am b63e4e88: am 5f32207a: Wrap ARM abort() to improve stack trace.

Merge commit 'b63e4e88b0d13afa431864190b3515a549c16d85'

* commit 'b63e4e88b0d13afa431864190b3515a549c16d85':
  Wrap ARM abort() to improve stack trace.

libc/unistd/abort.c

@@ -39,8 +39,13 @@
 #define debug_log(format, ...)  \
     __libc_android_log_print(ANDROID_LOG_DEBUG, "libc-abort", (format), ##__VA_ARGS__ )
 
+#ifdef __arm__
+void
+__libc_android_abort(void)
+#else
 void
 abort(void)
+#endif
 {
 	struct atexit *p = __atexit;
 	static int cleanup_called = 0;
@@ -97,3 +102,29 @@ abort(void)
 	(void)kill(getpid(), SIGABRT);
 	_exit(1);
 }
+
+#ifdef __arm__
+/*
+ * abort() does not return, which gcc interprets to mean that it doesn't
+ * need to preserve any of the callee-save registers.  Unfortunately this
+ * includes the link register, so if LR is used there is no way to determine
+ * which function called abort().
+ *
+ * We work around this by inserting a trivial stub that doesn't alter
+ * any of the "interesting" registers and thus doesn't need to save them.
+ * We can't just call __libc_android_abort from C because gcc uses "bl"
+ * without first saving LR, so we use an asm statement.  This also has
+ * the side-effect of replacing abort() with __libc_android_abort() in
+ * the stack trace.
+ *
+ * Ideally __libc_android_abort would be static, but I haven't figured out
+ * how to tell gcc to call a static function from an asm statement.
+ */
+void
+abort(void)
+{
+    asm ("b __libc_android_abort");
+    _exit(1);       /* suppress gcc noreturn warnings */
+}
+#endif
+