From 8f17560fa2aede38729419792e0e6f5f09dfd040 Mon Sep 17 00:00:00 2001
From: Douglas Katzman <dougk@google.com>
Date: Thu, 14 Sep 2017 12:58:06 -0400
Subject: [PATCH] Reserve 1 bit in bignum header for GC mark

---
 contrib/sb-gmp/gmp.lisp           |  4 ++--
 src/code/bignum.lisp              |  5 +----
 src/compiler/generic/vm-type.lisp | 12 ++++++------
 3 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/contrib/sb-gmp/gmp.lisp b/contrib/sb-gmp/gmp.lisp
index c506c25b8..d054e29c6 100644
--- a/contrib/sb-gmp/gmp.lisp
+++ b/contrib/sb-gmp/gmp.lisp
@@ -330,7 +330,7 @@ pre-allocated bignum. The allocated bignum-length must be (1+ COUNT)."
 (defmacro with-mpz-results (pairs &body body)
   (loop for (gres size) in pairs
         for res = (gensym "RESULT")
-        collect `(when (> ,size sb-bignum::maximum-bignum-length)
+        collect `(when (> ,size sb-kernel::maximum-bignum-length)
                    (error "Size of result exceeds maxim bignum length")) into checks
         collect `(,gres (struct gmpint)) into declares
         collect `(,res (%allocate-bignum ,size))
@@ -390,7 +390,7 @@ pre-allocated bignum. The allocated bignum-length must be (1+ COUNT)."
         collect `(__gmpz_init (addr ,gres)) into inits
         collect `(,size (abs (slot ,gres 'mp_size)))
           into resinits
-        collect `(when (> ,size (1- sb-bignum::maximum-bignum-length))
+        collect `(when (> ,size (1- sb-kernel::maximum-bignum-length))
                    (error "Size of result exceeds maxim bignum length")) into checks
         collect `(,res (%allocate-bignum (1+ ,size)))
           into resallocs
diff --git a/src/code/bignum.lisp b/src/code/bignum.lisp
index f101ff8a6..722fea6d8 100644
--- a/src/code/bignum.lisp
+++ b/src/code/bignum.lisp
@@ -107,9 +107,6 @@
 
 (defconstant digit-size sb!vm:n-word-bits)
 
-(defconstant maximum-bignum-length (1- (ash 1 (- sb!vm:n-word-bits
-                                                 sb!vm:n-widetag-bits))))
-
 (defconstant all-ones-digit (1- (ash 1 sb!vm:n-word-bits)))
 
 ;;;; internal inline routines
@@ -1071,7 +1068,7 @@
     (multiple-value-bind (digits n-bits) (truncate x digit-size)
       (let* ((bignum-len (or bignum-len (%bignum-length bignum)))
              (res-len (+ digits bignum-len 1)))
-        (when (> res-len maximum-bignum-length)
+        (when (> res-len sb!kernel::maximum-bignum-length)
           (error "can't represent result of left shift"))
         (if (zerop n-bits)
           (bignum-ashift-left-digits bignum bignum-len digits)
diff --git a/src/compiler/generic/vm-type.lisp b/src/compiler/generic/vm-type.lisp
index 10d85999c..8ce85d400 100644
--- a/src/compiler/generic/vm-type.lisp
+++ b/src/compiler/generic/vm-type.lisp
@@ -69,12 +69,12 @@
     #.(- 62 (floor (log sb!xc:internal-time-units-per-second 2)))))
 
 (sb!xc:deftype bignum-element-type () 'sb!vm:word)
-;;; FIXME: see also DEFCONSTANT MAXIMUM-BIGNUM-LENGTH in
-;;; src/code/bignum.lisp.  -- CSR, 2004-07-19
-(sb!xc:deftype bignum-index ()
-  '(mod #.(1- (ash 1 (- sb!vm:n-word-bits sb!vm:n-widetag-bits)))))
-(sb!xc:deftype bignum-length ()
-  '(unsigned-byte #.(- sb!vm:n-word-bits sb!vm:n-widetag-bits)))
+(defconstant maximum-bignum-length
+  ;; Compute number of bits in the maximum length's representation
+  ;; leaving one bit for a GC mark bit.
+  (ldb (byte (- sb!vm:n-word-bits sb!vm:n-widetag-bits 1) 0) -1))
+(sb!xc:deftype bignum-index () `(mod ,maximum-bignum-length))
+(sb!xc:deftype bignum-length () `(mod ,(1+ maximum-bignum-length)))
 
 ;;; an index into an integer
 (sb!xc:deftype bit-index ()
-- 
2.11.4.GIT