c++ IRGen. In trivial cases that object is going into static

storage and thus is implicitly zero-initialized, no need to
do C++11 memory model. This patch unconditionally detects
such condition and zeroinitializer's the variable.
Patch has been commented on and OKed by Doug off-line.
// rdar://12897704

llvm-svn: 172144

clang/lib/CodeGen/CGExprConstant.cpp

@@ -1006,6 +1006,23 @@ public:
 
 llvm::Constant *CodeGenModule::EmitConstantInit(const VarDecl &D,
                                                 CodeGenFunction *CGF) {
+  // Make a quick check if variable can be default NULL initialized
+  // and avoid going through rest of code which may do, for c++11,
+  // initialization of memory to all NULLs.
+  if (!D.hasLocalStorage()) {
+    QualType Ty = D.getType();
+    if (Ty->isArrayType())
+      Ty = Context.getBaseElementType(Ty);
+    if (Ty->isRecordType())
+      if (const CXXConstructExpr *E =
+          dyn_cast_or_null<CXXConstructExpr>(D.getInit())) {
+        const CXXConstructorDecl *CD = E->getConstructor();
+        if (CD->isTrivial() && CD->isDefaultConstructor() &&
+            Ty->getAsCXXRecordDecl()->hasTrivialDestructor())
+          return EmitNullConstant(D.getType());
+      }
+  }
+  
   if (const APValue *Value = D.evaluateValue())
     return EmitConstantValueForMemory(*Value, D.getType(), CGF);
 

clang/test/CodeGenCXX/cxx11-trivial-initializer-struct.cpp

@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -S -emit-llvm -o %t.ll %s -triple x86_64-apple-darwin10 
+// RUN: %clang_cc1 -std=c++11 -S -emit-llvm -o %t-c++11.ll %s -triple x86_64-apple-darwin10 
+// RUN: diff %t.ll  %t-c++11.ll
+
+// rdar://12897704
+
+struct sAFSearchPos {
+    unsigned char *pos;
+    unsigned char count;
+};
+
+static volatile struct sAFSearchPos testPositions;
+
+static volatile struct sAFSearchPos arrayPositions[100][10][5];
+
+int main() {
+  return testPositions.count + arrayPositions[10][4][3].count; 
+}