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Don't use pointer arithmetic for pointer tagging (bug#65491)

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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.
This commit is contained in:
Mattias Engdegård 2023-09-18 19:16:05 +02:00
parent 7be5c8f47c
commit 321f2e1e4d
1 changed files with 3 additions and 12 deletions
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15
src/lisp.h
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@ -919,20 +919,11 @@ verify (GCALIGNED (struct Lisp_Symbol));
#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
@ -942,7 +933,7 @@ typedef EMACS_UINT Lisp_Word_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. */