Add GCC7 -Wimplicit-fallthrough support/fixes.

GCC7 will have a new -Wimplicit-fallthrough warning. It did catch one
small buglet in elflint option procession. So it seems useful to enable
to make sure all swatch case fallthroughs are deliberate.

Add configure check to detect whether gcc support -Wimplicit-fallthrough
and enable it. Add fixes and explicit fallthrough comments where necessary.

Signed-off-by: Mark Wielaard <mjw@redhat.com>

7 files changed, 18 insertions, 8 deletions

diff --git a/backends/ChangeLog b/backends/ChangeLog
index 53d29081..1c561b50 100644
--- a/backends/ChangeLog
+++ b/backends/ChangeLog
@@ -1,3 +1,14 @@
+2016-11-02  Mark Wielaard  <mjw@redhat.com>
+
+	* i386_regs.c (i386_register_info): Add fallthrough comment.
+	* i386_retval.c (i386_return_value_location): Move fallthrough
+	comment.
+	* linux-core-note.c (core_note): Adjust fallthrough comment.
+	* m68k_retval.c (m68k_return_value_location): Move fallthrough
+	comment.
+	* ppc_regs.c (ppc_register_info): Add fallthrough comment.
+	* x86_64_regs.c (x86_64_register_info): Likewise.
+
 2016-08-09  Jose E. Marchesi  <jose.marchesi@oracle.com>
 
 	* sparc_attrs.c (sparc_check_object_attribute): Fix the
diff --git a/backends/i386_regs.c b/backends/i386_regs.c
index fb8ded33..fd963a62 100644
--- a/backends/i386_regs.c
+++ b/backends/i386_regs.c
@@ -92,6 +92,7 @@ i386_register_info (Ebl *ebl __attribute__ ((unused)),
     case 5:
     case 8:
       *type = DW_ATE_address;
+      /* Fallthrough */
     case 0 ... 3:
     case 6 ... 7:
       name[0] = 'e';
diff --git a/backends/i386_retval.c b/backends/i386_retval.c
index 9da797d5..4aa646fe 100644
--- a/backends/i386_retval.c
+++ b/backends/i386_retval.c
@@ -122,9 +122,8 @@ i386_return_value_location (Dwarf_Die *functypedie, const Dwarf_Op **locp)
 	  return nloc_intreg;
 	if (size <= 8)
 	  return nloc_intregpair;
-
-	/* Else fall through.  */
       }
+    /* Fallthrough */
 
     case DW_TAG_structure_type:
     case DW_TAG_class_type:
diff --git a/backends/linux-core-note.c b/backends/linux-core-note.c
index a4ec0be0..67638d70 100644
--- a/backends/linux-core-note.c
+++ b/backends/linux-core-note.c
@@ -225,8 +225,8 @@ EBLHOOK(core_note) (const GElf_Nhdr *nhdr, const char *name,
     case sizeof "CORE":
       if (memcmp (name, "CORE", nhdr->n_namesz) == 0)
 	break;
-      /* Buggy old Linux kernels didn't terminate "LINUX".
-         Fall through.  */
+      /* Buggy old Linux kernels didn't terminate "LINUX".  */
+      /* Fall through. */
 
     case sizeof "LINUX":
       if (memcmp (name, "LINUX", nhdr->n_namesz) == 0)
diff --git a/backends/m68k_retval.c b/backends/m68k_retval.c
index 2dd285a0..c68ed022 100644
--- a/backends/m68k_retval.c
+++ b/backends/m68k_retval.c
@@ -134,10 +134,8 @@ m68k_return_value_location (Dwarf_Die *functypedie, const Dwarf_Op **locp)
 	  return nloc_intreg;
 	if (size <= 8)
 	  return nloc_intregpair;
-
-	/* Else fall through.  */
       }
-
+      /* Fallthrough */
     case DW_TAG_structure_type:
     case DW_TAG_class_type:
     case DW_TAG_union_type:
diff --git a/backends/ppc_regs.c b/backends/ppc_regs.c
index 4b92a9aa..bcf4f7a3 100644
--- a/backends/ppc_regs.c
+++ b/backends/ppc_regs.c
@@ -140,7 +140,7 @@ ppc_register_info (Ebl *ebl __attribute__ ((unused)),
     case 100:
       if (*bits == 32)
 	return stpcpy (name, "mq") + 1 - name;
-
+      /* Fallthrough */
     case 102 ... 107:
       name[0] = 's';
       name[1] = 'p';
diff --git a/backends/x86_64_regs.c b/backends/x86_64_regs.c
index 2172d9f1..84304407 100644
--- a/backends/x86_64_regs.c
+++ b/backends/x86_64_regs.c
@@ -87,6 +87,7 @@ x86_64_register_info (Ebl *ebl __attribute__ ((unused)),
 
     case 6 ... 7:
       *type = DW_ATE_address;
+      /* Fallthrough */
     case 0 ... 5:
       name[0] = 'r';
       name[1] = baseregs[regno][0];