New loplugin:classmemaccess

...to complement and improve upon GCC's -Wclass-memaccess.  See the comment at
the head of compilerplugins/clang/classmemaccess.cxx for details.  (It is not
yet clear to me whether we would want to get this upstreamed into Clang.)

35d21e4bf6f66b3bbc7a44fcf184cb721b524a94 "Remove redundant memsets" was a case
that benefited from looking through toplevel casts to void*.  (Though the code
in include/basic/sbxvar.hxx needs a slightly more verbose way to deliberately
silence the warning now.)

d03041e19215592f21ba1222d3cfa29e1f94260a "Drop bogus memsets" is one example of
various cases that GCC -Wclass-memaccess failed to catch due to the use of array
instead of pointer types.

Change-Id: I6a9bfc34e3536834af35fdf4fb7ceeb31f31f8c0
Reviewed-on: https://gerrit.libreoffice.org/80421
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

2 files changed, 169 insertions, 0 deletions

diff --git a/compilerplugins/clang/classmemaccess.cxx b/compilerplugins/clang/classmemaccess.cxx
new file mode 100644
index 000000000000..56fe3fe39ab0
--- /dev/null
+++ b/compilerplugins/clang/classmemaccess.cxx
@@ -0,0 +1,133 @@
+/* -*- 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/.
+ */
+
+#ifndef LO_CLANG_SHARED_PLUGINS
+
+#include "check.hxx"
+#include "plugin.hxx"
+
+// Similar to GCC "warning: ‘void* memset(void*, int, size_t)’ writing to an object of non-trivial
+// type ‘...’; use assignment instead [-Wclass-memaccess]", but looking through toplevel cast to
+// void* and taking arrays into account in addition to pointers.  (Clang has
+// -Wdynamic-class-memaccess, but that only warns about memset overwriting a vtable pointer.  GCC
+// deliberately does not warn when there is a toplevel cast to void*, see
+// <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81327> "[8 Regression] cast to void* does not
+// suppress -Wclass-memaccess".)
+
+namespace
+{
+class ClassMemAccess final : public loplugin::FilteringPlugin<ClassMemAccess>
+{
+public:
+    explicit ClassMemAccess(loplugin::InstantiationData const& data)
+        : FilteringPlugin(data)
+    {
+    }
+
+    bool VisitCallExpr(CallExpr* expr)
+    {
+        if (ignoreLocation(expr))
+        {
+            return true;
+        }
+        auto const fdecl = expr->getDirectCallee();
+        if (fdecl == nullptr)
+        {
+            return true;
+        }
+        auto dc = loplugin::DeclCheck(fdecl).Function("memset");
+        if (!(dc.GlobalNamespace() || dc.StdNamespace()))
+        {
+            return true;
+        }
+        if (expr->getNumArgs() != 3)
+        {
+            if (isDebugMode())
+            {
+                report(DiagnosticsEngine::Fatal,
+                       "unexpected call to %0 with %1 instead of 3 arguments", expr->getExprLoc())
+                    << fdecl << expr->getNumArgs() << expr->getSourceRange();
+                report(DiagnosticsEngine::Note, "%0 declared here", fdecl->getLocation())
+                    << fdecl << fdecl->getSourceRange();
+            }
+            return true;
+        }
+        auto e = expr->getArg(0)->IgnoreParenImpCasts();
+        while (auto const cast = dyn_cast<ExplicitCastExpr>(e))
+        {
+            if (!loplugin::TypeCheck(cast->getTypeAsWritten()).Pointer().Void())
+            {
+                break;
+            }
+            e = cast->getSubExprAsWritten()->IgnoreParenImpCasts();
+        }
+        QualType t;
+        if (auto const t1 = e->getType()->getAs<clang::PointerType>())
+        {
+            t = t1->getPointeeType();
+        }
+        else if (e->getType()->isArrayType())
+        {
+            t = e->getType();
+            while (auto const t2 = t->getAsArrayTypeUnsafe())
+            {
+                t = t2->getElementType();
+            }
+        }
+        else
+        {
+            if (isDebugMode())
+            {
+                report(DiagnosticsEngine::Fatal,
+                       "unexpected call to %0 with first argument of non-pointer type %1",
+                       expr->getExprLoc())
+                    << fdecl << e->getType() << expr->getSourceRange();
+                report(DiagnosticsEngine::Note, "%0 declared here", fdecl->getLocation())
+                    << fdecl << fdecl->getSourceRange();
+            }
+            return true;
+        }
+        auto const decl = t->getAsCXXRecordDecl();
+        if (decl == nullptr)
+        {
+            return true;
+        }
+        if (!decl->isCompleteDefinition())
+        {
+            return true; // conservatively assume it may be trivial
+        }
+        if (decl->isTrivial())
+        {
+            return true;
+        }
+        report(DiagnosticsEngine::Warning,
+               "%0 writing to an object of non-trivial type %1; use assignment instead",
+               expr->getExprLoc())
+            << fdecl << decl << expr->getSourceRange();
+        return true;
+    }
+
+    bool preRun() override { return compiler.getLangOpts().CPlusPlus; }
+
+private:
+    void run() override
+    {
+        if (preRun())
+        {
+            TraverseDecl(compiler.getASTContext().getTranslationUnitDecl());
+        }
+    }
+};
+
+loplugin::Plugin::Registration<ClassMemAccess> classmemaccess("classmemaccess");
+}
+
+#endif
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */
diff --git a/compilerplugins/clang/test/classmemaccess.cxx b/compilerplugins/clang/test/classmemaccess.cxx
new file mode 100644
index 000000000000..05bb457f87e3
--- /dev/null
+++ b/compilerplugins/clang/test/classmemaccess.cxx
@@ -0,0 +1,36 @@
+/* -*- 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 <cstring>
+
+void f(struct Incomplete* p1, struct S* p2);
+
+struct S
+{
+    S();
+};
+
+void f(struct Incomplete* p1, struct S* p2)
+{
+    S s;
+    // expected-error@+1 {{writing to an object of non-trivial type 'S'; use assignment instead [loplugin:classmemaccess]}}
+    std::memset(&s, 0, sizeof s);
+    // expected-error@+1 {{writing to an object of non-trivial type 'S'; use assignment instead [loplugin:classmemaccess]}}
+    std::memset(static_cast<void*>(&s), 0, sizeof s);
+    auto const disableWarning = static_cast<void*>(&s);
+    std::memset(disableWarning, 0, sizeof s);
+    S a[1][1];
+    // expected-error@+1 {{writing to an object of non-trivial type 'S'; use assignment instead [loplugin:classmemaccess]}}
+    std::memset(a, 0, sizeof a);
+    std::memset(p1, 0, 10); // conservatively assume Incomplete may be trivial
+    // expected-error@+1 {{writing to an object of non-trivial type 'S'; use assignment instead [loplugin:classmemaccess]}}
+    std::memset(p2, 0, 10);
+}
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */