From a60e78be5dfc2172345d140f2cd8df0a77bea0a1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mattias=20Engdeg=C3=A5rd?= <address@hidden>
Date: Thu, 28 Mar 2019 22:12:37 +0100
Subject: [PATCH] Add bounds for portable fixnums, and portable-fixnum-p

These are useful for macros that need to detect whether a number is a
fixnum on any machine, so that the bytecode becomes portable (Bug#34781).

* src/data.c (most-positive-portable-fixnum, most-negative-portable-fixnum):
* lisp/subr.el (portable-fixnum-p):
New.
* etc/NEWS (Lisp Changes): Mention portable-fixnum-p.
---
 etc/NEWS     |  5 +++++
 lisp/subr.el |  5 +++++
 src/data.c   | 24 ++++++++++++++++++++++++
 3 files changed, 34 insertions(+)

diff --git a/etc/NEWS b/etc/NEWS
index 7486d6bcfe..2ab3c5b4bb 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1424,6 +1424,11 @@ like 'file-attributes' that compute file sizes and other attributes,
 functions like 'process-id' that compute process IDs, and functions like
 'user-uid' and 'group-gid' that compute user and group IDs.
 
+Since the size of fixnums varies between platforms, the new predicate
+'portable-fixnum-p' determines whether a number is a fixnum on any
+machine running the current Emacs version. The corresponding bounds
+are 'most-negative-portable-fixnum' and 'most-negative-portable-fixnum'.
+
 +++
 ** Although the default timestamp format is still (HI LO US PS),
 it is planned to change in a future Emacs version, to exploit bignums.
diff --git a/lisp/subr.el b/lisp/subr.el
index f1a1dddd81..efa9cd4065 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -376,6 +376,11 @@ bignump
   "Return t if OBJECT is a bignum."
   (and (integerp object) (not (fixnump object))))
 
+(defun portable-fixnum-p (object)
+  "Return t if OBJECT is a fixnum on any machine running this Emacs version."
+  (and (integerp object)
+       (<= most-negative-portable-fixnum object most-positive-portable-fixnum)))
+
 (defun lsh (value count)
   "Return VALUE with its bits shifted left by COUNT.
 If COUNT is negative, shifting is actually to the right.
diff --git a/src/data.c b/src/data.c
index 15b6106cfe..6d50a14bde 100644
--- a/src/data.c
+++ b/src/data.c
@@ -4110,6 +4110,30 @@ This variable cannot be set; trying to do so will signal an error.  */);
   Vmost_negative_fixnum = make_fixnum (MOST_NEGATIVE_FIXNUM);
   make_symbol_constant (intern_c_string ("most-negative-fixnum"));
 
+  /* The smallest portable value of EMACS_INT_MAX.  */
+  int least_emacs_int_max = 2147483647;   /* 2**31 - 1 */
+
+  /* Largest and smallest values that are guaranteed to be representable
+     as fixnums on any machine.  */
+  int most_positive_portable_fixnum = least_emacs_int_max >> INTTYPEBITS;
+  int most_negative_portable_fixnum = -1 - most_positive_portable_fixnum;
+
+  DEFVAR_LISP ("most-positive-portable-fixnum",
+               Vmost_positive_portable_fixnum,
+               doc: /* The largest integer representable as a fixnum on any platform.
+This variable can be used to ensure portability of bytecode that works
+with fixnums.  It cannot be set; trying to do so will signal an error.  */);
+  Vmost_positive_portable_fixnum = make_fixnum (most_positive_portable_fixnum);
+  make_symbol_constant (intern_c_string ("most-positive-portable-fixnum"));
+
+  DEFVAR_LISP ("most-negative-portable-fixnum",
+               Vmost_negative_portable_fixnum,
+               doc: /* The least integer representable as a fixnum on any platform.
+This variable can be used to ensure portability of bytecode that works
+with fixnums.  It cannot be set; trying to do so will signal an error.  */);
+  Vmost_negative_portable_fixnum = make_fixnum (most_negative_portable_fixnum);
+  make_symbol_constant (intern_c_string ("most-negative-portable-fixnum"));
+
   DEFSYM (Qwatchers, "watchers");
   DEFSYM (Qmakunbound, "makunbound");
   DEFSYM (Qunlet, "unlet");
-- 
2.20.1