[clang-repl] Emit const variables only once

Disable internal linkage for const variables if IncrementalExtensions
are enabled. Otherwise the variables are emitted multiple times, with
multiple constructions at unique memory locations, during every PTU.

Author: hahnjo

clang/lib/AST/ASTContext.cpp

@@ -11764,6 +11764,16 @@ GVALinkage ASTContext::GetGVALinkageForFunction(const FunctionDecl *FD) const {
 
 static GVALinkage basicGVALinkageForVariable(const ASTContext &Context,
                                              const VarDecl *VD) {
+  // As an extension for interactive REPLs, make sure constant variables are
+  // only emitted once instead of LinkageComputer::getLVForNamespaceScopeDecl
+  // marking them as internal.
+  if (Context.getLangOpts().CPlusPlus &&
+      Context.getLangOpts().IncrementalExtensions &&
+      VD->getType().isConstQualified() &&
+      !VD->getType().isVolatileQualified() && !VD->isInline() &&
+      !isa<VarTemplateSpecializationDecl>(VD) && !VD->getDescribedVarTemplate())
+    return GVA_DiscardableODR;
+
   if (!VD->isExternallyVisible())
     return GVA_Internal;
 

clang/test/Interpreter/const.cpp

@@ -0,0 +1,29 @@
+// UNSUPPORTED: system-aix
+// RUN: cat %s | clang-repl | FileCheck %s
+// RUN: cat %s | clang-repl -Xcc -O2 | FileCheck %s
+
+extern "C" int printf(const char*, ...);
+
+struct A { int val; A(int v); ~A(); void f() const; };
+A::A(int v) : val(v) { printf("A(%d), this = %p\n", val, this); }
+A::~A() { printf("~A, this = %p, val = %d\n", this, val); }
+void A::f() const { printf("f: this = %p, val = %d\n", this, val); }
+
+const A a(1);
+// CHECK: A(1), this = [[THIS:0x[0-9a-f]+]]
+// The constructor must only be called once!
+// CHECK-NOT: A(1)
+
+a.f();
+// CHECK-NEXT: f: this = [[THIS]], val = 1
+a.f();
+// CHECK-NEXT: f: this = [[THIS]], val = 1
+
+%quit
+// There must still be no other constructor!
+// CHECK-NOT: A(1)
+
+// At the end, we expect exactly one destructor call
+// CHECK: ~A
+// CHECK-SAME: this = [[THIS]], val = 1
+// CHECK-NOT: ~A