1 //===--- CGExprConstant.cpp - Emit LLVM Code from Constant Expressions ----===//
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 // This contains code to emit Constant Expr nodes as LLVM code.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "CodeGenFunction.h"
15 #include "CodeGenModule.h"
16 #include "CGObjCRuntime.h"
17 #include "clang/AST/APValue.h"
18 #include "clang/AST/ASTContext.h"
19 #include "clang/AST/StmtVisitor.h"
20 #include "llvm/Constants.h"
21 #include "llvm/Function.h"
22 #include "llvm/GlobalVariable.h"
23 #include "llvm/Support/Compiler.h"
24 #include "llvm/Target/TargetData.h"
25 using namespace clang;
26 using namespace CodeGen;
27 
28 namespace  {
29 class VISIBILITY_HIDDEN ConstExprEmitter :
30   public StmtVisitor<ConstExprEmitter, llvm::Constant*> {
31   CodeGenModule &CGM;
32   CodeGenFunction *CGF;
33 public:
34   ConstExprEmitter(CodeGenModule &cgm, CodeGenFunction *cgf)
35     : CGM(cgm), CGF(cgf) {
36   }
37 
38   //===--------------------------------------------------------------------===//
39   //                            Visitor Methods
40   //===--------------------------------------------------------------------===//
41 
42   llvm::Constant *VisitStmt(Stmt *S) {
43     CGM.ErrorUnsupported(S, "constant expression");
44     QualType T = cast<Expr>(S)->getType();
45     return llvm::UndefValue::get(CGM.getTypes().ConvertType(T));
46   }
47 
48   llvm::Constant *VisitParenExpr(ParenExpr *PE) {
49     return Visit(PE->getSubExpr());
50   }
51 
52   llvm::Constant *VisitCompoundLiteralExpr(CompoundLiteralExpr *E) {
53     return Visit(E->getInitializer());
54   }
55 
56   llvm::Constant *VisitCastExpr(CastExpr* E) {
57     // GCC cast to union extension
58     if (E->getType()->isUnionType()) {
59       const llvm::Type *Ty = ConvertType(E->getType());
60       return EmitUnion(CGM.EmitConstantExpr(E->getSubExpr(), CGF), Ty);
61     }
62     if (CGM.getContext().getCanonicalType(E->getSubExpr()->getType()) ==
63         CGM.getContext().getCanonicalType(E->getType())) {
64       return Visit(E->getSubExpr());
65     }
66     return 0;
67   }
68 
69   llvm::Constant *VisitCXXDefaultArgExpr(CXXDefaultArgExpr *DAE) {
70     return Visit(DAE->getExpr());
71   }
72 
73   llvm::Constant *EmitArrayInitialization(InitListExpr *ILE) {
74     std::vector<llvm::Constant*> Elts;
75     const llvm::ArrayType *AType =
76         cast<llvm::ArrayType>(ConvertType(ILE->getType()));
77     unsigned NumInitElements = ILE->getNumInits();
78     // FIXME: Check for wide strings
79     // FIXME: Check for NumInitElements exactly equal to 1??
80     if (NumInitElements > 0 &&
81         (isa<StringLiteral>(ILE->getInit(0)) ||
82          isa<ObjCEncodeExpr>(ILE->getInit(0))) &&
83         ILE->getType()->getArrayElementTypeNoTypeQual()->isCharType())
84       return Visit(ILE->getInit(0));
85     const llvm::Type *ElemTy = AType->getElementType();
86     unsigned NumElements = AType->getNumElements();
87 
88     // Initialising an array requires us to automatically
89     // initialise any elements that have not been initialised explicitly
90     unsigned NumInitableElts = std::min(NumInitElements, NumElements);
91 
92     // Copy initializer elements.
93     unsigned i = 0;
94     bool RewriteType = false;
95     for (; i < NumInitableElts; ++i) {
96       llvm::Constant *C = CGM.EmitConstantExpr(ILE->getInit(i), CGF);
97       if (!C)
98         return 0;
99       RewriteType |= (C->getType() != ElemTy);
100       Elts.push_back(C);
101     }
102 
103     // Initialize remaining array elements.
