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 "clang/AST/AST.h"
17 #include "llvm/Constants.h"
18 #include "llvm/Function.h"
19 #include "llvm/GlobalVariable.h"
20 #include "llvm/Support/Compiler.h"
21 using namespace clang;
22 using namespace CodeGen;
23 
24 namespace  {
25 class VISIBILITY_HIDDEN ConstExprEmitter :
26   public StmtVisitor<ConstExprEmitter, llvm::Constant*> {
27   CodeGenModule &CGM;
28   CodeGenFunction *CGF;
29 public:
30   ConstExprEmitter(CodeGenModule &cgm, CodeGenFunction *cgf)
31     : CGM(cgm), CGF(cgf) {
32   }
33 
34   //===--------------------------------------------------------------------===//
35   //                            Visitor Methods
36   //===--------------------------------------------------------------------===//
37 
38   llvm::Constant *VisitStmt(Stmt *S) {
39     CGM.WarnUnsupported(S, "constant expression");
40     QualType T = cast<Expr>(S)->getType();
41     return llvm::UndefValue::get(CGM.getTypes().ConvertType(T));
42   }
43 
44   llvm::Constant *VisitParenExpr(ParenExpr *PE) {
45     return Visit(PE->getSubExpr());
46   }
47 
48   // Leaves
49   llvm::Constant *VisitIntegerLiteral(const IntegerLiteral *E) {
50     return llvm::ConstantInt::get(E->getValue());
51   }
52   llvm::Constant *VisitFloatingLiteral(const FloatingLiteral *E) {
53     return llvm::ConstantFP::get(ConvertType(E->getType()), E->getValue());
54   }
55   llvm::Constant *VisitCharacterLiteral(const CharacterLiteral *E) {
56     return llvm::ConstantInt::get(ConvertType(E->getType()), E->getValue());
57   }
58   llvm::Constant *VisitCXXBoolLiteralExpr(const CXXBoolLiteralExpr *E) {
59     return llvm::ConstantInt::get(ConvertType(E->getType()), E->getValue());
60   }
61 
62   llvm::Constant *VisitCompoundLiteralExpr(CompoundLiteralExpr *E) {
63     return Visit(E->getInitializer());
64   }
65 
66   llvm::Constant *VisitCastExpr(const CastExpr* E) {
67     llvm::Constant *C = Visit(E->getSubExpr());
68 
69     return EmitConversion(C, E->getSubExpr()->getType(), E->getType());
70   }
71 
72   llvm::Constant *EmitArrayInitialization(InitListExpr *ILE,
73                                           const llvm::ArrayType *AType) {
74     std::vector<llvm::Constant*> Elts;
75     unsigned NumInitElements = ILE->getNumInits();
76     // FIXME: Check for wide strings
77     if (NumInitElements > 0 && isa<StringLiteral>(ILE->getInit(0)) &&
78         ILE->getType()->getAsArrayType()->getElementType()->isCharType())
79       return Visit(ILE->getInit(0));
80     const llvm::Type *ElemTy = AType->getElementType();
81     unsigned NumElements = AType->getNumElements();
82 
83     // Initialising an array requires us to automatically
84     // initialise any elements that have not been initialised explicitly
85     unsigned NumInitableElts = std::min(NumInitElements, NumElements);
86 
87     // Copy initializer elements.
88     unsigned i = 0;
89     for (; i < NumInitableElts; ++i) {
90 
91       llvm::Constant *C = Visit(ILE->getInit(i));
92       // FIXME: Remove this when sema of initializers is finished (and the code
93       // above).
94       if (C == 0 && ILE->getInit(i)->getType()->isVoidType()) {
95         if (ILE->getType()->isVoidType()) return 0;
96         return llvm::UndefValue::get(AType);
97       }
98       assert (C && "Failed to create initializer expression");
99       Elts.push_back(C);
100     }
101 
102     // Initialize remaining array elements.
