Slight switch byte op speedup

* src/bytecode.c (exec_byte_code): Hoist symbols_with_pos_enabled check
from fast loop, and eliminate the initial index check.

1 files changed, 19 insertions, 18 deletions

diff --git a/src/bytecode.c b/src/bytecode.c
index dd805cbd97a..8d7240b9966 100644
--- a/src/bytecode.c
+++ b/src/bytecode.c
@@ -1737,28 +1737,29 @@ exec_byte_code (Lisp_Object fun, ptrdiff_t args_template,
             if (BYTE_CODE_SAFE && !HASH_TABLE_P (jmp_table))
               emacs_abort ();
             Lisp_Object v1 = POP;
-            ptrdiff_t i;
             struct Lisp_Hash_Table *h = XHASH_TABLE (jmp_table);
-
-            /* h->count is a faster approximation for HASH_TABLE_SIZE (h)
-               here. */
-            if (h->count <= 5 && !h->test->cmpfn)
-              { /* Do a linear search if there are not many cases
-                   FIXME: 5 is arbitrarily chosen.  */
-                for (i = h->count; 0 <= --i; )
-                  if (EQ (v1, HASH_KEY (h, i)))
-                    break;
+            /* Do a linear search if there are few cases and the test is `eq'.
+               (The table is assumed to be sized exactly; all entries are
+               consecutive at the beginning.)
+               FIXME: 5 is arbitrarily chosen.  */
+            if (h->count <= 5 && !h->test->cmpfn && !symbols_with_pos_enabled)
+              {
+                eassume (h->count >= 2);
+                for (ptrdiff_t i = h->count - 1; i >= 0; i--)
+                  if (BASE_EQ (v1, HASH_KEY (h, i)))
+                    {
+                      op = XFIXNUM (HASH_VALUE (h, i));
+                      goto op_branch;
+                    }
               }
             else
-              i = hash_lookup (h, v1);
-
-            if (i >= 0)
               {
-                Lisp_Object val = HASH_VALUE (h, i);
-                if (BYTE_CODE_SAFE && !FIXNUMP (val))
-                  emacs_abort ();
-                op = XFIXNUM (val);
-                goto op_branch;
+                ptrdiff_t i = hash_lookup (h, v1);
+                if (i >= 0)
+                  {
+                    op = XFIXNUM (HASH_VALUE (h, i));
+                    goto op_branch;
+                  }
               }
           }
           NEXT;