libbacktrace patch committed: Use __attribute__((__fallthrough__))

[Ian Lance Taylor via Gcc-patches]

This libbacktrace patch uses __attribute__((__fallthrough__)) rather
than relying on a /*fallthrough*/ comment.  Bootstrapped and ran
libbacktrace tests on x86_64-pc-linux-gnu.  Committed to mainline.

Ian

* internal.h (ATTRIBUTE_FALLTHROUGH): Define.
* elf.c (elf_zlib_inflate): Use ATTRIBUTE_FALLTHROUGH.

diff --git a/libbacktrace/elf.c b/libbacktrace/elf.c
index 941f820d944..d52b86cdeb5 100644
--- a/libbacktrace/elf.c
+++ b/libbacktrace/elf.c
@@ -2081,10 +2081,10 @@ elf_zlib_inflate (const unsigned char *pin, size_t sin, 
uint16_t *zdebug_table,
                        {
                        case 6:
                          *plen++ = prev;
-                         /* fallthrough */
+                         ATTRIBUTE_FALLTHROUGH;
                        case 5:
                          *plen++ = prev;
-                         /* fallthrough */
+                         ATTRIBUTE_FALLTHROUGH;
                        case 4:
                          *plen++ = prev;
                        }
@@ -2115,22 +2115,22 @@ elf_zlib_inflate (const unsigned char *pin, size_t sin, 
uint16_t *zdebug_table,
                        {
                        case 10:
                          *plen++ = 0;
-                         /* fallthrough */
+                         ATTRIBUTE_FALLTHROUGH;
                        case 9:
                          *plen++ = 0;
-                         /* fallthrough */
+                         ATTRIBUTE_FALLTHROUGH;
                        case 8:
                          *plen++ = 0;
-                         /* fallthrough */
+                         ATTRIBUTE_FALLTHROUGH;
                        case 7:
                          *plen++ = 0;
-                         /* fallthrough */
+                         ATTRIBUTE_FALLTHROUGH;
                        case 6:
                          *plen++ = 0;
-                         /* fallthrough */
+                         ATTRIBUTE_FALLTHROUGH;
                        case 5:
                          *plen++ = 0;
-                         /* fallthrough */
+                         ATTRIBUTE_FALLTHROUGH;
                        case 4:
                          *plen++ = 0;
                        }
diff --git a/libbacktrace/internal.h b/libbacktrace/internal.h
index 047a700c0ce..659db9e21e2 100644
--- a/libbacktrace/internal.h
+++ b/libbacktrace/internal.h
@@ -56,6 +56,14 @@ POSSIBILITY OF SUCH DAMAGE.  */
 # endif
 #endif
 
+#ifndef ATTRIBUTE_FALLTHROUGH
+# if (GCC_VERSION >= 7000)
+#  define ATTRIBUTE_FALLTHROUGH __attribute__ ((__fallthrough__))
+# else
+#  define ATTRIBUTE_FALLTHROUGH
+# endif
+#endif
+
 #ifndef HAVE_SYNC_FUNCTIONS
 
 /* Define out the sync functions.  These should never be called if