103     for (; i < NumElements; ++i)
104       Elts.push_back(llvm::Constant::getNullValue(ElemTy));
105 
106     return llvm::ConstantArray::get(AType, Elts);
107   }
108 
109   llvm::Constant *EmitStructInitialization(InitListExpr *ILE,
110                                            const llvm::StructType *SType) {
111 
112     TagDecl *TD = ILE->getType()->getAsRecordType()->getDecl();
113     std::vector<llvm::Constant*> Elts;
114     const CGRecordLayout *CGR = CGM.getTypes().getCGRecordLayout(TD);
115     unsigned NumInitElements = ILE->getNumInits();
116     unsigned NumElements = SType->getNumElements();
117 
118     // Initialising an structure requires us to automatically
119     // initialise any elements that have not been initialised explicitly
120     unsigned NumInitableElts = std::min(NumInitElements, NumElements);
121 
122     // Copy initializer elements. Skip padding fields.
123     unsigned EltNo = 0;  // Element no in ILE
124     unsigned FieldNo = 0; // Field no in  SType
125     while (EltNo < NumInitableElts) {
126 
127       // Zero initialize padding field.
128       if (CGR->isPaddingField(FieldNo)) {
129         const llvm::Type *FieldTy = SType->getElementType(FieldNo);
130         Elts.push_back(llvm::Constant::getNullValue(FieldTy));
131         FieldNo++;
132         continue;
133       }
134 
135       llvm::Constant *C = Visit(ILE->getInit(EltNo));
136       // FIXME: Remove this when sema of initializers is finished (and the code
137       // above).
138       if (C == 0 && ILE->getInit(EltNo)->getType()->isVoidType()) {
139         if (ILE->getType()->isVoidType()) return 0;
140         return llvm::UndefValue::get(SType);
141       }
142       assert (C && "Failed to create initializer expression");
143       Elts.push_back(C);
144       EltNo++;
145       FieldNo++;
146     }
147 
148     // Initialize remaining structure elements.
149     for (unsigned i = Elts.size(); i < NumElements; ++i) {
150       const llvm::Type *FieldTy = SType->getElementType(i);
151       Elts.push_back(llvm::Constant::getNullValue(FieldTy));
152     }
153 
154     return llvm::ConstantStruct::get(SType, Elts);
155   }
156 
157   llvm::Constant *EmitVectorInitialization(InitListExpr *ILE,
158                                            const llvm::VectorType *VType) {
159 
160     std::vector<llvm::Constant*> Elts;
161     unsigned NumInitElements = ILE->getNumInits();
162     unsigned NumElements = VType->getNumElements();
163 
164     assert (NumInitElements == NumElements
165             && "Unsufficient vector init elelments");
166     // Copy initializer elements.
167     unsigned i = 0;
168     for (; i < NumElements; ++i) {
169 
170       llvm::Constant *C = Visit(ILE->getInit(i));
171       // FIXME: Remove this when sema of initializers is finished (and the code
172       // above).
173       if (C == 0 && ILE->getInit(i)->getType()->isVoidType()) {
174         if (ILE->getType()->isVoidType()) return 0;
175         return llvm::UndefValue::get(VType);
176       }
177       assert (C && "Failed to create initializer expression");
178       Elts.push_back(C);
179     }
180 
181     return llvm::ConstantVector::get(VType, Elts);
182   }
183 
184   llvm::Constant *VisitInitListExpr(InitListExpr *ILE) {
185     const llvm::CompositeType *CType =
186       dyn_cast<llvm::CompositeType>(ConvertType(ILE->getType()));
187 
188     if (!CType) {
189         // We have a scalar in braces. Just use the first element.
190         return Visit(ILE->getInit(0));
191     }
192 
193     if (const llvm::ArrayType *AType = dyn_cast<llvm::ArrayType>(CType))
194       return EmitArrayInitialization(ILE, AType);
195 
196     if (const llvm::StructType *SType = dyn_cast<llvm::StructType>(CType))
197       return EmitStructInitialization(ILE, SType);
198 
199     if (const llvm::VectorType *VType = dyn_cast<llvm::VectorType>(CType))
200       return EmitVectorInitialization(ILE, VType);
201 
202     // Make sure we have an array at this point
203     assert(0 && "Unable to handle InitListExpr");
204     // Get rid of control reaches end of void function warning.
