63-bit fixnums on 64-bit targets.

[sbcl.git] / src / compiler / generic / early-vm.lisp

diff --git a/src/compiler/generic/early-vm.lisp b/src/compiler/generic/early-vm.lisp
index 8810860..304637d 100644 (file)
--- a/src/compiler/generic/early-vm.lisp
+++ b/src/compiler/generic/early-vm.lisp
@@ -19,7 +19,24 @@
 ;;; pointer
 (def!constant lowtag-limit (ash 1 n-lowtag-bits))
 ;;; the number of tag bits used for a fixnum
-(def!constant n-fixnum-tag-bits (1- n-lowtag-bits))
+(def!constant n-fixnum-tag-bits
+    (if (= 64 sb!vm:n-word-bits)
+        ;; On 64-bit targets, this may be as low as 1 (for 63-bit
+        ;; fixnums) and as high as 3 (for 61-bit fixnums).  The
+        ;; constraint on the low end is that we need at least one bit
+        ;; to determine if a value is a fixnum or not, and the
+        ;; constraint on the high end is that it must not exceed
+        ;; WORD-SHIFT (defined below) due to the use of unboxed
+        ;; word-aligned byte pointers as boxed values in various
+        ;; places.  FIXME: This should possibly be exposed for
+        ;; configuration via customize-target-features.
+        1
+        ;; On 32-bit targets, this may be as low as 2 (for 30-bit
+        ;; fixnums) and as high as 2 (for 30-bit fixnums).  The
+        ;; constraint on the low end is simple overcrowding of the
+        ;; lowtag space, and the constraint on the high end is that it
+        ;; must not exceed WORD-SHIFT.
+        (1- n-lowtag-bits)))
 ;;; the fixnum tag mask
 (def!constant fixnum-tag-mask (1- (ash 1 n-fixnum-tag-bits)))
 ;;; the bit width of fixnums