tcg: Add tcg_debug_assert

Like the C assert macro, except only enabled for CONFIG_DEBUG_TCG,
and without having to set _NDEBUG and disable all other asserts at
the same time.

The use of __builtin_unreachable (when available) gives the compiler
the same information, which may (or may not) help it optimize better.
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

tcg/tcg.h

@@ -530,6 +530,15 @@ do {\
     abort();\
 } while (0)
 
+#ifdef CONFIG_DEBUG_TCG
+# define tcg_debug_assert(X) do { assert(X); } while (0)
+#elif QEMU_GNUC_PREREQ(4, 5)
+# define tcg_debug_assert(X) \
+    do { if (!(X)) { __builtin_unreachable(); } } while (0)
+#else
+# define tcg_debug_assert(X) do { (void)(X); } while (0)
+#endif
+
 void tcg_add_target_add_op_defs(const TCGTargetOpDef *tdefs);
 
 #if TCG_TARGET_REG_BITS == 32