205     // Not reached.
206     return 0;
207   }
208 
209   llvm::Constant *VisitImplicitCastExpr(ImplicitCastExpr *ICExpr) {
210     Expr* SExpr = ICExpr->getSubExpr();
211     QualType SType = SExpr->getType();
212     llvm::Constant *C; // the intermediate expression
213     QualType T;        // the type of the intermediate expression
214     if (SType->isArrayType()) {
215       // Arrays decay to a pointer to the first element
216       // VLAs would require special handling, but they can't occur here
217       C = EmitLValue(SExpr);
218       llvm::Constant *Idx0 = llvm::ConstantInt::get(llvm::Type::Int32Ty, 0);
219       llvm::Constant *Ops[] = {Idx0, Idx0};
220       C = llvm::ConstantExpr::getGetElementPtr(C, Ops, 2);
221 
222       QualType ElemType = SType->getAsArrayType()->getElementType();
223       T = CGM.getContext().getPointerType(ElemType);
224     } else if (SType->isFunctionType()) {
225       // Function types decay to a pointer to the function
226       C = EmitLValue(SExpr);
227       T = CGM.getContext().getPointerType(SType);
228     } else {
229       C = Visit(SExpr);
230       T = SType;
231     }
232 
233     // Perform the conversion; note that an implicit cast can both promote
234     // and convert an array/function
235     return EmitConversion(C, T, ICExpr->getType());
236   }
237 
238   llvm::Constant *VisitStringLiteral(StringLiteral *E) {
239     const char *StrData = E->getStrData();
240     unsigned Len = E->getByteLength();
241     assert(!E->getType()->isPointerType() && "Strings are always arrays");
242 
243     // Otherwise this must be a string initializing an array in a static
244     // initializer.  Don't emit it as the address of the string, emit the string
245     // data itself as an inline array.
246     const ConstantArrayType *CAT = E->getType()->getAsConstantArrayType();
247     assert(CAT && "String isn't pointer or array!");
248 
249     std::string Str(StrData, StrData + Len);
250     // Null terminate the string before potentially truncating it.
251     // FIXME: What about wchar_t strings?
252     Str.push_back(0);
253 
254     uint64_t RealLen = CAT->getSize().getZExtValue();
255     // String or grow the initializer to the required size.
256     if (RealLen != Str.size())
257       Str.resize(RealLen);
258 
259     return llvm::ConstantArray::get(Str, false);
260   }
261 
262   llvm::Constant *VisitDeclRefExpr(DeclRefExpr *E) {
263     const ValueDecl *Decl = E->getDecl();
264     if (const EnumConstantDecl *EC = dyn_cast<EnumConstantDecl>(Decl))
265       return llvm::ConstantInt::get(EC->getInitVal());
266     assert(0 && "Unsupported decl ref type!");
267     return 0;
268   }
269 
270   llvm::Constant *VisitSizeOfAlignOfTypeExpr(const SizeOfAlignOfTypeExpr *E) {
271     return EmitSizeAlignOf(E->getArgumentType(), E->getType(), E->isSizeOf());
272   }
273 
274   // Unary operators
275   llvm::Constant *VisitUnaryPlus(const UnaryOperator *E) {
276     return Visit(E->getSubExpr());
277   }
278   llvm::Constant *VisitUnaryMinus(const UnaryOperator *E) {
279     return llvm::ConstantExpr::getNeg(Visit(E->getSubExpr()));
280   }
281   llvm::Constant *VisitUnaryNot(const UnaryOperator *E) {
282     return llvm::ConstantExpr::getNot(Visit(E->getSubExpr()));
283   }
284   llvm::Constant *VisitUnaryLNot(const UnaryOperator *E) {
285     llvm::Constant *SubExpr = Visit(E->getSubExpr());
286 
287     if (E->getSubExpr()->getType()->isRealFloatingType()) {
288       // Compare against 0.0 for fp scalars.
