X-Git-Url: http://repo.macrolet.net/gitweb/?a=blobdiff_plain;f=src%2Fcompiler%2Fgeneric%2Fearly-objdef.lisp;h=fb4deda0b41dd0aa279f8e3b1e1b23dca9d2485e;hb=e049902f5e7c30501d2dbb7a41d058a0c717fc1f;hp=b40ee3a75fdd2f3763263ffd20d54fddadb36163;hpb=f143939b1dbaf38ebd4f92c851fbc4ecddf37af1;p=sbcl.git

diff --git a/src/compiler/generic/early-objdef.lisp b/src/compiler/generic/early-objdef.lisp
index b40ee3a..fb4deda 100644
--- a/src/compiler/generic/early-objdef.lisp
+++ b/src/compiler/generic/early-objdef.lisp
@@ -14,21 +14,37 @@
 ;;; FIXME: It's clever using :SUFFIX -TYPE for these things, but it's
 ;;; a pain for people just learning to find their way around the code
 ;;; who want to use lexical search to figure out where things like
-;;; EVEN-FIXNUM-TYPE are defined. Remove the :SUFFIXes and just expand
-;;; out the full names. Or even define them in DEF-FROB EVEN-FIXNUM-TYPE
-;;; style so searches like 'def.*even-fixnum-type' can find them.
+;;; EVEN-FIXNUM-LOWTAG are defined. Remove the :SUFFIXes and just expand
+;;; out the full names. Or even define them in DEF EVEN-FIXNUM-LOWTAG
+;;; style so searches like 'def.*even-fixnum-lowtag' can find them.
 
-;;; the main types. These types are represented by the low three bits
-;;; of the pointer or immediate object.
+;;; Tags for the main low-level types are stored in the low three
+;;; bits to identify the type of a machine word.  Certain constraints 
+;;; apply:
+;;;   * EVEN-FIXNUM-LOWTAG and ODD-FIXNUM-LOWTAG must be 0 and 4: code
+;;;     which shifts left two places to convert raw integers to tagged
+;;;     fixnums is ubiquitous.
+;;;   * LIST-POINTER-LOWTAG + 4 = OTHER-POINTER-LOWTAG: NIL is both a
+;;;     cons and a symbol (at the same address) and depends on this.
+;;;     See the definition of SYMBOL in objdef.lisp
+;;;   * OTHER-POINTER-LOWTAG > 4: Some code in the SPARC backend,
+;;;     which uses bit 2 of the ALLOC register to indicate that
+;;;     PSEUDO-ATOMIC is on, doesn't strip the low bits of reg_ALLOC
+;;;     before ORing in OTHER-POINTER-LOWTAG within a PSEUDO-ATOMIC
+;;;     section.
+;;; (These are just the ones we know about as of sbcl-0.7.1.22. There
+;;; might easily be more, since these values have stayed highly
+;;; constrained for more than a decade, an inviting target for
+;;; inventive abstraction-phobic maintainers.:-)
 (eval-when (:compile-toplevel :load-toplevel :execute)
   ;; The EVAL-WHEN is necessary (at least for Lispworks), because the
-  ;; second DEFENUM uses the value of OTHER-IMMEDIATE-0-TYPE, which is
+  ;; second DEFENUM uses the value of OTHER-IMMEDIATE-0-LOWTAG, which is
   ;; defined in the first DEFENUM. -- AL 20000216
-  (defenum (:suffix -type)
+  (defenum (:suffix -lowtag)
     even-fixnum
-    ;; Note: CMU CL, and SBCL < 0.pre7.39, had FUN-POINTER-TYPE
-    ;; here. We swapped FUN-POINTER-TYPE and
-    ;; INSTANCE-POINTER-TYPE in sbcl-0.pre7.39 in order to help with a
+    ;; Note: CMU CL, and SBCL < 0.pre7.39, had FUN-POINTER-LOWTAG
+    ;; here. We swapped FUN-POINTER-LOWTAG and
+    ;; INSTANCE-POINTER-LOWTAG in sbcl-0.pre7.39 in order to help with a
     ;; low-level pun in the function call sequence on the PPC port.
     ;; For more information, see the PPC port code. -- WHN 2001-10-03
     instance-pointer
@@ -39,11 +55,12 @@
     other-immediate-1
     other-pointer))
 
-;;; the heap types. Each of these types is in the header of objects in
-;;; the heap.
-(defenum (:suffix -type
-	  :start (+ (ash 1 lowtag-bits) other-immediate-0-type)
-	  :step (ash 1 (1- lowtag-bits)))
+;;; 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)
+(defenum (:suffix -widetag
+	  :start (+ (ash 1 n-lowtag-bits) other-immediate-0-lowtag)
+	  :step (ash 1 (1- n-lowtag-bits)))
   bignum
   ratio
   single-float