104     for (; i < NumElements; ++i)
105       Elts.push_back(llvm::Constant::getNullValue(ElemTy));
106 
107     if (RewriteType) {
108       // FIXME: Try to avoid packing the array
109       std::vector<const llvm::Type*> Types;
110       for (unsigned i = 0; i < Elts.size(); ++i)
111         Types.push_back(Elts[i]->getType());
112       const llvm::StructType *SType = llvm::StructType::get(Types, true);
113       return llvm::ConstantStruct::get(SType, Elts);
114     }
115 
116     return llvm::ConstantArray::get(AType, Elts);
117   }
118 
119   void InsertBitfieldIntoStruct(std::vector<llvm::Constant*>& Elts,
120                                 FieldDecl* Field, Expr* E) {
121     // Calculate the value to insert
122     llvm::Constant *C = CGM.EmitConstantExpr(E, CGF);
123     if (!C)
124       return;
125 
126     llvm::ConstantInt *CI = dyn_cast<llvm::ConstantInt>(C);
127     if (!CI) {
128       CGM.ErrorUnsupported(E, "bitfield initialization");
129       return;
130     }
131     llvm::APInt V = CI->getValue();
132 
133     // Calculate information about the relevant field
134     const llvm::Type* Ty = CI->getType();
135     const llvm::TargetData &TD = CGM.getTypes().getTargetData();
136     unsigned size = TD.getTypePaddedSizeInBits(Ty);
137     unsigned fieldOffset = CGM.getTypes().getLLVMFieldNo(Field) * size;
138     CodeGenTypes::BitFieldInfo bitFieldInfo =
139         CGM.getTypes().getBitFieldInfo(Field);
140     fieldOffset += bitFieldInfo.Begin;
141 
142     // Find where to start the insertion
143     // FIXME: This is O(n^2) in the number of bit-fields!
144     // FIXME: This won't work if the struct isn't completely packed!
145     unsigned offset = 0, i = 0;
146     while (offset < (fieldOffset & -8))
147       offset += TD.getTypePaddedSizeInBits(Elts[i++]->getType());
148 
149     // Advance over 0 sized elements (must terminate in bounds since
150     // the bitfield must have a size).
151     while (TD.getTypePaddedSizeInBits(Elts[i]->getType()) == 0)
152       ++i;
153 
154     // Promote the size of V if necessary
155     // FIXME: This should never occur, but currently it can because
156     // initializer constants are cast to bool, and because clang is
157     // not enforcing bitfield width limits.
158     if (bitFieldInfo.Size > V.getBitWidth())
159       V.zext(bitFieldInfo.Size);
160 
161     // Insert the bits into the struct
162     // FIXME: This algorthm is only correct on X86!
163     // FIXME: THis algorthm assumes bit-fields only have byte-size elements!
164     unsigned bitsToInsert = bitFieldInfo.Size;
165     unsigned curBits = std::min(8 - (fieldOffset & 7), bitsToInsert);
166     unsigned byte = V.getLoBits(curBits).getZExtValue() << (fieldOffset & 7);
167     do {
168       llvm::Constant* byteC = llvm::ConstantInt::get(llvm::Type::Int8Ty, byte);
169       Elts[i] = llvm::ConstantExpr::getOr(Elts[i], byteC);
170       ++i;
171       V = V.lshr(curBits);
172       bitsToInsert -= curBits;
173 
174       if (!bitsToInsert)
175         break;
176 
177       curBits = bitsToInsert > 8 ? 8 : bitsToInsert;
178       byte = V.getLoBits(curBits).getZExtValue();
179     } while (true);
180   }
181 
182   llvm::Constant *EmitStructInitialization(InitListExpr *ILE) {
183     const llvm::StructType *SType =
184         cast<llvm::StructType>(ConvertType(ILE->getType()));
185     RecordDecl *RD = ILE->getType()->getAsRecordType()->getDecl();
186     std::vector<llvm::Constant*> Elts;
187 
188     // Initialize the whole structure to zero.
189     for (unsigned i = 0; i < SType->getNumElements(); ++i) {
190       const llvm::Type *FieldTy = SType->getElementType(i);
191       Elts.push_back(llvm::Constant::getNullValue(FieldTy));
192     }
193 
194     // Copy initializer elements. Skip padding fields.
