Make use of __builtin_unreachable

The purpose of __builtin_unreachable() is to assist the compiler in:
- Eliminating dead code that the programmer knows will never be executed.
- Linearizing the code by indicating to the compiler that the path is
 'cold' (a similar effect can be achieved by calling a noreturn function).

src/common.h

@@ -24,6 +24,19 @@
 #define __ALIGNED(x)
 #endif
 
+/* The purpose of __builtin_unreachable() is to assist the compiler in:
+ * - Eliminating dead code that the programmer knows will never be executed.
+ * - Linearizing the code by indicating to the compiler that the path is 'cold'
+ *   (a similar effect can be achieved by calling a noreturn function).
+ */
+#if defined(__GNUC__) || defined(__clang__)
+#define __UNREACHABLE __builtin_unreachable()
+#else /* unspported compilers */
+/* clang-format off */
+#define __UNREACHABLE do { /* nop */ } while (0)
+/* clang-format on */
+#endif
+
 /* Non-optimized builds do not have tail-call optimization (TCO). To work
  * around this, the compiler attribute 'musttail' is used, which forces TCO
  * even without optimizations enabled.

src/decode.c

@@ -1411,7 +1411,7 @@ static inline bool op_cmisc_alu(rv_insn_t *ir, const uint32_t insn)
             assert(!"Instruction reserved");
             break;
         default:
-            assert(!"Should not be reachable");
+            __UNREACHABLE;
             break;
         }
         break;
@@ -1598,7 +1598,7 @@ static inline bool op_ccr(rv_insn_t *ir, const uint32_t insn)
         }
         break;
     default:
-        assert(!"Should be unreachable.");
+        __UNREACHABLE;
         break;
     }
     return true;