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