warning cleanup: suspicious switch fallthrough cases.

This is the seventh round of an effort to enable GCC's -Wextra
option. Warnings about switch fallthrough situations are
addressed. GCC now has a diagnostic for this that is enabled
by -Wextra in such a way that if a fallthrough comment is
present, the diagnostic is suppressed. In much of the code,
we have such a comment. It's missing in a few places, or
misplaced. There are also some real bugs.

* hash.c (hash_buf): Add fallthrough comments to intentional
fallthrough cases.
(hash_hash_op): bugfix: add break statement. The 32 and 64
bit cases are independent (at compile time).

* lib.c (cdr, nullify, list_collect, empty): Add fallthrough
comment.
(int_str): Add missing break. This has not caused a
bug though because setting the octzero flag in the zerox
case is harmless to the logic which follows.

* linenoise.c (edit): Move misplaced fallthrough.

* sysif.c (fcntl_wrap): Bugfix: add missing break, without
which errno is tampered to hold EINVAL, in spite of a
successful F_SETLK, F_SETLKW or F_GETLK operation.

* unwind.h (jmp_restore): Declare noreturn, so that GCC
does not issue a false positive warning about a fallthrough
in uw_unwind_to_exit_point.

* utf8.c (utf8_from_buf, utf8_decode): Move a fallthrough
comment outside of preprocessing, so it is properly processed
by GCC's diagnostic.

6 files changed, 16 insertions, 4 deletions

diff --git a/hash.c b/hash.c
index 9f4df00b..8a298de2 100644
--- a/hash.c
+++ b/hash.c
@@ -155,10 +155,13 @@ static u32_t hash_buf(const mem_t *ptr, ucnum size, u32_t seed, int *pcount)
     switch (size) {
     case 3:
       in |= convert(u32_t, tail[2]) << 16;
+      /* fallthrough */
     case 2:
       in |= convert(u32_t, tail[1]) << 8;
+      /* fallthrough */
     case 1:
       in |= convert(u32_t, tail[0]);
+      break;
     }
 
     acc ^= in;
@@ -472,8 +475,10 @@ static ucnum hash_hash_op(val obj, int *count, ucnum seed)
   switch (CHAR_BIT * sizeof (mem_t *)) {
   case 32:
     out += coerce(ucnum, h->hops) >> 4;
+    break;
   case 64: default:
     out += coerce(ucnum, h->hops) >> 5;
+    break;
   }
 
   out += equal_hash(h->userdata, count, seed);
diff --git a/lib.c b/lib.c
index c61cf60d..6fae0008 100644
--- a/lib.c
+++ b/lib.c
@@ -601,6 +601,7 @@ val cdr(val cons)
           return funcall2(lambda_meth, cons, rcons(one, t));
       }
     }
+    /* fallthrough */
   default:
     type_mismatch(lit("cdr: ~s is not a cons"), cons, nao);
   }
@@ -1061,6 +1062,7 @@ val nullify(val seq)
       if (nullify_meth)
         return funcall1(nullify_meth, seq);
     }
+    /* fallthrough */
   default:
     return seq;
   }
@@ -1107,6 +1109,7 @@ again:
       replace_obj(tailobj, items, t, t);
       return ptail;
     }
+    /* fallthrough */
   default:
     uw_throwf(error_s, lit("cannot append ~s"), deref(ptail), nao);
   }
@@ -4568,8 +4571,10 @@ val int_str(val str, val base)
     case 'x': case 'X':
       zerox = 1;
       wcs += 2;
+      break;
     default:
       octzero = 1;
+      break;
     }
     break;
   }
@@ -10415,6 +10420,7 @@ val empty(val seq)
         return eq(funcall1(length_meth, seq), zero);
       return if3(nullify_meth && funcall1(nullify_meth, seq), nil, seq);
     }
+    /* fallthrough */
   default:
     type_mismatch(lit("empty: ~s is not a sequence"), seq, nao);
   }
diff --git a/linenoise/linenoise.c b/linenoise/linenoise.c
index 75721075..5418aeda 100644
--- a/linenoise/linenoise.c
+++ b/linenoise/linenoise.c
@@ -2212,8 +2212,8 @@ static int edit(lino_t *l, const wchar_t *prompt)
                     }
                     break;
                 }
-                /* fallthrough */
                 l->save_hist_idx = l->history_index;
+                /* fallthrough */
             case CTL('F'):
                 ret = l->len;
                 if (l->mlmode)
diff --git a/sysif.c b/sysif.c
index dd563b7b..a3b3ff91 100644
--- a/sysif.c
+++ b/sysif.c
@@ -895,6 +895,7 @@ static val fcntl_wrap(val fd_in, val cmd_in, val arg_in)
       if (cmd == F_GETLK)
         flock_unpack(arg_in, &fl);
     }
+    break;
   default:
     errno = EINVAL;
     break;
diff --git a/unwind.h b/unwind.h
index 768e71e1..ad438ca9 100644
--- a/unwind.h
+++ b/unwind.h
@@ -150,7 +150,7 @@ extern "C" {
 #endif
 
 int jmp_save(struct jmp *);
-void jmp_restore(struct jmp *, int);
+noreturn void jmp_restore(struct jmp *, int);
 
 #ifdef __cplusplus
 }
diff --git a/utf8.c b/utf8.c
index 8afae187..9c286756 100644
--- a/utf8.c
+++ b/utf8.c
@@ -90,10 +90,10 @@ size_t utf8_from_buf(wchar_t *wdst, const unsigned char *src, size_t nbytes)
           wch_min = 0x10000;
           break;
         }
-        /* fallthrough */
 #else
         conversion_error();
 #endif
+        /* fallthrough */
       default:
         if (wdst)
           *wdst++ = 0xDC00 | ch;
@@ -321,10 +321,10 @@ wint_t utf8_decode(utf8_decoder_t *ud, int (*get)(mem_t *ctx), mem_t *ctx)
           ud->wch_min = 0x10000;
           break;
         }
-        /* fallthrough */
 #else
         conversion_error();
 #endif
+        /* fallthrough */
       default:
         ud->back = ud->tail;
         return 0xDC00 | ch;