Add support for GCC-4.5's __builtin_unreachable() to compiler.h (v2)

Starting with version 4.5, GCC has a new built-in function
__builtin_unreachable() that can be used in places like the kernel's
BUG() where inline assembly is used to transfer control flow.  This
eliminated the need for an endless loop in these places.

The patch adds a new macro 'unreachable()' that will expand to either
__builtin_unreachable() or an endless loop depending on the compiler
version.

Change from v1: Simplify unreachable() for non-GCC 4.5 case.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 19 insertions, 0 deletions

diff --git a/include/linux/compiler-gcc4.h b/include/linux/compiler-gcc4.h
index 450fa597c94d..ab3af40a53c6 100644
--- a/include/linux/compiler-gcc4.h
+++ b/include/linux/compiler-gcc4.h
@@ -36,4 +36,18 @@
    the kernel context */
 #define __cold                  __attribute__((__cold__))
 
+
+#if __GNUC_MINOR__ >= 5
+/*
+ * Mark a position in code as unreachable.  This can be used to
+ * suppress control flow warnings after asm blocks that transfer
+ * control elsewhere.
+ *
+ * Early snapshots of gcc 4.5 don't support this and we can't detect
+ * this in the preprocessor, but we can live with this because they're
+ * unreleased.  Really, we need to have autoconf for the kernel.
+ */
+#define unreachable() __builtin_unreachable()
+#endif
+
 #endif
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 04fb5135b4e1..59f208926d13 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -144,6 +144,11 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 # define barrier() __memory_barrier()
 #endif
 
+/* Unreachable code */
+#ifndef unreachable
+# define unreachable() do { } while (1)
+#endif
+
 #ifndef RELOC_HIDE
 # define RELOC_HIDE(ptr, off)                                   \
   ({ unsigned long __ptr;                                       \