195     unsigned EltNo = 0;  // Element no in ILE
196     int FieldNo = 0; // Field no in RecordDecl
197     bool RewriteType = false;
198     for (RecordDecl::field_iterator Field = RD->field_begin(),
199                                  FieldEnd = RD->field_end();
200          EltNo < ILE->getNumInits() && Field != FieldEnd; ++Field) {
201       FieldNo++;
202       if (!Field->getIdentifier())
203         continue;
204 
205       if (Field->isBitField()) {
206         InsertBitfieldIntoStruct(Elts, *Field, ILE->getInit(EltNo));
207       } else {
208         unsigned FieldNo = CGM.getTypes().getLLVMFieldNo(*Field);
209         llvm::Constant *C = CGM.EmitConstantExpr(ILE->getInit(EltNo), CGF);
210         if (!C) return 0;
211         RewriteType |= (C->getType() != Elts[FieldNo]->getType());
212         Elts[FieldNo] = C;
213       }
214       EltNo++;
215     }
216 
217     if (RewriteType) {
218       // FIXME: Make this work for non-packed structs
219       assert(SType->isPacked() && "Cannot recreate unpacked structs");
220       std::vector<const llvm::Type*> Types;
221       for (unsigned i = 0; i < Elts.size(); ++i)
222         Types.push_back(Elts[i]->getType());
223       SType = llvm::StructType::get(Types, true);
224     }
225 
226     return llvm::ConstantStruct::get(SType, Elts);
227   }
228 
229   llvm::Constant *EmitUnion(llvm::Constant *C, const llvm::Type *Ty) {
230     if (!C)
231       return 0;
232 
233     // Build a struct with the union sub-element as the first member,
234     // and padded to the appropriate size
235     std::vector<llvm::Constant*> Elts;
236     std::vector<const llvm::Type*> Types;
237     Elts.push_back(C);
238     Types.push_back(C->getType());
239     unsigned CurSize = CGM.getTargetData().getTypePaddedSize(C->getType());
240     unsigned TotalSize = CGM.getTargetData().getTypePaddedSize(Ty);
241     while (CurSize < TotalSize) {
242       Elts.push_back(llvm::Constant::getNullValue(llvm::Type::Int8Ty));
243       Types.push_back(llvm::Type::Int8Ty);
244       CurSize++;
245     }
246 
247     // This always generates a packed struct
248     // FIXME: Try to generate an unpacked struct when we can
249     llvm::StructType* STy = llvm::StructType::get(Types, true);
250     return llvm::ConstantStruct::get(STy, Elts);
251   }
252 
253   llvm::Constant *EmitUnionInitialization(InitListExpr *ILE) {
254     const llvm::Type *Ty = ConvertType(ILE->getType());
255 
256     // If this is an empty initializer list, we value-initialize the
257     // union.
258     if (ILE->getNumInits() == 0)
259       return llvm::Constant::getNullValue(Ty);
260 
261     FieldDecl* curField = ILE->getInitializedFieldInUnion();
262     if (!curField) {
263       // There's no field to initialize, so value-initialize the union.
264 #ifndef NDEBUG
265       // Make sure that it's really an empty and not a failure of
266       // semantic analysis.
267       RecordDecl *RD = ILE->getType()->getAsRecordType()->getDecl();
268       for (RecordDecl::field_iterator Field = RD->field_begin(),
269                                    FieldEnd = RD->field_end();
270            Field != FieldEnd; ++Field)
271         assert(Field->isUnnamedBitfield() && "Only unnamed bitfields allowed");
272 #endif
273       return llvm::Constant::getNullValue(Ty);
274     }
275 
276     if (curField->isBitField()) {
277       // Create a dummy struct for bit-field insertion
278       unsigned NumElts = CGM.getTargetData().getTypePaddedSize(Ty) / 8;
279       llvm::Constant* NV = llvm::Constant::getNullValue(llvm::Type::Int8Ty);
280       std::vector<llvm::Constant*> Elts(NumElts, NV);
281 
282       InsertBitfieldIntoStruct(Elts, curField, ILE->getInit(0));
283       const llvm::ArrayType *RetTy =
284           llvm::ArrayType::get(NV->getType(), NumElts);
285       return llvm::ConstantArray::get(RetTy, Elts);
286     }
287 
288     return EmitUnion(CGM.EmitConstantExpr(ILE->getInit(0), CGF), Ty);
289   }
290 
291   llvm::Constant *EmitVectorInitialization(InitListExpr *ILE) {
292     const llvm::VectorType *VType =
293         cast<llvm::VectorType>(ConvertType(ILE->getType()));
294     const llvm::Type *ElemTy = VType->getElementType();
295     std::vector<llvm::Constant*> Elts;
296     unsigned NumElements = VType->getNumElements();
297     unsigned NumInitElements = ILE->getNumInits();
298 
299     unsigned NumInitableElts = std::min(NumInitElements, NumElements);
300 
301     // Copy initializer elements.
