+(def!constant nil-value
+ (+ static-space-start n-word-bytes other-pointer-lowtag))
+
+;;; the heap types, stored in 8 bits of the header of an object on the
+;;; heap, to identify the type of the heap object (which'll be at
+;;; least two machine words, often more)
+;;;
+;;; Note: the order specified here is not critical for correctness,
+;;; but (FIXME) with %TEST-HEADERS as currently defined, BIGNUM must
+;;; be first, and COMPLEX-ARRAY must be last.
+;;;
+;;; However, for efficiency, we prefer contiguous sets of widetags for
+;;; "similar" objects, so that type checking can be done with a range
+;;; check, rather than several individual checks.
+;;;
+;;; * BIGNUM + RATIO (+ FIXNUM) = RATIONAL
+;;;
+;;; * SINGLE-FLOAT + DOUBLE-FLOAT + LONG-FLOAT = FLOAT
+;;;
+;;; * RATIONAL + FLOAT = REAL
+;;;
+;;; * (FIXME: COMPLEX example, which needs fixing anyway -- see
+;;; UPGRADED-COMPLEX-PART-TYPE)
+;;;
+;;; * SIMPLE-ARRAY-* = (SIMPLE-ARRAY * (*))
+;;;
+;;; * SIMPLE-ARRAY-NIL + SIMPLE-BASE-STRING = SIMPLE-STRING
+;;;
+;;; * SIMPLE-ARRAY + COMPLEX-ARRAYOID = (SATISFIES ARRAY-HEADER-P)
+;;;
+;;; In addition, with
+;;; sufficient care we can cause extra combinations to appear with
+;;; differences in only one bit, permitting a more efficient type
+;;; test. As an example, if SIMPLE-BASE-STRING = 0xA6 and
+;;; COMPLEX-BASE-STRING = 0xE6, then the type test for BASE-STRING is
+;;;
+;;; AND tag, ~0x40, tag
+;;; ANDcc tag, 0xA6, tag
+;;; JNE tag, label
+;;;
+;;; rather than two separate tests and jumps
+(defenum (:suffix -widetag
+ ;; The first widetag must be greater than SB!VM:LOWTAG-LIMIT
+ ;; otherwise code in generic/early-type-vops will suffer
+ ;; a long, horrible death. --njf, 2004-08-09
+ :start (+ (ash 1 n-lowtag-bits) other-immediate-0-lowtag)
+ :step 4)
+ ;; NOTE: the binary numbers off to the side are only valid for 32-bit
+ ;; ports; add #b1000 if you want to know the values for 64-bit ports.
+ ;; And note that the numbers get a little scrambled further down.
+ ;; --njf, 2004-08-09
+ bignum ; 00001010
+ ratio ; 00001110
+ single-float ; 00010010
+ double-float ; 00010110
+ complex ; 00011010
+ complex-single-float ; 00011110
+ complex-double-float ; 00100010
+
+ code-header ; 00100110
+
+ simple-fun-header ; 00101010
+ closure-header ; 00101110
+ funcallable-instance-header ; 00110010
+
+ return-pc-header ; 00110110
+ value-cell-header ; 00111010
+ symbol-header ; 00111110
+ character ; 01000010
+ sap ; 01000110
+ unbound-marker ; 01001010
+ weak-pointer ; 01001110
+ instance-header ; 01010010
+ fdefn ; 01010110
+
+ no-tls-value-marker ; 01011010
+ #!-(and sb-lutex sb-thread)
+ unused01
+ #!+(and sb-lutex sb-thread)
+ lutex ; 01011110
+ unused02 ; 01100010
+ unused03 ; 01100110
+ unused04 ; 01101010
+ unused05 ; 01101110
+ unused06 ; 01110010
+ unused07 ; 01110110
+ #!+#.(cl:if (cl:= 32 sb!vm:n-word-bits) '(and) '(or))
+ unused08 ; 01111010
+ #!+#.(cl:if (cl:= 32 sb!vm:n-word-bits) '(and) '(or))
+ unused09 ; 01111110