From 366c1ceb038b42ed8590486e1a3c3cebba87e9da Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Fri, 28 Nov 2014 23:38:41 -0800
Subject: [PATCH] Improve clarity of USE_LSB_TAG definition.
Problem reported by Lee Duhem. Suggestion by Andreas Schwab in:
http://lists.gnu.org/archive/html/emacs-devel/2014-11/msg02222.html
* lisp.h (USE_LSB_TAG): Define in terms of the (simpler)
VAL_MAX / 2 rather than in terms of the (more complicated)
EMACS_INT_MAX >> GCTYPEBITS, and adjust commentary to match.
---
src/ChangeLog | 9 +++++++++
src/lisp.h | 7 ++++---
2 files changed, 13 insertions(+), 3 deletions(-)
diff --git a/src/ChangeLog b/src/ChangeLog
index ed111bf3814..53834edffa8 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,12 @@
+2014-11-29 Paul Eggert <eggert@cs.ucla.edu>
+
+ Improve clarity of USE_LSB_TAG definition.
+ Problem reported by Lee Duhem. Suggestion by Andreas Schwab in:
+ http://lists.gnu.org/archive/html/emacs-devel/2014-11/msg02222.html
+ * lisp.h (USE_LSB_TAG): Define in terms of the (simpler)
+ VAL_MAX / 2 rather than in terms of the (more complicated)
+ EMACS_INT_MAX >> GCTYPEBITS, and adjust commentary to match.
+
2014-11-29 Eli Zaretskii <eliz@gnu.org>
* xdisp.c (handle_single_display_spec): When ignoring a fringe
diff --git a/src/lisp.h b/src/lisp.h
index bdff0195eac..42bb33704fa 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -257,16 +257,17 @@ enum Lisp_Bits
/* The maximum value that can be stored in a EMACS_INT, assuming all
bits other than the type bits contribute to a nonnegative signed value.
- This can be used in #if, e.g., '#if VAL_MAX < UINTPTR_MAX' below. */
+ This can be used in #if, e.g., '#if USB_TAG' below expands to an
+ expression involving VAL_MAX. */
#define VAL_MAX (EMACS_INT_MAX >> (GCTYPEBITS - 1))
/* Whether the least-significant bits of an EMACS_INT contain the tag.
- On hosts where pointers-as-ints do not exceed VAL_MAX, USE_LSB_TAG is:
+ On hosts where pointers-as-ints do not exceed VAL_MAX / 2, USE_LSB_TAG is:
a. unnecessary, because the top bits of an EMACS_INT are unused, and
b. slower, because it typically requires extra masking.
So, USE_LSB_TAG is true only on hosts where it might be useful. */
DEFINE_GDB_SYMBOL_BEGIN (bool, USE_LSB_TAG)
-#define USE_LSB_TAG (EMACS_INT_MAX >> GCTYPEBITS < INTPTR_MAX)
+#define USE_LSB_TAG (VAL_MAX / 2 < INTPTR_MAX)
DEFINE_GDB_SYMBOL_END (USE_LSB_TAG)
#if !USE_LSB_TAG && !defined WIDE_EMACS_INT
--
2.39.5