Implementing C99 partial re-initialization behavior (DR-253)

Based on previous discussion on the mailing list, clang currently lacks support
for C99 partial re-initialization behavior:
Reference: http://lists.cs.uiuc.edu/pipermail/cfe-dev/2013-April/029188.html
Reference: http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_253.htm

This patch attempts to fix this problem.

Given the following code snippet,

struct P1 { char x[6]; };
struct LP1 { struct P1 p1; };

struct LP1 l = { .p1 = { "foo" }, .p1.x[2] = 'x' };
// this example is adapted from the example for "struct fred x[]" in DR-253;
// currently clang produces in l: { "\0\0x" },
//   whereas gcc 4.8 produces { "fox" };
// with this fix, clang will also produce: { "fox" };


Differential Review: http://reviews.llvm.org/D5789

llvm-svn: 239446

1 files changed, 156 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index b90b3ab61d8..acfb9b6dbb2 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -33,6 +33,7 @@ using namespace CodeGen;
 //===----------------------------------------------------------------------===//
 
 namespace {
+class ConstExprEmitter;
 class ConstStructBuilder {
   CodeGenModule &CGM;
   CodeGenFunction *CGF;
@@ -42,6 +43,10 @@ class ConstStructBuilder {
   CharUnits LLVMStructAlignment;
   SmallVector<llvm::Constant *, 32> Elements;
 public:
+  static llvm::Constant *BuildStruct(CodeGenModule &CGM, CodeGenFunction *CFG,
+                                     ConstExprEmitter *Emitter,
+                                     llvm::ConstantStruct *Base,
+                                     InitListExpr *Updater);
   static llvm::Constant *BuildStruct(CodeGenModule &CGM, CodeGenFunction *CGF,
                                      InitListExpr *ILE);
   static llvm::Constant *BuildStruct(CodeGenModule &CGM, CodeGenFunction *CGF,
@@ -68,6 +73,8 @@ private:
   void ConvertStructToPacked();
 
   bool Build(InitListExpr *ILE);
+  bool Build(ConstExprEmitter *Emitter, llvm::ConstantStruct *Base,
+             InitListExpr *Updater);
   void Build(const APValue &Val, const RecordDecl *RD, bool IsPrimaryBase,
              const CXXRecordDecl *VTableClass, CharUnits BaseOffset);
   llvm::Constant *Finalize(QualType Ty);
@@ -547,6 +554,17 @@ llvm::Constant *ConstStructBuilder::Finalize(QualType Ty) {
 
 llvm::Constant *ConstStructBuilder::BuildStruct(CodeGenModule &CGM,
                                                 CodeGenFunction *CGF,
+                                                ConstExprEmitter *Emitter,
+                                                llvm::ConstantStruct *Base,
+                                                InitListExpr *Updater) {
+  ConstStructBuilder Builder(CGM, CGF);
+  if (!Builder.Build(Emitter, Base, Updater))
+    return nullptr;
+  return Builder.Finalize(Updater->getType());
+}
+
+llvm::Constant *ConstStructBuilder::BuildStruct(CodeGenModule &CGM,
+                                                CodeGenFunction *CGF,
                                                 InitListExpr *ILE) {
   ConstStructBuilder Builder(CGM, CGF);
 
