new loplugin:sequenceloop

look for places we should be using std::as_const on for-range
loops over uno::Sequence, to avoid triggering a copy

Change-Id: I7efb641bf09d37c87946f03428ee4eec90298c8a
Reviewed-on: https://gerrit.libreoffice.org/77441
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

compilerplugins/clang/sequenceloop.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 <cassert>
+#include <string>
+#include <iostream>
+#include <fstream>
+#include <set>
+
+#include <clang/AST/CXXInheritance.h>
+#include "plugin.hxx"
+#include "check.hxx"
+
+/**
+  When used in "for" loops, css::uno::Sequence objects tend to end up calling the non-const begin()/end(),
+  which is considerably more expensive than the const variants because it forces a local copy
+  of the internal ref-counted impl object.
+*/
+
+namespace
+{
+class SequenceLoop : public loplugin::FilteringPlugin<SequenceLoop>
+{
+public:
+    explicit SequenceLoop(loplugin::InstantiationData const& data)
+        : FilteringPlugin(data)
+    {
+    }
+
+    virtual void run() override
+    {
+        if (preRun())
+            TraverseDecl(compiler.getASTContext().getTranslationUnitDecl());
+    }
+
+    bool VisitCXXForRangeStmt(CXXForRangeStmt const*);
+};
+
+bool SequenceLoop::VisitCXXForRangeStmt(CXXForRangeStmt const* forStmt)
+{
+    if (ignoreLocation(forStmt))
+        return true;
+
+    auto tc = loplugin::TypeCheck(forStmt->getRangeInit()->getType());
+    if (tc.Const())
+        return true;
+    if (!tc.Class("Sequence")
+             .Namespace("uno")
+             .Namespace("star")
+             .Namespace("sun")
+             .Namespace("com")
+             .GlobalNamespace())
+        return true;
+    const VarDecl* varDecl = forStmt->getLoopVariable();
+    auto tc2 = loplugin::TypeCheck(varDecl->getType());
+    if (!tc2.LvalueReference().Const())
+        return true;
+
+    report(DiagnosticsEngine::Warning,
+           "use std::as_const, or make range var const, to avoid creating a copy of the Sequence",
+           compat::getBeginLoc(forStmt))
+        << forStmt->getSourceRange();
+    return true;
+}
+
+loplugin::Plugin::Registration<SequenceLoop> X("sequenceloop");
+
+} // namespace
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab: */