Check for __builtin_unreachable at configure time

Add a configure check for __builtin_unreachable instead of basing its
availability on the __GNUC__ version. On OS X using gcc (a real gcc, not the
bundled version that's just a gcc front-end) leads to a linker assertion:

    https://github.com/jemalloc/jemalloc/issues/266

It turns out that this is caused by a gcc bug resulting from the use of
__builtin_unreachable():

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57438

To work around this bug, check that __builtin_unreachable() actually works at
configure time, and if it doesn't use abort() instead. The check is based on
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57438#c21.

With this `make check` passes with a homebrew installed gcc-5 and gcc-6.

configure.ac

@@ -1038,6 +1038,23 @@ if test "x$enable_cache_oblivious" = "x1" ; then
 fi
 AC_SUBST([enable_cache_oblivious])
 
+
+
+JE_COMPILABLE([a program using __builtin_unreachable], [
+void foo (void) {
+  __builtin_unreachable();
+}
+], [
+	{
+		foo();
+	}
+], [je_cv_gcc_builtin_unreachable])
+if test "x${je_cv_gcc_builtin_ffsl}" = "xyes" ; then
+  AC_DEFINE([JEMALLOC_INTERNAL_UNREACHABLE], [__builtin_unreachable])
+else
+  AC_DEFINE([JEMALLOC_INTERNAL_UNREACHABLE], [abort])
+fi
+
 dnl ============================================================================
 dnl Check for  __builtin_ffsl(), then ffsl(3), and fail if neither are found.
 dnl One of those two functions should (theoretically) exist on all platforms

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -185,6 +185,12 @@
 /* TLS is used to map arenas and magazine caches to threads. */
 #undef JEMALLOC_TLS
 
+/*
+ * Used to mark unreachable code to quiet "end of non-void" compiler warnings.
+ * Don't use this directly; instead use unreachable() from util.h
+ */
+#undef JEMALLOC_INTERNAL_UNREACHABLE
+
 /*
  * ffs*() functions to use for bitmapping.  Don't use these directly; instead,
  * use ffs_*() from util.h.

include/jemalloc/internal/util.h

@@ -61,30 +61,20 @@
 #	define JEMALLOC_CC_SILENCE_INIT(v)
 #endif
 
-#define	JEMALLOC_GNUC_PREREQ(major, minor)				\
-    (!defined(__clang__) &&						\
-    (__GNUC__ > (major) || (__GNUC__ == (major) && __GNUC_MINOR__ >= (minor))))
-#ifndef __has_builtin
-#  define __has_builtin(builtin) (0)
-#endif
-#define	JEMALLOC_CLANG_HAS_BUILTIN(builtin)				\
-    (defined(__clang__) && __has_builtin(builtin))
-
 #ifdef __GNUC__
 #	define likely(x)   __builtin_expect(!!(x), 1)
 #	define unlikely(x) __builtin_expect(!!(x), 0)
-#  if JEMALLOC_GNUC_PREREQ(4, 6) ||					\
-      JEMALLOC_CLANG_HAS_BUILTIN(__builtin_unreachable)
-#	define unreachable() __builtin_unreachable()
-#  else
-#	define unreachable() abort()
-#  endif
 #else
 #	define likely(x)   !!(x)
 #	define unlikely(x) !!(x)
-#	define unreachable() abort()
 #endif
 
+#if !defined(JEMALLOC_INTERNAL_UNREACHABLE)
+#  error JEMALLOC_INTERNAL_UNREACHABLE should have been defined by configure
+#endif
+
+#define unreachable() JEMALLOC_INTERNAL_UNREACHABLE()
+
 #include "jemalloc/internal/assert.h"
 
 /* Use to assert a particular configuration, e.g., cassert(config_debug). */