289       llvm::Constant *Zero = llvm::Constant::getNullValue(SubExpr->getType());
290       SubExpr = llvm::ConstantExpr::getFCmp(llvm::FCmpInst::FCMP_UEQ, SubExpr,
291                                             Zero);
292     } else {
293       assert((E->getSubExpr()->getType()->isIntegerType() ||
294               E->getSubExpr()->getType()->isPointerType()) &&
295              "Unknown scalar type to convert");
296       // Compare against an integer or pointer null.
297       llvm::Constant *Zero = llvm::Constant::getNullValue(SubExpr->getType());
298       SubExpr = llvm::ConstantExpr::getICmp(llvm::ICmpInst::ICMP_EQ, SubExpr,
299                                             Zero);
300     }
301 
302     return llvm::ConstantExpr::getZExt(SubExpr, ConvertType(E->getType()));
303   }
304   llvm::Constant *VisitUnarySizeOf(const UnaryOperator *E) {
305     return EmitSizeAlignOf(E->getSubExpr()->getType(), E->getType(), true);
306   }
307   llvm::Constant *VisitUnaryAlignOf(const UnaryOperator *E) {
308     return EmitSizeAlignOf(E->getSubExpr()->getType(), E->getType(), false);
309   }
310   llvm::Constant *VisitUnaryAddrOf(const UnaryOperator *E) {
311     return EmitLValue(E->getSubExpr());
312   }
313   llvm::Constant *VisitUnaryOffsetOf(const UnaryOperator *E) {
314     int64_t Val = E->evaluateOffsetOf(CGM.getContext());
315 
316     assert(E->getType()->isIntegerType() && "Result type must be an integer!");
317 
318     uint32_t ResultWidth =
319       static_cast<uint32_t>(CGM.getContext().getTypeSize(E->getType()));
320     return llvm::ConstantInt::get(llvm::APInt(ResultWidth, Val));
321   }
322 
323   // Binary operators
324   llvm::Constant *VisitBinOr(const BinaryOperator *E) {
325     llvm::Constant *LHS = Visit(E->getLHS());
326     llvm::Constant *RHS = Visit(E->getRHS());
327 
328     return llvm::ConstantExpr::getOr(LHS, RHS);
329   }
330   llvm::Constant *VisitBinSub(const BinaryOperator *E) {
331     llvm::Constant *LHS = Visit(E->getLHS());
332     llvm::Constant *RHS = Visit(E->getRHS());
333 
334     if (!isa<llvm::PointerType>(RHS->getType())) {
335       // pointer - int
336       if (isa<llvm::PointerType>(LHS->getType())) {
337         llvm::Constant *Idx = llvm::ConstantExpr::getNeg(RHS);
338 
339         return llvm::ConstantExpr::getGetElementPtr(LHS, &Idx, 1);
340       }
341 
342       // int - int
343       return llvm::ConstantExpr::getSub(LHS, RHS);
344     }
345 
346     assert(0 && "Unhandled bin sub case!");
347     return 0;
348   }
349 
350   llvm::Constant *VisitBinShl(const BinaryOperator *E) {
351     llvm::Constant *LHS = Visit(E->getLHS());
352     llvm::Constant *RHS = Visit(E->getRHS());
353 
354     // LLVM requires the LHS and RHS to be the same type: promote or truncate the
355     // RHS to the same size as the LHS.
