;;; 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