ash: less hackish implementation of evaltreenr()

Defining a function alias with __attribute__ ((alias("evaltree"),__noreturn__))
is not that usual, and clang had a bug which made it misunderstand
this construct.

Switch to:
ALWAYS_INLINE NORETURN evaltreenr() { evaltree(); unreachable(); }

Older gcc's do not know unreachable(), on them we pay the price of having
a few extra calls to abort():

function                                             old     new   delta
evalsubshell                                         151     156      +5
evalpipe                                             357     362      +5
argstr                                              1141    1144      +3

On newer gcc, code size does not change.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/platform.h

@@ -45,6 +45,13 @@
 
 #define UNUSED_PARAM __attribute__ ((__unused__))
 #define NORETURN __attribute__ ((__noreturn__))
+
+#if __GNUC_PREREQ(4,5)
+# define bb_unreachable(altcode) __builtin_unreachable()
+#else
+# define bb_unreachable(altcode) altcode
+#endif
+
 /* "The malloc attribute is used to tell the compiler that a function
  * may be treated as if any non-NULL pointer it returns cannot alias
  * any other pointer valid when the function returns. This will often

shell/ash.c

@@ -6137,6 +6137,19 @@ struct backcmd {                /* result of evalbackcmd */
 #define EV_TESTED  02           /* exit status is checked; ignore -e flag */
 static int evaltree(union node *, int);
 
+/* An evaltree() which is known to never return.
+ * Used to use an alias:
+ * static int evaltreenr(union node *, int) __attribute__((alias("evaltree"),__noreturn__));
+ * but clang was reported to "transfer" noreturn-ness to evaltree() as well.
+ */
+static ALWAYS_INLINE NORETURN void
+evaltreenr(union node *n, int flags)
+{
+	evaltree(n, flags);
+	bb_unreachable(abort());
+	/* NOTREACHED */
+}
+
 static void FAST_FUNC
 evalbackcmd(union node *n, struct backcmd *result)
 {
@@ -6173,7 +6186,7 @@ evalbackcmd(union node *n, struct backcmd *result)
  */
 		eflag = 0;
 		ifsfree();
-		evaltree(n, EV_EXIT); /* actually evaltreenr... */
+		evaltreenr(n, EV_EXIT);
 		/* NOTREACHED */
 	}
 	/* parent */
@@ -8796,11 +8809,6 @@ evaltree(union node *n, int flags)
 	return exitstatus;
 }
 
-#if !defined(__alpha__) || (defined(__GNUC__) && __GNUC__ >= 3)
-static
-#endif
-int evaltreenr(union node *, int) __attribute__ ((alias("evaltree"),__noreturn__));
-
 static int
 skiploop(void)
 {