356     if (LHS->getType() != RHS->getType())
357       RHS = llvm::ConstantExpr::getIntegerCast(RHS, LHS->getType(), false);
358 
359     return llvm::ConstantExpr::getShl(LHS, RHS);
360   }
361 
362   llvm::Constant *VisitBinMul(const BinaryOperator *E) {
363     llvm::Constant *LHS = Visit(E->getLHS());
364     llvm::Constant *RHS = Visit(E->getRHS());
365 
366     return llvm::ConstantExpr::getMul(LHS, RHS);
367   }
368 
369   llvm::Constant *VisitBinDiv(const BinaryOperator *E) {
370     llvm::Constant *LHS = Visit(E->getLHS());
371     llvm::Constant *RHS = Visit(E->getRHS());
372 
373     if (LHS->getType()->isFPOrFPVector())
374       return llvm::ConstantExpr::getFDiv(LHS, RHS);
375     else if (E->getType()->isUnsignedIntegerType())
376       return llvm::ConstantExpr::getUDiv(LHS, RHS);
377     else
378       return llvm::ConstantExpr::getSDiv(LHS, RHS);
379   }
380 
381   llvm::Constant *VisitBinAdd(const BinaryOperator *E) {
382     llvm::Constant *LHS = Visit(E->getLHS());
383     llvm::Constant *RHS = Visit(E->getRHS());
384 
385     if (!E->getType()->isPointerType())
386       return llvm::ConstantExpr::getAdd(LHS, RHS);
387 
388     llvm::Constant *Ptr, *Idx;
389     if (isa<llvm::PointerType>(LHS->getType())) { // pointer + int
390       Ptr = LHS;
391       Idx = RHS;
392     } else { // int + pointer
393       Ptr = RHS;
394       Idx = LHS;
395     }
396 
397     return llvm::ConstantExpr::getGetElementPtr(Ptr, &Idx, 1);
398   }
399 
400   llvm::Constant *VisitBinAnd(const BinaryOperator *E) {
401     llvm::Constant *LHS = Visit(E->getLHS());
402     llvm::Constant *RHS = Visit(E->getRHS());
403 
404     return llvm::ConstantExpr::getAnd(LHS, RHS);
405   }
406 
407   // Utility methods
408   const llvm::Type *ConvertType(QualType T) {
409     return CGM.getTypes().ConvertType(T);
410   }
411 
412   llvm::Constant *EmitConversionToBool(llvm::Constant *Src, QualType SrcType) {
413     assert(SrcType->isCanonical() && "EmitConversion strips typedefs");
414 
415     if (SrcType->isRealFloatingType()) {
416       // Compare against 0.0 for fp scalars.
417       llvm::Constant *Zero = llvm::Constant::getNullValue(Src->getType());
418       return llvm::ConstantExpr::getFCmp(llvm::FCmpInst::FCMP_UNE, Src, Zero);
419     }
420 
421     assert((SrcType->isIntegerType() || SrcType->isPointerType()) &&
422            "Unknown scalar type to convert");
423 
424     // Compare against an integer or pointer null.
425     llvm::Constant *Zero = llvm::Constant::getNullValue(Src->getType());
426     return llvm::ConstantExpr::getICmp(llvm::ICmpInst::ICMP_NE, Src, Zero);
427   }
428 
429   llvm::Constant *EmitConversion(llvm::Constant *Src, QualType SrcType,
430                                  QualType DstType) {
431     SrcType = SrcType.getCanonicalType();
432     DstType = DstType.getCanonicalType();
433     if (SrcType == DstType) return Src;
434 
435     // Handle conversions to bool first, they are special: comparisons against 0.
436     if (DstType->isBooleanType())
437       return EmitConversionToBool(Src, SrcType);
438 
439     const llvm::Type *DstTy = ConvertType(DstType);
440 
441     // Ignore conversions like int -> uint.
442     if (Src->getType() == DstTy)
443       return Src;
444 
445     // Handle pointer conversions next: pointers can only be converted to/from
446     // other pointers and integers.
447     if (isa<PointerType>(DstType)) {
448       // The source value may be an integer, or a pointer.
449       if (isa<llvm::PointerType>(Src->getType()))
450         return llvm::ConstantExpr::getBitCast(Src, DstTy);
451       assert(SrcType->isIntegerType() &&"Not ptr->ptr or int->ptr conversion?");
452       return llvm::ConstantExpr::getIntToPtr(Src, DstTy);
453     }
454 
455     if (isa<PointerType>(SrcType)) {
456       // Must be an ptr to int cast.