302     unsigned i = 0;
303     for (; i < NumInitableElts; ++i) {
304       llvm::Constant *C = CGM.EmitConstantExpr(ILE->getInit(i), CGF);
305       if (!C)
306         return 0;
307       Elts.push_back(C);
308     }
309 
310     for (; i < NumElements; ++i)
311       Elts.push_back(llvm::Constant::getNullValue(ElemTy));
312 
313     return llvm::ConstantVector::get(VType, Elts);
314   }
315 
316   llvm::Constant *VisitImplicitValueInitExpr(ImplicitValueInitExpr* E) {
317     const llvm::Type* RetTy = CGM.getTypes().ConvertType(E->getType());
318     return llvm::Constant::getNullValue(RetTy);
319   }
320 
321   llvm::Constant *VisitInitListExpr(InitListExpr *ILE) {
322     if (ILE->getType()->isScalarType()) {
323       // We have a scalar in braces. Just use the first element.
324       if (ILE->getNumInits() > 0)
325         return CGM.EmitConstantExpr(ILE->getInit(0), CGF);
326 
327       const llvm::Type* RetTy = CGM.getTypes().ConvertType(ILE->getType());
328       return llvm::Constant::getNullValue(RetTy);
329     }
330 
331     if (ILE->getType()->isArrayType())
332       return EmitArrayInitialization(ILE);
333 
334     if (ILE->getType()->isStructureType())
335       return EmitStructInitialization(ILE);
336 
337     if (ILE->getType()->isUnionType())
338       return EmitUnionInitialization(ILE);
339 
340     if (ILE->getType()->isVectorType())
341       return EmitVectorInitialization(ILE);
342 
343     assert(0 && "Unable to handle InitListExpr");
344     // Get rid of control reaches end of void function warning.
345     // Not reached.
346     return 0;
347   }
348 
349   llvm::Constant *VisitStringLiteral(StringLiteral *E) {
350     assert(!E->getType()->isPointerType() && "Strings are always arrays");
351 
352     // This must be a string initializing an array in a static initializer.
353     // Don't emit it as the address of the string, emit the string data itself
354     // as an inline array.
355     return llvm::ConstantArray::get(CGM.GetStringForStringLiteral(E), false);
356   }
357 
358   llvm::Constant *VisitObjCEncodeExpr(ObjCEncodeExpr *E) {
359     // This must be an @encode initializing an array in a static initializer.
360     // Don't emit it as the address of the string, emit the string data itself
361     // as an inline array.
362     std::string Str;
363     CGM.getContext().getObjCEncodingForType(E->getEncodedType(), Str);
364     const ConstantArrayType *CAT = cast<ConstantArrayType>(E->getType());
365 
366     // Resize the string to the right size, adding zeros at the end, or
367     // truncating as needed.
368     Str.resize(CAT->getSize().getZExtValue(), '\0');
369     return llvm::ConstantArray::get(Str, false);
370   }
371 
372   llvm::Constant *VisitUnaryExtension(const UnaryOperator *E) {
373     return Visit(E->getSubExpr());
374   }
375 
376   // Utility methods
377   const llvm::Type *ConvertType(QualType T) {
378     return CGM.getTypes().ConvertType(T);
379   }
380 
381 public:
382   llvm::Constant *EmitLValue(Expr *E) {
383     switch (E->getStmtClass()) {
384     default: break;
385     case Expr::CompoundLiteralExprClass: {
386       // Note that due to the nature of compound literals, this is guaranteed
387       // to be the only use of the variable, so we just generate it here.
