Fix overenthusiastic header size check

Problem reported by Eli Zaretskii in:
https://lists.gnu.org/r/emacs-devel/2018-09/msg00222.html
* doc/lispref/internals.texi (Garbage Collection):
Document vector sizes and slot counts more accurately.
* src/lisp.h: Omit header_size sanity check that was too picky.
Add some less-picky checks.

2 files changed, 22 insertions, 8 deletions

diff --git a/doc/lispref/internals.texi b/doc/lispref/internals.texi
index 3fe28446eaf..d42e2444e68 100644
--- a/doc/lispref/internals.texi
+++ b/doc/lispref/internals.texi
@@ -382,7 +382,7 @@ This is used for convenience and equals to @code{sizeof (char)}.
 The total size of all string data in bytes.
 
 @item vector-size
-Internal size of a vector header, i.e., @code{sizeof (struct Lisp_Vector)}.
+Size in bytes of a vector of length 1, including its header.
 
 @item used-vectors
 The number of vector headers allocated from the vector blocks.
@@ -392,6 +392,8 @@ Internal size of a vector slot, always equal to @code{sizeof (Lisp_Object)}.
 
 @item used-slots
 The number of slots in all used vectors.
+Slot counts might include some or all overhead from vector headers,
+depending on the platform.
 
 @item free-slots
 The number of free slots in all vector blocks.
diff --git a/src/lisp.h b/src/lisp.h
index 7e365e8f478..56623a75f74 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -1619,7 +1619,16 @@ struct Lisp_Bool_Vector
   } GCALIGNED_STRUCT;
 
 /* Some handy constants for calculating sizes
-   and offsets, mostly of vectorlike objects.   */
+   and offsets, mostly of vectorlike objects.
+
+   The garbage collector assumes that the initial part of any struct
+   that starts with a union vectorlike_header followed by N
+   Lisp_Objects (some possibly in arrays and/or a trailing flexible
+   array) will be laid out like a struct Lisp_Vector with N
+   Lisp_Objects.  This assumption is true in practice on known Emacs
+   targets even though the C standard does not guarantee it.  This
+   header contains a few sanity checks that should suffice to detect
+   violations of this assumption on plausible practical hosts.  */
 
 enum
   {
@@ -1627,7 +1636,6 @@ enum
     bool_header_size = offsetof (struct Lisp_Bool_Vector, data),
     word_size = sizeof (Lisp_Object)
   };
-verify (header_size == sizeof (union vectorlike_header));
 
 /* The number of data words and bytes in a bool vector with SIZE bits.  */
 
@@ -1989,6 +1997,13 @@ enum char_table_specials
     SUB_CHAR_TABLE_OFFSET = PSEUDOVECSIZE (struct Lisp_Sub_Char_Table, contents)
   };
 
+/* Sanity-check pseudovector layout.  */
+verify (offsetof (struct Lisp_Char_Table, defalt) == header_size);
+verify (offsetof (struct Lisp_Char_Table, extras)
+        == header_size + CHAR_TABLE_STANDARD_SLOTS * sizeof (Lisp_Object));
+verify (offsetof (struct Lisp_Sub_Char_Table, contents)
+        == header_size + SUB_CHAR_TABLE_OFFSET * sizeof (Lisp_Object));
+
 /* Return the number of "extra" slots in the char table CT.  */
 
 INLINE int
@@ -1998,11 +2013,6 @@ CHAR_TABLE_EXTRA_SLOTS (struct Lisp_Char_Table *ct)
           - CHAR_TABLE_STANDARD_SLOTS);
 }
 
-/* Make sure that sub char-table contents slot is where we think it is.  */
-verify (offsetof (struct Lisp_Sub_Char_Table, contents)
-        == (offsetof (struct Lisp_Vector, contents)
-            + SUB_CHAR_TABLE_OFFSET * sizeof (Lisp_Object)));
-
 
 /* Save and restore the instruction and environment pointers,
    without affecting the signal mask.  */
@@ -2216,6 +2226,8 @@ struct Lisp_Hash_Table
   struct Lisp_Hash_Table *next_weak;
 } GCALIGNED_STRUCT;
 
+/* Sanity-check pseudovector layout.  */
+verify (offsetof (struct Lisp_Hash_Table, weak) == header_size);
 
 INLINE bool
 HASH_TABLE_P (Lisp_Object a)