457       assert(isa<llvm::IntegerType>(DstTy) && "not ptr->int?");
458       return llvm::ConstantExpr::getPtrToInt(Src, DstTy);
459     }
460 
461     // A scalar source can be splatted to a vector of the same element type
462     if (isa<llvm::VectorType>(DstTy) && !isa<VectorType>(SrcType)) {
463       const llvm::VectorType *VT = cast<llvm::VectorType>(DstTy);
464       assert((VT->getElementType() == Src->getType()) &&
465              "Vector element type must match scalar type to splat.");
466       unsigned NumElements = DstType->getAsVectorType()->getNumElements();
467       llvm::SmallVector<llvm::Constant*, 16> Elements;
468       for (unsigned i = 0; i < NumElements; i++)
469         Elements.push_back(Src);
470 
471       return llvm::ConstantVector::get(&Elements[0], NumElements);
472     }
473 
474     if (isa<llvm::VectorType>(Src->getType()) ||
475         isa<llvm::VectorType>(DstTy)) {
476       return llvm::ConstantExpr::getBitCast(Src, DstTy);
477     }
478 
479     // Finally, we have the arithmetic types: real int/float.
480     if (isa<llvm::IntegerType>(Src->getType())) {
481       bool InputSigned = SrcType->isSignedIntegerType();
482       if (isa<llvm::IntegerType>(DstTy))
483         return llvm::ConstantExpr::getIntegerCast(Src, DstTy, InputSigned);
484       else if (InputSigned)
485         return llvm::ConstantExpr::getSIToFP(Src, DstTy);
486       else
487         return llvm::ConstantExpr::getUIToFP(Src, DstTy);
488     }
489 
490     assert(Src->getType()->isFloatingPoint() && "Unknown real conversion");
491     if (isa<llvm::IntegerType>(DstTy)) {
492       if (DstType->isSignedIntegerType())
493         return llvm::ConstantExpr::getFPToSI(Src, DstTy);
494       else
495         return llvm::ConstantExpr::getFPToUI(Src, DstTy);
496     }
497 
498     assert(DstTy->isFloatingPoint() && "Unknown real conversion");
499     if (DstTy->getTypeID() < Src->getType()->getTypeID())
500       return llvm::ConstantExpr::getFPTrunc(Src, DstTy);
501     else
502       return llvm::ConstantExpr::getFPExtend(Src, DstTy);
503   }
504 
505   llvm::Constant *EmitSizeAlignOf(QualType TypeToSize,
506                                   QualType RetType, bool isSizeOf) {
507     std::pair<uint64_t, unsigned> Info =
508       CGM.getContext().getTypeInfo(TypeToSize);
509 
510     uint64_t Val = isSizeOf ? Info.first : Info.second;
511     Val /= 8;  // Return size in bytes, not bits.
512 
513     assert(RetType->isIntegerType() && "Result type must be an integer!");
514 
515     uint32_t ResultWidth =
516       static_cast<uint32_t>(CGM.getContext().getTypeSize(RetType));
517     return llvm::ConstantInt::get(llvm::APInt(ResultWidth, Val));
518   }
519 
520   llvm::Constant *EmitLValue(Expr *E) {
521     switch (E->getStmtClass()) {
522     default: break;
523     case Expr::ParenExprClass:
524       // Elide parenthesis
525       return EmitLValue(cast<ParenExpr>(E)->getSubExpr());
526     case Expr::CompoundLiteralExprClass: {
527       // Note that due to the nature of compound literals, this is guaranteed
528       // to be the only use of the variable, so we just generate it here.
