X-Git-Url: http://repo.macrolet.net/gitweb/?a=blobdiff_plain;f=src%2Fcompiler%2Fgeneric%2Fearly-objdef.lisp;h=fb4deda0b41dd0aa279f8e3b1e1b23dca9d2485e;hb=24bc431a3403af05c5df601d09c0d0c27cb500b2;hp=18596f410b91a03476e22ec22a8a1cfea1f3b56e;hpb=0c7ffa8fb85a94482814835c9f28abfd0400ab99;p=sbcl.git

diff --git a/src/compiler/generic/early-objdef.lisp b/src/compiler/generic/early-objdef.lisp
index 18596f4..fb4deda 100644
--- a/src/compiler/generic/early-objdef.lisp
+++ b/src/compiler/generic/early-objdef.lisp
@@ -18,8 +18,24 @@
 ;;; out the full names. Or even define them in DEF EVEN-FIXNUM-LOWTAG
 ;;; style so searches like 'def.*even-fixnum-lowtag' can find them.
 
-;;; tags for the main low-level types, to be stored in the low three
-;;; bits to identify the type of a machine word 
+;;; 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-LOWTAG, which is