From: Christophe Rhodes <c.rhodes@gold.ac.uk>
Date: Mon, 30 Sep 2013 14:46:26 +0000 (+0100)
Subject: improve SXHASH on fixnums
X-Git-Url: http://repo.macrolet.net/gitweb/?a=commitdiff_plain;h=236acc699ec1b5996e04f3f30d2859803e54e5ea;p=sbcl.git

improve SXHASH on fixnums

Remove one redundant LOGAND; also, use the whole of the fixnum
range on 64-bit platforms.
---

diff --git a/NEWS b/NEWS
index 4bdd1df..cd406a0 100644
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,8 @@
 changes relative to sbcl-1.1.12:
   * optimization: better distribution of SXHASH over small conses of related
     values.  (lp#309443)
+  * other improvements to SXHASH:
+    ** use the whole of the positive-fixnum range for SXHASH of fixnums
 
 changes in sbcl-1.1.12 relative to sbcl-1.1.11:
   * enhancement: Add sb-bsd-sockets:socket-shutdown, for calling
diff --git a/src/code/sxhash.lisp b/src/code/sxhash.lisp
index abe8199..6b09f00 100644
--- a/src/code/sxhash.lisp
+++ b/src/code/sxhash.lisp
@@ -38,10 +38,9 @@
 ;;; SXHASH of FIXNUM values is defined as a DEFTRANSFORM because it's so
 ;;; simple.
 (deftransform sxhash ((x) (fixnum))
-  '(logand most-positive-fixnum
-           (logxor (ash (logand x (ash most-positive-fixnum -4)) 4)
-                   (logand (ash x -1) most-positive-fixnum) ; to get sign bit into hash
-                   361475658)))
+  (let ((c (logand 1193941380939624010 sb!xc:most-positive-fixnum)))
+    ;; shift by -1 to get sign bit into hash
+    `(logand (logxor (ash x 4) (ash x -1) ,c) sb!xc:most-positive-fixnum)))
 
 ;;; SXHASH of SIMPLE-BIT-VECTOR values is defined as a DEFTRANSFORM
 ;;; because it is endian-dependent.