Add DOHASH_SAFE, make DOHASH faster (bug#68690)

Revert DOHASH to the fast (field-caching) implementation but with
an assertion to detect misuses.  Add DOHASH_SAFE for use in
code that must tolerate arbitrary mutation of the table being
iterated through.

* src/lisp.h (DOHASH): Go back to fast design that only allows
restricted mutation, but with a checking assertion.
(DOHASH_SAFE): New macro that tolerates arbitrary mutation while being
much simpler (and acceptably fast).
* src/fns.c (Fmaphash):
* src/comp.c (compile_function, Fcomp__compile_ctxt_to_file):
Use DOHASH_SAFE.

1 files changed, 30 insertions, 24 deletions

diff --git a/src/lisp.h b/src/lisp.h
index d07d9d14e2f..c2dfd1afad5 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -2604,32 +2604,38 @@ hash_from_key (struct Lisp_Hash_Table *h, Lisp_Object key)
 }
 
 /* Iterate K and V as key and value of valid entries in hash table H.
-   The body may mutate the hash-table.  */
-#define DOHASH(h, k, v)							 \
-  for (Lisp_Object *dohash_##k##_##v##_base = (h)->key_and_value,	 \
-                   *dohash_##k##_##v##_kv   = dohash_##k##_##v##_base,	 \
-                   *dohash_##k##_##v##_end  = dohash_##k##_##v##_base	 \
-                                              + 2 * HASH_TABLE_SIZE (h), \
-                   k, v;						 \
-       dohash_##k##_##v##_kv < dohash_##k##_##v##_end			 \
-       && (dohash_##k##_##v##_base == (h)->key_and_value                 \
-           /* The `key_and_value` table has been reallocated!  */        \
-           || (dohash_##k##_##v##_kv                                     \
-                  = (dohash_##k##_##v##_kv - dohash_##k##_##v##_base)	 \
-                    + (h)->key_and_value,                                \
-               dohash_##k##_##v##_base = (h)->key_and_value,             \
-               dohash_##k##_##v##_end  = dohash_##k##_##v##_base	 \
-                                         + 2 * HASH_TABLE_SIZE (h),      \
-               /* Check again, in case the table has shrunk.  */         \
-               dohash_##k##_##v##_kv < dohash_##k##_##v##_end))          \
-       && (k = dohash_##k##_##v##_kv[0],                                 \
-           v = dohash_##k##_##v##_kv[1], /*maybe unused*/ (void)v,       \
-           true);			                                 \
-        dohash_##k##_##v##_kv += 2)				         \
-    if (hash_unused_entry_key_p (k))				         \
-      ;								         \
+   The body may remove the current entry or alter its value slot, but not
+   mutate TABLE in any other way.  */
+#define DOHASH(h, k, v)							\
+  for (Lisp_Object *dohash_##k##_##v##_kv = (h)->key_and_value,		\
+                   *dohash_##k##_##v##_end = dohash_##k##_##v##_kv	\
+                                             + 2 * HASH_TABLE_SIZE (h),	\
+	           *dohash_##k##_##v##_base = dohash_##k##_##v##_kv,	\
+                   k, v;						\
+       dohash_##k##_##v##_kv < dohash_##k##_##v##_end			\
+       && (k = dohash_##k##_##v##_kv[0],				\
+           v = dohash_##k##_##v##_kv[1], /*maybe unused*/ (void)v,      \
+           true);			                                \
+       eassert (dohash_##k##_##v##_base == (h)->key_and_value		\
+		&& dohash_##k##_##v##_end				\
+		   == dohash_##k##_##v##_base				\
+	              + 2 * HASH_TABLE_SIZE (h)),			\
+       dohash_##k##_##v##_kv += 2)					\
+    if (hash_unused_entry_key_p (k))					\
+      ;									\
     else
 
+/* Iterate I as index of valid entries in hash table H.
+   Unlike DOHASH, this construct copes with arbitrary table mutations
+   in the body.  The consequences of such mutations are limited to
+   whether and in what order entries are encountered by the loop
+   (which is usually bad enough), but not crashing or corrupting the
+   Lisp state.  */
+#define DOHASH_SAFE(h, i)					\
+  for (ptrdiff_t i = 0; i < HASH_TABLE_SIZE (h); i++)		\
+    if (hash_unused_entry_key_p (HASH_KEY (h, i)))		\
+      ;								\
+    else
 
 void hash_table_thaw (Lisp_Object hash_table);