@@ -818,6 +836,82 @@ public:
     return nullptr;
   }
 
+  llvm::Constant *EmitDesignatedInitUpdater(llvm::Constant *Base,
+                                            InitListExpr *Updater) {
+    QualType ExprType = Updater->getType();
+
+    if (ExprType->isArrayType()) {
+      llvm::ArrayType *AType = cast<llvm::ArrayType>(ConvertType(ExprType));
+      llvm::Type *ElemType = AType->getElementType();
+
+      unsigned NumInitElements = Updater->getNumInits();
+      unsigned NumElements = AType->getNumElements();
+      
+      std::vector<llvm::Constant *> Elts;
+      Elts.reserve(NumElements);
+
+      if (llvm::ConstantDataArray *DataArray =
+            dyn_cast<llvm::ConstantDataArray>(Base))
+        for (unsigned i = 0; i != NumElements; ++i)
+          Elts.push_back(DataArray->getElementAsConstant(i));
+      else if (llvm::ConstantArray *Array =
+                 dyn_cast<llvm::ConstantArray>(Base))
+        for (unsigned i = 0; i != NumElements; ++i)
+          Elts.push_back(Array->getOperand(i));
+      else
+        return nullptr; // FIXME: other array types not implemented
+
+      llvm::Constant *fillC = nullptr;
+      if (Expr *filler = Updater->getArrayFiller())
+        if (!isa<NoInitExpr>(filler))
+          fillC = CGM.EmitConstantExpr(filler, filler->getType(), CGF);
+      bool RewriteType = (fillC && fillC->getType() != ElemType);
+
+      for (unsigned i = 0; i != NumElements; ++i) {
+        Expr *Init = nullptr;
+        if (i < NumInitElements)
+          Init = Updater->getInit(i);
+
+        if (!Init && fillC)
+          Elts[i] = fillC;
+        else if (!Init || isa<NoInitExpr>(Init))
+          ; // Do nothing.
+        else if (InitListExpr *ChildILE = dyn_cast<InitListExpr>(Init))
+          Elts[i] = EmitDesignatedInitUpdater(Elts[i], ChildILE);
+        else
+          Elts[i] = CGM.EmitConstantExpr(Init, Init->getType(), CGF);
+ 
+       if (!Elts[i])
+          return nullptr;
+        RewriteType |= (Elts[i]->getType() != ElemType);
+      }
+
+      if (RewriteType) {
+        std::vector<llvm::Type *> Types;
+        Types.reserve(NumElements);
+        for (unsigned i = 0; i != NumElements; ++i)
+          Types.push_back(Elts[i]->getType());
+        llvm::StructType *SType = llvm::StructType::get(AType->getContext(),
+                                                        Types, true);
+        return llvm::ConstantStruct::get(SType, Elts);
+      }
+
+      return llvm::ConstantArray::get(AType, Elts);
+    }
+
+    if (ExprType->isRecordType())
+      return ConstStructBuilder::BuildStruct(CGM, CGF, this,
+                 dyn_cast<llvm::ConstantStruct>(Base), Updater);
+
+    return nullptr;
+  }
+
+  llvm::Constant *VisitDesignatedInitUpdateExpr(DesignatedInitUpdateExpr *E) {
+    return EmitDesignatedInitUpdater(
+               CGM.EmitConstantExpr(E->getBase(), E->getType(), CGF),
+               E->getUpdater());
+  }  
+
   llvm::Constant *VisitCXXConstructExpr(CXXConstructExpr *E) {
     if (!E->getConstructor()->isTrivial())
       return nullptr;
@@ -1003,6 +1097,68 @@ public:
 
 }  // end anonymous namespace.
 
+bool ConstStructBuilder::Build(ConstExprEmitter *Emitter,
+                               llvm::ConstantStruct *Base,
+                               InitListExpr *Updater) {
+  assert(Base && "base expression should not be empty");
+
+  QualType ExprType = Updater->getType();
+  RecordDecl *RD = ExprType->getAs<RecordType>()->getDecl();
+  const ASTRecordLayout &Layout = CGM.getContext().getASTRecordLayout(RD);
+  const llvm::StructLayout *BaseLayout = CGM.getDataLayout().getStructLayout(
+                                           Base->getType());
+  unsigned FieldNo = -1;
+  unsigned ElementNo = 0;
+
+  for (FieldDecl *Field : RD->fields()) {
+    ++FieldNo;
+
+    if (RD->isUnion() && Updater->getInitializedFieldInUnion() != Field)
+      continue;
+
+    // Skip anonymous bitfields.
+    if (Field->isUnnamedBitfield())
+      continue;
+
+    llvm::Constant *EltInit = Base->getOperand(ElementNo);
+
+    // Bail out if the type of the ConstantStruct does not have the same layout
+    // as the type of the InitListExpr.
+    if (CGM.getTypes().ConvertType(Field->getType()) != EltInit->getType() ||
+        Layout.getFieldOffset(ElementNo) !=
+          BaseLayout->getElementOffsetInBits(ElementNo))
+      return false;
+
+    // Get the initializer. If we encounter an empty field or a NoInitExpr,
+    // we use values from the base expression.
+    Expr *Init = nullptr;
+    if (ElementNo < Updater->getNumInits())
+      Init = Updater->getInit(ElementNo);
+
+    if (!Init || isa<NoInitExpr>(Init))
+      ; // Do nothing.
+    else if (InitListExpr *ChildILE = dyn_cast<InitListExpr>(Init))
+      EltInit = Emitter->EmitDesignatedInitUpdater(EltInit, ChildILE);
+    else
+      EltInit = CGM.EmitConstantExpr(Init, Field->getType(), CGF);
+
+    ++ElementNo;
+
+    if (!EltInit)
+      return false;
+
+    if (!Field->isBitField())
+      AppendField(Field, Layout.getFieldOffset(FieldNo), EltInit);
+    else if (llvm::ConstantInt *CI = dyn_cast<llvm::ConstantInt>(EltInit))
+      AppendBitField(Field, Layout.getFieldOffset(FieldNo), CI);
+    else
+      // Initializing a bitfield with a non-trivial constant?
+      return false;
+  }
+
+  return true;
+}
+
 llvm::Constant *CodeGenModule::EmitConstantInit(const VarDecl &D,
                                                 CodeGenFunction *CGF) {
   // Make a quick check if variable can be default NULL initialized