388       CompoundLiteralExpr *CLE = cast<CompoundLiteralExpr>(E);
389       llvm::Constant* C = Visit(CLE->getInitializer());
390       // FIXME: "Leaked" on failure.
391       if (C)
392         C = new llvm::GlobalVariable(C->getType(),
393                                      E->getType().isConstQualified(),
394                                      llvm::GlobalValue::InternalLinkage,
395                                      C, ".compoundliteral", &CGM.getModule());
396       return C;
397     }
398     case Expr::DeclRefExprClass:
399     case Expr::QualifiedDeclRefExprClass: {
400       NamedDecl *Decl = cast<DeclRefExpr>(E)->getDecl();
401       if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(Decl))
402         return CGM.GetAddrOfFunction(FD);
403       if (const VarDecl* VD = dyn_cast<VarDecl>(Decl)) {
404         // We can never refer to a variable with local storage.
405         if (!VD->hasLocalStorage()) {
406           if (VD->isFileVarDecl() || VD->hasExternalStorage())
407             return CGM.GetAddrOfGlobalVar(VD);
408           else if (VD->isBlockVarDecl()) {
409             assert(CGF && "Can't access static local vars without CGF");
410             return CGF->GetAddrOfStaticLocalVar(VD);
411           }
412         }
413       }
414       break;
415     }
416     case Expr::StringLiteralClass:
417       return CGM.GetAddrOfConstantStringFromLiteral(cast<StringLiteral>(E));
418     case Expr::ObjCEncodeExprClass:
419       return CGM.GetAddrOfConstantStringFromObjCEncode(cast<ObjCEncodeExpr>(E));
420     case Expr::ObjCStringLiteralClass: {
421       ObjCStringLiteral* SL = cast<ObjCStringLiteral>(E);
422       std::string S(SL->getString()->getStrData(),
423                     SL->getString()->getByteLength());
424       llvm::Constant *C = CGM.getObjCRuntime().GenerateConstantString(S);
425       return llvm::ConstantExpr::getBitCast(C, ConvertType(E->getType()));
426     }
427     case Expr::PredefinedExprClass: {
428       // __func__/__FUNCTION__ -> "".  __PRETTY_FUNCTION__ -> "top level".
429       std::string Str;
430       if (cast<PredefinedExpr>(E)->getIdentType() ==
431           PredefinedExpr::PrettyFunction)
432         Str = "top level";
433 
434       return CGM.GetAddrOfConstantCString(Str, ".tmp");
435     }
436     case Expr::AddrLabelExprClass: {
437       assert(CGF && "Invalid address of label expression outside function.");
438       unsigned id = CGF->GetIDForAddrOfLabel(cast<AddrLabelExpr>(E)->getLabel());
439       llvm::Constant *C = llvm::ConstantInt::get(llvm::Type::Int32Ty, id);
440       return llvm::ConstantExpr::getIntToPtr(C, ConvertType(E->getType()));
441     }
442     case Expr::CallExprClass: {
443       CallExpr* CE = cast<CallExpr>(E);
444       if (CE->isBuiltinCall(CGM.getContext()) !=
445             Builtin::BI__builtin___CFStringMakeConstantString)
446         break;
447       const Expr *Arg = CE->getArg(0)->IgnoreParenCasts();
448       const StringLiteral *Literal = cast<StringLiteral>(Arg);
449       std::string S(Literal->getStrData(), Literal->getByteLength());
450       return CGM.GetAddrOfConstantCFString(S);
451     }
452     case Expr::BlockExprClass: {
453       std::string FunctionName;
454       if (CGF)
455         FunctionName = CGF->CurFn->getName();
456       else
457         FunctionName = "global";
458 
459       return CGM.GetAddrOfGlobalBlock(cast<BlockExpr>(E), FunctionName.c_str());
460     }
461     }
462 
463     return 0;
464   }
465 };
466 
467 }  // end anonymous namespace.
