diff --git a/module/Makefile.am b/module/Makefile.am
index 587d7b5ce..65f7f5a94 100644
--- a/module/Makefile.am
+++ b/module/Makefile.am
@@ -158,6 +158,7 @@ CPS2_LANG_SOURCES =						\
   language/cps2/optimize.scm					\
   language/cps2/simplify.scm					\
   language/cps2/spec.scm					\
+  language/cps2/specialize-primcalls.scm			\
   language/cps2/types.scm					\
   language/cps2/utils.scm					\
   language/cps2/with-cps.scm
diff --git a/module/language/cps2/optimize.scm b/module/language/cps2/optimize.scm
index bfc43c122..d6400ed4c 100644
--- a/module/language/cps2/optimize.scm
+++ b/module/language/cps2/optimize.scm
@@ -29,6 +29,7 @@
   #:use-module (language cps2 dce)
   #:use-module (language cps2 prune-top-level-scopes)
   #:use-module (language cps2 simplify)
+  #:use-module (language cps2 specialize-primcalls)
   #:export (optimize))
 
 (define (kw-arg-ref args kw default)
@@ -60,7 +61,7 @@
   (run-pass! simplify #:simplify? #t)
   (run-pass! contify #:contify? #t)
   (run-pass! inline-constructors #:inline-constructors? #t)
-  ;; (run-pass! specialize-primcalls #:specialize-primcalls? #t)
+  (run-pass! specialize-primcalls #:specialize-primcalls? #t)
   ;; (run-pass! elide-values #:elide-values? #t)
   ;; (run-pass! prune-bailouts #:prune-bailouts? #t)
   ;; (run-pass! eliminate-common-subexpressions #:cse? #t)
diff --git a/module/language/cps2/specialize-primcalls.scm b/module/language/cps2/specialize-primcalls.scm
new file mode 100644
index 000000000..00d2149d7
--- /dev/null
+++ b/module/language/cps2/specialize-primcalls.scm
@@ -0,0 +1,59 @@
+;;; Continuation-passing style (CPS) intermediate language (IL)
+
+;; Copyright (C) 2013, 2014, 2015 Free Software Foundation, Inc.
+
+;;;; This library is free software; you can redistribute it and/or
+;;;; modify it under the terms of the GNU Lesser General Public
+;;;; License as published by the Free Software Foundation; either
+;;;; version 3 of the License, or (at your option) any later version.
+;;;;
+;;;; This library is distributed in the hope that it will be useful,
+;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;;; Lesser General Public License for more details.
+;;;;
+;;;; You should have received a copy of the GNU Lesser General Public
+;;;; License along with this library; if not, write to the Free Software
+;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+;;; Commentary:
+;;;
+;;; Some bytecode operations can encode an immediate as an operand.
+;;; This pass tranforms generic primcalls to these specialized
+;;; primcalls, if possible.
+;;;
+;;; Code:
+
+(define-module (language cps2 specialize-primcalls)
+  #:use-module (ice-9 match)
+  #:use-module (language cps2)
+  #:use-module (language cps2 utils)
+  #:use-module (language cps intmap)
+  #:export (specialize-primcalls))
+
+(define (specialize-primcalls conts)
+  (let ((constants (compute-constant-values conts)))
+    (define (immediate-u8? var)
+      (let ((val (intmap-ref constants var (lambda (_) #f))))
+        (and (exact-integer? val) (<= 0 val 255))))
+    (define (specialize-primcall name args)
+      (match (cons name args)
+        (('make-vector (? immediate-u8? n) init) 'make-vector/immediate)
+        (('vector-ref v (? immediate-u8? n)) 'vector-ref/immediate)
+        (('vector-set! v (? immediate-u8? n) x) 'vector-set!/immediate)
+        (('allocate-struct v (? immediate-u8? n)) 'allocate-struct/immediate)
+        (('struct-ref s (? immediate-u8? n)) 'struct-ref/immediate)
+        (('struct-set! s (? immediate-u8? n) x) 'struct-set!/immediate)
+        (_ #f)))
+    (intmap-map
+     (lambda (label cont)
+       (match cont
+         (($ $kargs names vars ($ $continue k src ($ $primcall name args)))
+          (let ((name* (specialize-primcall name args)))
+            (if name*
+                (build-cont
+                  ($kargs names vars
+                    ($continue k src ($primcall name* args))))
+                cont)))
+         (_ cont)))
+     conts)))