529       CompoundLiteralExpr *CLE = cast<CompoundLiteralExpr>(E);
530       llvm::Constant* C = Visit(CLE->getInitializer());
531       C = new llvm::GlobalVariable(C->getType(),E->getType().isConstQualified(),
532                                    llvm::GlobalValue::InternalLinkage,
533                                    C, ".compoundliteral", &CGM.getModule());
534       return C;
535     }
536     case Expr::DeclRefExprClass: {
537       ValueDecl *Decl = cast<DeclRefExpr>(E)->getDecl();
538       if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(Decl))
539         return CGM.GetAddrOfFunctionDecl(FD, false);
540       if (const FileVarDecl* VD = dyn_cast<FileVarDecl>(Decl))
541         return CGM.GetAddrOfGlobalVar(VD, false);
542       if (const BlockVarDecl* BVD = dyn_cast<BlockVarDecl>(Decl)) {
543         assert(CGF && "Can't access static local vars without CGF");
544         return CGF->GetAddrOfStaticLocalVar(BVD);
545       }
546       break;
547     }
548     case Expr::MemberExprClass: {
549       MemberExpr* ME = cast<MemberExpr>(E);
550       llvm::Constant *Base;
551       if (ME->isArrow())
552         Base = Visit(ME->getBase());
553       else
554         Base = EmitLValue(ME->getBase());
555 
556       unsigned FieldNumber = CGM.getTypes().getLLVMFieldNo(ME->getMemberDecl());
557       llvm::Constant *Zero = llvm::ConstantInt::get(llvm::Type::Int32Ty, 0);
558       llvm::Constant *Idx = llvm::ConstantInt::get(llvm::Type::Int32Ty,
559                                                    FieldNumber);
560       llvm::Value *Ops[] = {Zero, Idx};
561       return llvm::ConstantExpr::getGetElementPtr(Base, Ops, 2);
562     }
563     case Expr::ArraySubscriptExprClass: {
564       ArraySubscriptExpr* ASExpr = cast<ArraySubscriptExpr>(E);
565       llvm::Constant *Base = Visit(ASExpr->getBase());
566       llvm::Constant *Index = Visit(ASExpr->getIdx());
567       assert(!ASExpr->getBase()->getType()->isVectorType() &&
568              "Taking the address of a vector component is illegal!");
569       return llvm::ConstantExpr::getGetElementPtr(Base, &Index, 1);
570     }
571     case Expr::StringLiteralClass: {
572       StringLiteral *String = cast<StringLiteral>(E);
573       assert(!String->isWide() && "Cannot codegen wide strings yet");
574       const char *StrData = String->getStrData();
575       unsigned Len = String->getByteLength();
576 
577       return CGM.GetAddrOfConstantString(std::string(StrData, StrData + Len));
578     }
579     case Expr::UnaryOperatorClass: {
580       UnaryOperator *Exp = cast<UnaryOperator>(E);
581       switch (Exp->getOpcode()) {
582       default: break;
583       case UnaryOperator::Extension:
584         // Extension is just a wrapper for expressions
585         return EmitLValue(Exp->getSubExpr());
586       case UnaryOperator::Real:
587       case UnaryOperator::Imag: {
588         // The address of __real or __imag is just a GEP off the address
589         // of the internal expression
590         llvm::Constant* C = EmitLValue(Exp->getSubExpr());
591         llvm::Constant *Zero = llvm::ConstantInt::get(llvm::Type::Int32Ty, 0);
592         llvm::Constant *Idx  = llvm::ConstantInt::get(llvm::Type::Int32Ty,
593                                        Exp->getOpcode() == UnaryOperator::Imag);
594         llvm::Value *Ops[] = {Zero, Idx};
595         return llvm::ConstantExpr::getGetElementPtr(C, Ops, 2);
596       }
597       case UnaryOperator::Deref:
598         // The address of a deref is just the value of the expression
599         return Visit(Exp->getSubExpr());
600       }
601       break;
602     }
603     }
604     CGM.WarnUnsupported(E, "constant l-value expression");
605     llvm::Type *Ty = llvm::PointerType::getUnqual(ConvertType(E->getType()));
606     return llvm::UndefValue::get(Ty);
607   }
608 
609 };
610 
611 }  // end anonymous namespace.
612 
613 
614 llvm::Constant *CodeGenModule::EmitConstantExpr(const Expr *E,
615                                                 CodeGenFunction *CGF)
616 {
617   QualType type = E->getType().getCanonicalType();
618 
619   if (type->isIntegerType()) {
620     llvm::APSInt Value(static_cast<uint32_t>(Context.getTypeSize(type)));
621     if (E->isIntegerConstantExpr(Value, Context)) {
622       return llvm::ConstantInt::get(Value);
623     }
624   }
625 
626   return ConstExprEmitter(*this, CGF).Visit(const_cast<Expr*>(E));
627 }
628