468 
469 llvm::Constant *CodeGenModule::EmitConstantExpr(const Expr *E,
470                                                 CodeGenFunction *CGF) {
471   Expr::EvalResult Result;
472 
473   if (E->Evaluate(Result, Context)) {
474     assert(!Result.HasSideEffects &&
475            "Constant expr should not have any side effects!");
476     switch (Result.Val.getKind()) {
477     case APValue::Uninitialized:
478       assert(0 && "Constant expressions should be initialized.");
479       return 0;
480     case APValue::LValue: {
481       const llvm::Type *DestType = getTypes().ConvertTypeForMem(E->getType());
482       llvm::Constant *Offset =
483         llvm::ConstantInt::get(llvm::Type::Int64Ty,
484                                Result.Val.getLValueOffset());
485 
486       llvm::Constant *C;
487       if (const Expr *LVBase = Result.Val.getLValueBase()) {
488         C = ConstExprEmitter(*this, CGF).EmitLValue(const_cast<Expr*>(LVBase));
489 
490         // Apply offset if necessary.
491         if (!Offset->isNullValue()) {
492           const llvm::Type *Type =
493             llvm::PointerType::getUnqual(llvm::Type::Int8Ty);
494           llvm::Constant *Casted = llvm::ConstantExpr::getBitCast(C, Type);
495           Casted = llvm::ConstantExpr::getGetElementPtr(Casted, &Offset, 1);
496           C = llvm::ConstantExpr::getBitCast(Casted, C->getType());
497         }
498 
499         // Convert to the appropriate type; this could be an lvalue for
500         // an integer.
501         if (isa<llvm::PointerType>(DestType))
502           return llvm::ConstantExpr::getBitCast(C, DestType);
503 
504         return llvm::ConstantExpr::getPtrToInt(C, DestType);
505       } else {
506         C = Offset;
507 
508         // Convert to the appropriate type; this could be an lvalue for
509         // an integer.
510         if (isa<llvm::PointerType>(DestType))
511           return llvm::ConstantExpr::getIntToPtr(C, DestType);
512 
513         // If the types don't match this should only be a truncate.
514         if (C->getType() != DestType)
515           return llvm::ConstantExpr::getTrunc(C, DestType);
516 
517         return C;
518       }
519     }
520     case APValue::Int: {
521       llvm::Constant *C = llvm::ConstantInt::get(Result.Val.getInt());
522 
523       if (C->getType() == llvm::Type::Int1Ty) {
524         const llvm::Type *BoolTy = getTypes().ConvertTypeForMem(E->getType());
525         C = llvm::ConstantExpr::getZExt(C, BoolTy);
526       }
527       return C;
528     }
529     case APValue::ComplexInt: {
530       llvm::Constant *Complex[2];
531 
532       Complex[0] = llvm::ConstantInt::get(Result.Val.getComplexIntReal());
533       Complex[1] = llvm::ConstantInt::get(Result.Val.getComplexIntImag());
534 
535       return llvm::ConstantStruct::get(Complex, 2);
536     }
537     case APValue::Float:
538       return llvm::ConstantFP::get(Result.Val.getFloat());
539     case APValue::ComplexFloat: {
540       llvm::Constant *Complex[2];
541 
542       Complex[0] = llvm::ConstantFP::get(Result.Val.getComplexFloatReal());
543       Complex[1] = llvm::ConstantFP::get(Result.Val.getComplexFloatImag());
544 
545       return llvm::ConstantStruct::get(Complex, 2);
546     }
547     case APValue::Vector: {
548       llvm::SmallVector<llvm::Constant *, 4> Inits;
549       unsigned NumElts = Result.Val.getVectorLength();
550 
551       for (unsigned i = 0; i != NumElts; ++i) {
552         APValue &Elt = Result.Val.getVectorElt(i);
553         if (Elt.isInt())
554           Inits.push_back(llvm::ConstantInt::get(Elt.getInt()));
555         else
556           Inits.push_back(llvm::ConstantFP::get(Elt.getFloat()));
557       }
558       return llvm::ConstantVector::get(&Inits[0], Inits.size());
559     }
560     }
561   }
562 
563   llvm::Constant* C = ConstExprEmitter(*this, CGF).Visit(const_cast<Expr*>(E));
564   if (C && C->getType() == llvm::Type::Int1Ty) {
565     const llvm::Type *BoolTy = getTypes().ConvertTypeForMem(E->getType());
566     C = llvm::ConstantExpr::getZExt(C, BoolTy);
567   }
568   return C;
569 }
570