------------------------------------------------------------
revno: 107377
committer: Paul Eggert <eggert@xxxxxxxxxxx>
branch nick: trunk
timestamp: Wed 2012-02-22 07:55:46 -0800
message:
* src/lisp.h: Improve comment about USE_LSB_TAG.
modified:
src/lisp.h
=== modified file 'src/lisp.h'
--- a/src/lisp.h 2012-02-20 23:09:58 +0000
+++ b/src/lisp.h 2012-02-22 15:55:46 +0000
@@ -195,12 +195,25 @@
|| defined DARWIN_OS || defined __sun)
/* We also need to be able to specify mult-of-8 alignment on static vars. */
# if defined DECL_ALIGN
-/* mark_maybe_object assumes that EMACS_INT values are contiguous,
- but this is not true on some hosts where EMACS_INT is wider than a pointer,
- as they may allocate the halves of an EMACS_INT separately.
- On these hosts USE_LSB_TAG is not needed because the top bits of an
- EMACS_INT are unused, so define USE_LSB_TAG only on hosts where it
- might be useful. */
+/* On hosts where VALBITS is greater than the pointer width in bits,
+ USE_LSB_TAG is:
+
+ a. unnecessary, because the top bits of an EMACS_INT are unused,
+
+ b. slower, because it typically requires extra masking, and
+
+ c. harmful, because it can create Lisp_Object values that are so scrambled
+ that mark_maybe_object cannot decipher them. mark_maybe_object assumes
+ that EMACS_INT values are contiguous, but a host where EMACS_INT is
+ wider than a pointer might allocate the top half of an EMACS_INT in
+ (say) a 32-bit word on the stack, putting the bottom half in a 32-bit
+ register that is saved elsewhere in a jmp_buf. When this happens,
+ since USE_LSB_TAG is not defined the bottom half alone is a valid
+ pointer that mark_maybe_pointer can follow; but if USE_LSB_TAG were
+ defined, the bottom half would not be a valid pointer and neither
+ mark_maybe_object nor mark_maybe_pointer would follow it.
+
+ So, define USE_LSB_TAG only on hosts where it might be useful. */
# if UINTPTR_MAX >> VALBITS != 0
# define USE_LSB_TAG
# endif

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