new loplugin:returnconstval

look for code that thwarts copy-elision/move optimisations by
returning const values.

Change-Id: Iafa038d15051e06ee76d8a1c0ce8b7d234cd8488
Reviewed-on: https://gerrit.libreoffice.org/77964
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

2 files changed, 97 insertions, 0 deletions

diff --git a/compilerplugins/clang/returnconstval.cxx b/compilerplugins/clang/returnconstval.cxx
new file mode 100644
index 000000000000..beab3c654972
--- /dev/null
+++ b/compilerplugins/clang/returnconstval.cxx
@@ -0,0 +1,76 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/*
+ * This file is part of the LibreOffice project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+#include <string>
+#include <set>
+
+#include "plugin.hxx"
+
+/**
+  Find code where we are return a const value type from a function.
+  Which makes no sense.
+  Either we should return by non-const value, or by const ref.
+  e.g.
+    struct S2 {
+        OUString mv1;
+        const OUString get13() { return mv1; }
+    }
+
+  Specifically, this code pattern will prevent copy/move optimisations.
+*/
+
+namespace
+{
+class ReturnConstVal : public loplugin::FilteringPlugin<ReturnConstVal>
+{
+public:
+    explicit ReturnConstVal(loplugin::InstantiationData const& data)
+        : FilteringPlugin(data)
+    {
+    }
+
+    virtual void run() override { TraverseDecl(compiler.getASTContext().getTranslationUnitDecl()); }
+
+    bool VisitFunctionDecl(const FunctionDecl* decl);
+};
+
+bool ReturnConstVal::VisitFunctionDecl(const FunctionDecl* functionDecl)
+{
+    if (ignoreLocation(functionDecl))
+        return true;
+    if (!functionDecl->hasBody())
+        return true;
+    // ignore stuff that forms part of the stable URE interface
+    if (isInUnoIncludeFile(functionDecl))
+        return true;
+    QualType t1{ functionDecl->getReturnType() };
+    if (!t1.isConstQualified())
+        return true;
+    if (t1->isReferenceType())
+        return true;
+    report(DiagnosticsEngine::Warning, "either return non-const, or by const ref",
+           functionDecl->getSourceRange().getBegin())
+        << functionDecl->getSourceRange();
+
+    // display the location of the class member declaration so I don't have to search for it by hand
+    auto canonicalDecl = functionDecl->getCanonicalDecl();
+    if (canonicalDecl != functionDecl)
+    {
+        report(DiagnosticsEngine::Note, "either return non-const, or by const ref",
+               canonicalDecl->getSourceRange().getBegin())
+            << canonicalDecl->getSourceRange();
+    }
+
+    return true;
+}
+
+loplugin::Plugin::Registration<ReturnConstVal> X("returnconstval", false);
+}
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab: */
diff --git a/compilerplugins/clang/test/returnconstval.cxx b/compilerplugins/clang/test/returnconstval.cxx
new file mode 100644
index 000000000000..481ae84dff48
--- /dev/null
+++ b/compilerplugins/clang/test/returnconstval.cxx
@@ -0,0 +1,21 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4; fill-column: 100 -*- */
+/*
+ * This file is part of the LibreOffice project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+#include <rtl/ustring.hxx>
+
+struct S2
+{
+    OUString mv1;
+    // expected-error@+1 {{either return non-const, or by const ref [loplugin:returnconstval]}}
+    const OUString get1() { return mv1; }
+    const OUString& get2() { return mv1; }
+    OUString get3() { return mv1; }
+};
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */