master b1881d7dab5 1/2: More accurate static vector block size assertion

[Mattias Engdegård]

branch: master
commit b1881d7dab53b490148b6a19bb6a3fb62f52ad22
Author: Mattias Engdegård <mattiase@acm.org>
Commit: Mattias Engdegård <mattiase@acm.org>

    More accurate static vector block size assertion
    
    * src/alloc.c: The size of a vector block is bound by the number of
    words, not bytes, represented by the pseudovector header RESTSIZE
    field, because that limits how big a PVEC_FREE object can be.
---
 src/alloc.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/alloc.c b/src/alloc.c
index fbb1c6ed6c3..addbb54e01f 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -3054,9 +3054,8 @@ enum { VECTOR_BLOCK_SIZE = 4096 };
 /* Vector size requests are a multiple of this.  */
 enum { roundup_size = COMMON_MULTIPLE (LISP_ALIGNMENT, word_size) };
 
-/* Verify assumptions described above.  */
+/* Verify assumption described above.  */
 verify (VECTOR_BLOCK_SIZE % roundup_size == 0);
-verify (VECTOR_BLOCK_SIZE <= (1 << PSEUDOVECTOR_SIZE_BITS));
 
 /* Round up X to nearest mult-of-ROUNDUP_SIZE --- use at compile time.  */
 #define vroundup_ct(x) ROUNDUP (x, roundup_size)
@@ -3067,6 +3066,11 @@ verify (VECTOR_BLOCK_SIZE <= (1 << 
PSEUDOVECTOR_SIZE_BITS));
 
 enum {VECTOR_BLOCK_BYTES = VECTOR_BLOCK_SIZE - vroundup_ct (sizeof (void *))};
 
+/* The current code expects to be able to represent an unused block by
+   a single PVEC_FREE object, whose size is limited by the header word.
+   (Of course we could use multiple such objects.)  */
+verify (VECTOR_BLOCK_BYTES <= (word_size << PSEUDOVECTOR_REST_BITS));
+
 /* Size of the minimal vector allocated from block.  */
 
 enum { VBLOCK_BYTES_MIN = vroundup_ct (header_size + sizeof (Lisp_Object)) };