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