Introduce O3TL_UNREACHABLE as a better alternative to assert(false)

The code in sw/source/core/fields/reffld.cxx had turned into whack-a-mole:  MSVC
needed the unreachable default:assert(false); to fall through to init some vars
to avoid C4701 ("potentially uninitialized local variable").  But Clang (with
!NDEBUG, and an implementation of assert marked as noreturn) reported that as
"fallthrough annotation in unreachable code", requiring the #ifdef NDEBUG guards
to be added.  And now clang-cl (also with !NDEBUG, but with the MSVC
implementation of assert apparently not marked as noreturn) complained about
"unannotated fall-through between switch labels", which wold require the #ifdef
NDEBUG guards to be removed again.

O3TL_UNREACHABLE (inspired by LLVM's llvm_unreachable and underyling
LLVM_BUILTIN_UNREACHABLE) hopefully improves on that for all compilers involved.
For non-MSVC (i.e., Clang or GCC) it assumes that __builtin_unreachable is
supported (which it is at least for GCC 7, which is our current baseline per
README.md).

Change-Id: I4474f95664d97c3d0e506bab90f80d5ab252ec6e
Reviewed-on: https://gerrit.libreoffice.org/67232
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

1 files changed, 39 insertions, 0 deletions

diff --git a/include/o3tl/unreachable.hxx b/include/o3tl/unreachable.hxx
new file mode 100644
index 000000000000..bc33d34b534e
--- /dev/null
+++ b/include/o3tl/unreachable.hxx
@@ -0,0 +1,39 @@
+/* -*- 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 INCLUDED_O3TL_UNREACHABLE_HXX
+#define INCLUDED_O3TL_UNREACHABLE_HXX
+
+#include <sal/config.h>
+
+#include <cassert>
+
+// A better replacement for assert(false) to indicate a place in the code that should not be
+// reachable.  This should improve on the sometimes poor false-positive warnings emitted by
+// compilers when they cannot detect that some condition flagged by assert(false) cannot occur,
+// either because assert is reduced to a no-op by NDEBUG or because assert is not marked as noreturn
+// in the MSVC headers.  This is inspired by LLVM's LLVM_BUILTIN_UNREACHABLE
+// (llvm/include/llvm/Support/Compiler.h).
+
+#if defined _MSC_VER
+#define O3TL_UNREACHABLE_detail __assume(false)
+#else // assuming Clang or GCC with support for:
+#define O3TL_UNREACHABLE_detail __builtin_unreachable()
+#endif
+
+#define O3TL_UNREACHABLE                                                                           \
+    do                                                                                             \
+    {                                                                                              \
+        assert(false);                                                                             \
+        O3TL_UNREACHABLE_detail;                                                                   \
+    } while (false)
+
+#endif
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */