/Constants.h"
16 #include "llvm/IR/GlobalValue.h"
17 #include "llvm/IR/GlobalVariable.h"
18 #include "llvm/IR/InstrTypes.h"
19 #include "llvm/IR/Instruction.h"
20 #include "llvm/IR/Instructions.h"
21 #include "llvm/IR/IntrinsicInst.h"
22 #include "llvm/IR/Use.h"
23 #include "llvm/IR/User.h"
24 #include "llvm/IR/Value.h"
25 #include "llvm/Support/AtomicOrdering.h"
26 #include "llvm/Support/Casting.h"
27 #include <algorithm>
28 #include <cassert>
29 
30 using namespace llvm;
31 
32 /// Return the stronger of the two ordering. If the two orderings are acquire
33 /// and release, then return AcquireRelease.
34 ///
35 static AtomicOrdering strongerOrdering(AtomicOrdering X, AtomicOrdering Y) {
36   if ((X == AtomicOrdering::Acquire && Y == AtomicOrdering::Release) ||
37       (Y == AtomicOrdering::Acquire && X == AtomicOrdering::Release))
38     return AtomicOrdering::AcquireRelease;
39   return (AtomicOrdering)std::max((unsigned)X, (unsigned)Y);
40 }
41 
42 /// It is safe to destroy a constant iff it is only used by constants itself.
43 /// Note that constants cannot be cyclic, so this test is pretty easy to
44 /// implement recursively.
45 ///
46 bool llvm::isSafeToDestroyConstant(const Constant *C) {
47   if (isa<GlobalValue>(C))
48     return false;
49 
50   if (isa<ConstantData>(C))
51     return false;
52 
53   for (const User *U : C->users())
54     if (const Constant *CU = dyn_cast<Constant>(U)) {
55       if (!isSafeToDestroyConstant(CU))
56         return false;
57     } else
58       return false;
59   return true;
60 }
61 
62 static bool analyzeGlobalAux(const Value *V, GlobalStatus &GS,
63                              SmallPtrSetImpl<const PHINode *> &PhiUsers) {
64   if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
65     if (GV->isExternallyInitialized())
66       GS.StoredType = GlobalStatus::StoredOnce;
67 
68   for (const Use &U : V->uses()) {
69     const User *UR = U.getUser();
70     if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(UR)) {
71       GS.HasNonInstructionUser = true;
72 
73       // If the result of the constantexpr isn't pointer type, then we won't
74       // know to expect it in various places.  Just reject early.
75       if (!isa<PointerType>(CE->getType()))
76         return true;
77 
78       if (analyzeGlobalAux(CE, GS, PhiUsers))
79         return true;
80     } else if (const Instruction *I = dyn_cast<Instruction>(UR)) {
81       if (!GS.HasMultipleAccessingFunctions) {
82         const Function *F = I->getParent()->getParent();
83         if (!GS.AccessingFunction)
84           GS.AccessingFunction = F;
85         else if (GS.AccessingFunction != F)
86           GS.HasMultipleAccessingFunctions = true;
87       }
88       if (const LoadInst *LI = dyn_cast<LoadInst>(I)) {
89         GS.IsLoaded = true;
90         // Don't hack on volatile loads.
91         if (LI->isVolatile())
92           return true;
93         GS.Ordering = strongerOrdering(GS.Ordering, LI->getOrdering());
94       } else if (const StoreInst *SI = dyn_cast<StoreInst>(I)) {
95         // Don't allow a store OF the address, only stores TO the address.
96         if (SI->getOperand(0) == V)
97           return true;
98 
99         // Don't hack on volatile stores.
100         if (SI->isVolatile())
101           return true;
102 
103         GS.Ordering = strongerOrdering(GS.Ordering, SI->getOrdering());
104 
105         // If this is a direct store to the global (i.e., the global is a scalar
106         // value, not an aggregate), keep more specific information about
107         // stores.
108         if (GS.StoredType != GlobalStatus::Stored) {
109           if (const GlobalVariable *GV =
110                   dyn_cast<GlobalVariable>(SI->getOperand(1))) {
111             Value *StoredVal = SI->getOperand(0);
112 
113             if (Constant *C = dyn_cast<Constant>(StoredVal)) {
114               if (C->isThreadDependent()) {
115                 // The stored value changes between threads; don't track it.
116                 return true;
117               }
118             }
119 
120             if (GV->hasInitializer() && StoredVal == GV->getInitializer()) {
121               if (GS.StoredType < GlobalStatus::InitializerStored)
122                 GS.StoredType = GlobalStatus::InitializerStored;
123             } else if (isa<LoadInst>(StoredVal) &&
124                        cast<LoadInst>(StoredVal)->getOperand(0) == GV) {
125               if (GS.StoredType < GlobalStatus::InitializerStored)
126                 GS.StoredType = GlobalStatus::InitializerStored;
127             } else if (GS.StoredType < GlobalStatus::StoredOnce) {
128               GS.StoredType = GlobalStatus::StoredOnce;
129               GS.StoredOnceValue = StoredVal;
130             } else if (GS.StoredType == GlobalStatus::StoredOnce &&
131                        GS.StoredOnceValue == StoredVal) {
132               // noop.
133             } else {
134               GS.StoredType = GlobalStatus::Stored;
135             }
136           } else {
137             GS.StoredType = GlobalStatus::Stored;
138           }
139         }
140       } else if (isa<BitCastInst>(I)) {
141         if (analyzeGlobalAux(I, GS, PhiUsers))
142           return true;
143       } else if (isa<GetElementPtrInst>(I)) {
144         if (analyzeGlobalAux(I, GS, PhiUsers))
145           return true;
146       } else if (isa<SelectInst>(I)) {
147         if (analyzeGlobalAux(I, GS, PhiUsers))
148           return true;
149       } else if (const PHINode *PN = dyn_cast<PHINode>(I)) {
150         // PHI nodes we can check just like select or GEP instructions, but we
151         // have to be careful about infinite recursion.
152         if (PhiUsers.insert(PN).second) // Not already visited.
153           if (analyzeGlobalAux(I, GS, PhiUsers))
154             return true;
155       } else if (isa<CmpInst>(I)) {
156         GS.IsCompared = true;
157       } else if (const MemTransferInst *MTI = dyn_cast<MemTransferInst>(I)) {
158         if (MTI->isVolatile())
159           return true;
160         if (MTI->getArgOperand(0) == V)
161           GS.StoredType = GlobalStatus::Stored;
162         if (MTI->getArgOperand(1) == V)
163           GS.IsLoaded = true;
164       } else if (const MemSetInst *MSI = dyn_cast<MemSetInst>(I)) {
165         assert(MSI->getArgOperand(0) == V && "Memset only takes one pointer!");
166         if (MSI->isVolatile())
167           return true;
168         GS.StoredType = GlobalStatus::Stored;
169       } else if (auto C = ImmutableCallSite(I)) {
170         if (!C.isCallee(&U))
171           return true;
172         GS.IsLoaded = true;
173       } else {
174         return true; // Any other non-load instruction might take address!
175       }
176     } else if (const Constant *C = dyn_cast<Constant>(UR)) {
177       GS.HasNonInstructionUser = true;
178       // We might have a dead and dangling constant hanging off of here.
179       if (!isSafeToDestroyConstant(C))
180         return true;
181     } else {
182       GS.HasNonInstructionUser = true;
183       // Otherwise must be some other user.
184       return true;
185     }
186   }
187 
188   return false;
189 }
190 
191 GlobalStatus::GlobalStatus() = default;
192 
193 bool GlobalStatus::analyzeGlobal(const Value *V, GlobalStatus &GS) {
194   SmallPtrSet<const PHINode *, 16> PhiUsers;
195   return analyzeGlobalAux(V, GS, PhiUsers);
196 }
197