bug#65491: [PATCH] Improve performance allocating vectors

["Mattias Engdegård" <mattias.engdegard@gmail.com>]

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> The conversion at the other end, TAG_PTR, suffers from the same potential problem but in reverse, which is perhaps less obvious to the casual C programmer but nevertheless a danger of the same kind, so we'd better do something about it as well. I'll be back with a proposed patch.

Here it is. Very straightforward: just flip a cast and remove unused definitions.


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From 9072db6ca24df50dc8dc66cd50ef638b8aac8a1b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mattias=20Engdeg=C3=A5rd?= <mattiase@acm.org>
Date: Mon, 18 Sep 2023 19:16:05 +0200
Subject: [PATCH] Don't use pointer arithmetic for pointer tagging (bug#65491)

This makes for safer code when tagging null pointers in particular,
since pointer arithmetic on NULL is undefined and therefore can be
assumed, by the compiler, not to occur.

* src/lisp.h (untagged_ptr): Remove.
(TAG_PTR): Cast to uintptr_t instead of untagged_ptr.
---
 src/lisp.h | 15 +++------------
 1 file changed, 3 insertions(+), 12 deletions(-)

diff --git a/src/lisp.h b/src/lisp.h
index de6746f1c07..f0eea0c1f28 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -920,20 +920,11 @@ #define DEFUN_ARGS_7	(Lisp_Object, Lisp_Object, Lisp_Object, Lisp_Object, \
 #define DEFUN_ARGS_8	(Lisp_Object, Lisp_Object, Lisp_Object, Lisp_Object, \
 			 Lisp_Object, Lisp_Object, Lisp_Object, Lisp_Object)
 
-/* untagged_ptr represents a pointer before tagging, and Lisp_Word_tag
-   contains a possibly-shifted tag to be added to an untagged_ptr to
-   convert it to a Lisp_Word.  */
+/* Lisp_Word_tag is big enough for a possibly-shifted tag, to be
+   added to a pointer value for conversion to a Lisp_Word.  */
 #if LISP_WORDS_ARE_POINTERS
-/* untagged_ptr is a pointer so that the compiler knows that TAG_PTR
-   yields a pointer.  It is char * so that adding a tag uses simple
-   machine addition.  */
-typedef char *untagged_ptr;
 typedef uintptr_t Lisp_Word_tag;
 #else
-/* untagged_ptr is an unsigned integer instead of a pointer, so that
-   it can be added to the possibly-wider Lisp_Word_tag type without
-   losing information.  */
-typedef uintptr_t untagged_ptr;
 typedef EMACS_UINT Lisp_Word_tag;
 #endif
 
@@ -943,7 +934,7 @@ #define LISP_WORD_TAG(tag) \
 
 /* An initializer for a Lisp_Object that contains TAG along with PTR.  */
 #define TAG_PTR(tag, ptr) \
-  LISP_INITIALLY ((Lisp_Word) ((untagged_ptr) (ptr) + LISP_WORD_TAG (tag)))
+  LISP_INITIALLY ((Lisp_Word) ((uintptr_t) (ptr) + LISP_WORD_TAG (tag)))
 
 /* LISPSYM_INITIALLY (Qfoo) is equivalent to Qfoo except it is
    designed for use as an initializer, even for a constant initializer.  */
-- 
2.32.0 (Apple Git-132)


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