1 //===--- ConstantEmitter.h - IR constant emission ---------------*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // A helper class for emitting expressions and values as llvm::Constants
10 // and as initializers for global variables.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #ifndef LLVM_CLANG_LIB_CODEGEN_CONSTANTEMITTER_H
15 #define LLVM_CLANG_LIB_CODEGEN_CONSTANTEMITTER_H
16 
17 #include "CodeGenFunction.h"
18 #include "CodeGenModule.h"
19 
20 namespace clang {
21 namespace CodeGen {
22 
23 class ConstantEmitter {
24 public:
25   CodeGenModule &CGM;
26   CodeGenFunction *const CGF;
27 
28 private:
29   bool Abstract = false;
30 
31   /// Whether non-abstract components of the emitter have been initialized.
32   bool InitializedNonAbstract = false;
33 
34   /// Whether the emitter has been finalized.
35   bool Finalized = false;
36 
37   /// Whether the constant-emission failed.
38   bool Failed = false;
39 
40   /// Whether we're in a constant context.
41   bool InConstantContext = false;
42 
43   /// The AST address space where this (non-abstract) initializer is going.
44   /// Used for generating appropriate placeholders.
45   LangAS DestAddressSpace;
46 
47   llvm::SmallVector<std::pair<llvm::Constant *, llvm::GlobalVariable*>, 4>
48     PlaceholderAddresses;
49 
50 public:
51   ConstantEmitter(CodeGenModule &CGM, CodeGenFunction *CGF = nullptr)
52     : CGM(CGM), CGF(CGF) {}
53 
54   /// Initialize this emission in the context of the given function.
55   /// Use this if the expression might contain contextual references like
56   /// block addresses or PredefinedExprs.
57   ConstantEmitter(CodeGenFunction &CGF)
58     : CGM(CGF.CGM), CGF(&CGF) {}
59 
60   ConstantEmitter(const ConstantEmitter &other) = delete;
61   ConstantEmitter &operator=(const ConstantEmitter &other) = delete;
62 
63   ~ConstantEmitter();
64 
65   /// Is the current emission context abstract?
66   bool isAbstract() const {
67     return Abstract;
68   }
69 
70   bool isInConstantContext() const { return InConstantContext; }
71   void setInConstantContext(bool var) { InConstantContext = var; }
72 
73   /// Try to emit the initiaizer of the given declaration as an abstract
74   /// constant.  If this succeeds, the emission must be finalized.
75   llvm::Constant *tryEmitForInitializer(const VarDecl &D);
76   llvm::Constant *tryEmitForInitializer(const Expr *E, LangAS destAddrSpace,
77                                         QualType destType);
78   llvm::Constant *emitForInitializer(const APValue &value, LangAS destAddrSpace,
79                                      QualType destType);
80 
81   void finalize(llvm::GlobalVariable *global);
82 
83   // All of the "abstract" emission methods below permit the emission to
84   // be immediately discarded without finalizing anything.  Therefore, they
85   // must also promise not to do anything that will, in the future, require
86   // finalization:
87   //
88   //   - using the CGF (if present) for anything other than establishing
89   //     semantic context; for example, an expression with ignored
90   //     side-effects must not be emitted as an abstract expression
91   //
92   //   - doing anything that would not be safe to duplicate within an
93   //     initializer or to propagate to another context; for example,
94   //     side effects, or emitting an initialization that requires a
95   //     reference to its current location.
96 
97   /// Try to emit the initializer of the given declaration as an abstract
98   /// constant.
99   llvm::Constant *tryEmitAbstractForInitializer(const VarDecl &D);
100 
101   /// Emit the result of the given expression as an abstract constant,
102   /// asserting that it succeeded.  This is only safe to do when the
103   /// expression is known to be a constant expression with either a fairly
104   /// simple type or a known simple form.
105   llvm::Constant *emitAbstract(const Expr *E, QualType T);
106   llvm::Constant *emitAbstract(SourceLocation loc, const APValue &value,
107                                QualType T);
108 
109   /// Try to emit the result of the given expression as an abstract constant.
110   llvm::Constant *tryEmitAbstract(const Expr *E, QualType T);
111   llvm::Constant *tryEmitAbstractForMemory(const Expr *E, QualType T);
112 
113   llvm::Constant *tryEmitAbstract(const APValue &value, QualType T);
114   llvm::Constant *tryEmitAbstractForMemory(const APValue &value, QualType T);
115 
116   llvm::Constant *tryEmitConstantExpr(const ConstantExpr *CE);
117 
118   llvm::Constant *emitNullForMemory(QualType T) {
119     return emitNullForMemory(CGM, T);
120   }
121   llvm::Constant *emitForMemory(llvm::Constant *C, QualType T) {
122     return emitForMemory(CGM, C, T);
123   }
124 
125   static llvm::Constant *emitNullForMemory(CodeGenModule &CGM, QualType T);
126   static llvm::Constant *emitForMemory(CodeGenModule &CGM, llvm::Constant *C,
127                                        QualType T);
128 
129   // These are private helper routines of the constant emitter that
130   // can't actually be private because things are split out into helper
131   // functions and classes.
132 
133   llvm::Constant *tryEmitPrivateForVarInit(const VarDecl &D);
134 
135   llvm::Constant *tryEmitPrivate(const Expr *E, QualType T);
136   llvm::Constant *tryEmitPrivateForMemory(const Expr *E, QualType T);
137 
138   llvm::Constant *tryEmitPrivate(const APValue &value, QualType T);
139   llvm::Constant *tryEmitPrivateForMemory(const APValue &value, QualType T);
140 
141   /// Get the address of the current location.  This is a constant
142   /// that will resolve, after finalization, to the address of the
143   /// 'signal' value that is registered with the emitter later.
144   llvm::GlobalValue *getCurrentAddrPrivate();
145 
146   /// Register a 'signal' value with the emitter to inform it where to
147   /// resolve a placeholder.  The signal value must be unique in the
148   /// initializer; it might, for example, be the address of a global that
149   /// refers to the current-address value in its own initializer.
150   ///
151   /// Uses of the placeholder must be properly anchored before finalizing
152   /// the emitter, e.g. by being installed as the initializer of a global
153   /// variable.  That is, it must be possible to replaceAllUsesWith
154   /// the placeholder with the proper address of the signal.
155   void registerCurrentAddrPrivate(llvm::Constant *signal,
156                                   llvm::GlobalValue *placeholder);
157 
158 private:
159   void initializeNonAbstract(LangAS destAS) {
160     assert(!InitializedNonAbstract);
161     InitializedNonAbstract = true;
162     DestAddressSpace = destAS;
163   }
164   llvm::Constant *markIfFailed(llvm::Constant *init) {
165     if (!init)
166       Failed = true;
167     return init;
168   }
169 
170   struct AbstractState {
171     bool OldValue;
172     size_t OldPlaceholdersSize;
173   };
174   AbstractState pushAbstract() {
175     AbstractState saved = { Abstract, PlaceholderAddresses.size() };
176     Abstract = true;
177     return saved;
178   }
179   llvm::Constant *validateAndPopAbstract(llvm::Constant *C, AbstractState save);
180 };
181 
182 }
183 }
184 
185 #endif
186