From 1f770d97b4338846dd75006bbcf229c1952780f6 Mon Sep 17 00:00:00 2001
From: Marko Lindqvist <cazfi74@gmail.com>
Date: Mon, 10 Apr 2023 19:03:53 +0300
Subject: [PATCH 39/39] Add fc__unreachable()

See osdn #47835

Signed-off-by: Marko Lindqvist <cazfi74@gmail.com>
---
 configure.ac                          |  1 +
 gen_headers/meson_freeciv_config.h.in |  3 +++
 m4/compiler.m4                        | 11 +++++++++++
 meson.build                           |  7 +++++++
 utility/support.h                     |  9 +++++++++
 5 files changed, 31 insertions(+)

diff --git a/configure.ac b/configure.ac
index 70e6810c10..c971e39b42 100644
--- a/configure.ac
+++ b/configure.ac
@@ -804,6 +804,7 @@ FC_CXX20_CAPTURE_THIS
 _CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS $WERROR_C_TEST_FLAGS"
 FC_SIZE_T_FORMAT
+FC_BUILTIN_UNREACHABLE
 CFLAGS="$_CFLAGS"
 
 dnl BeOS-specific settings
diff --git a/gen_headers/meson_freeciv_config.h.in b/gen_headers/meson_freeciv_config.h.in
index 7e231d7737..466ff08827 100644
--- a/gen_headers/meson_freeciv_config.h.in
+++ b/gen_headers/meson_freeciv_config.h.in
@@ -39,6 +39,9 @@
 /* C++20 capture this supported */
 #mesondefine FREECIV_HAVE_CXX20_CAPTURE_THIS
 
+/* __builtin_unreachable() available */
+#mesondefine FREECIV_HAVE_UNREACHABLE
+
 /* Use pthreads as thread implementation */
 #mesondefine FREECIV_HAVE_PTHREAD
 
diff --git a/m4/compiler.m4 b/m4/compiler.m4
index bc813977d9..873c79beda 100644
--- a/m4/compiler.m4
+++ b/m4/compiler.m4
@@ -167,3 +167,14 @@ AC_DEFUN([FC_SIZE_T_FORMAT],
 
   AC_MSG_RESULT([$SIZE_T_PRINTF])
 ])
+
+# Check for __builtin_unreachable()
+AC_DEFUN([FC_BUILTIN_UNREACHABLE],
+[
+  AC_MSG_CHECKING([__builtin_unreachable()])
+
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[ ]],
+[[__builtin_unreachable();]])],[
+  AC_DEFINE([FREECIV_HAVE_UNREACHABLE], [1], [__builtin_unreachable() available])
+  AC_MSG_RESULT([yes])], [AC_MSG_RESULT([no])])
+])
diff --git a/meson.build b/meson.build
index 59d0129d83..d58036002e 100644
--- a/meson.build
+++ b/meson.build
@@ -633,6 +633,13 @@ int main(void) { size_t var = 0; fr("''' + format + '''", var); return 0; }''',
   endif
 endforeach
 
+if c_compiler.compiles('''int main(void) { __builtin_unreachable(); return 0; }''',
+  include_directories: include_directories(cross_inc_path),
+  name: '__builtin_unreachable()',
+  args: ['-Werror', '-Wall'])
+    pub_conf_data.set('FREECIV_HAVE_UNREACHABLE', 1)
+endif
+
 if get_option('audio') or get_option('clients').contains('sdl2')
   if host_system == 'windows'
     sdl2main_dep = [c_compiler.find_library('mingw32', dirs: cross_lib_path),
diff --git a/utility/support.h b/utility/support.h
index b112ad93d8..49f2c5fe9c 100644
--- a/utility/support.h
+++ b/utility/support.h
@@ -123,6 +123,15 @@ extern "C" {
 #define fc__noreturn
 #endif
 
+#ifdef FREECIV_HAVE_UNREACHABLE
+#define fc__unreachable(_cond_) \
+  if (_cond_) {                 \
+    __builtin_unreachable();    \
+  }
+#else  /* FREECIV_HAVE_UNREACHABLE */
+#define fc__unreachable(_cond_) fc_assert(!(_cond_))
+#endif /* FREECIV_HAVE_UNREACHABLE */
+
 #ifdef FREECIV_MSWINDOWS
 typedef long int fc_errno;
 #else
-- 
2.39.2