Move 'likely' macros to the compiler support header

This makes it easier to move away from autoconfig in the future.

Also, use '!!' C trick to prevent accidental incorrect usage of
these macros.

acinclude.m4

@@ -335,17 +335,8 @@ AH_BOTTOM([#if C_ATTRIBUTE_FASTCALL
 #define DB_FASTCALL
 #endif])
 
-
 AH_BOTTOM([#if C_HAS_ATTRIBUTE
 #define GCC_ATTRIBUTE(x) __attribute__ ((x))
 #else
 #define GCC_ATTRIBUTE(x) /* attribute not supported */
 #endif])
-
-AH_BOTTOM([#if C_HAS_BUILTIN_EXPECT
-#define GCC_UNLIKELY(x) __builtin_expect((x),0)
-#define GCC_LIKELY(x) __builtin_expect((x),1)
-#else
-#define GCC_UNLIKELY(x) (x)
-#define GCC_LIKELY(x) (x)
-#endif])

include/compiler.h

@@ -0,0 +1,42 @@
+/*
+ *  Copyright (C) 2019  The DOSBox Team
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License along
+ *  with this program; if not, write to the Free Software Foundation, Inc.,
+ *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#ifndef DOSBOX_COMPILER_H
+#define DOSBOX_COMPILER_H
+
+// This header wraps compiler-specific features, so they won't need to
+// be hacked into the buildsystem.
+
+// GCC_LIKELY macro is incorrectly named, because other compilers support
+// this feature as well (e.g. Clang, Intel); leave it be for now, at
+// least until full support for C++20 [[likely]] attribute will start arriving
+// in new compilers.
+//
+// Note: '!!' trick is used, to convert non-boolean values to 1 or 0
+// to prevent accidental incorrect usage (e.g. when user wraps macro
+// around a pointer or an integer, expecting usual C semantics).
+
+#if C_HAS_BUILTIN_EXPECT
+#define GCC_LIKELY(x)   __builtin_expect(!!(x), 1)
+#define GCC_UNLIKELY(x) __builtin_expect(!!(x), 0)
+#else
+#define GCC_LIKELY
+#define GCC_UNLIKELY
+#endif
+
+#endif /* DOSBOX_COMPILER_H */

include/dosbox.h

@@ -21,6 +21,7 @@
 #define DOSBOX_DOSBOX_H
 
 #include "config.h"
+#include "compiler.h"
 #include "types.h"
 
 GCC_ATTRIBUTE(noreturn) void E_Exit(const char * message,...) GCC_ATTRIBUTE( __format__(__printf__, 1, 2));

src/platform/visualc/config.h

@@ -64,8 +64,6 @@
 #define C_DIRECTSERIAL 1
 
 #define GCC_ATTRIBUTE(x) /* attribute not supported */
-#define GCC_UNLIKELY(x) (x)
-#define GCC_LIKELY(x) (x)
 
 #define INLINE __forceinline
 #define DB_FASTCALL __fastcall