Subject: bug#62009: 29.0.60; Emacs crashes on setf symbol-name

From: Po Lu <[email-masked]>
Date: Sat, 11 Mar 2023 15:07:56 +0800

Robert Pluim <[email-masked]> writes:

>>>>>> On Fri, 10 Mar 2023 12:09:59 +0100, Daniel Mendler <[email-masked]> said:
>
>     Daniel> One could check if the string is located in read-only memory. Or one
>     Daniel> could add a flag bit to the string data structure (and possibly to other
>     Daniel> data structures too). Freezing data structures such that they become
>     Daniel> read-only is a generally useful feature. There won't be any performance
>     Daniel> overhead of the check since a branch not taken is fast thanks to the
>     Daniel> branch predictor.
>
> We already have such a flag:
>
>       /* Number of characters in string; MSB is used as the mark bit.  */
>       ptrdiff_t size;
>       /* If nonnegative, number of bytes in the string (which is multibyte).
> 	 If negative, the string is unibyte:
> 	 -1 for data normally allocated
> 	 -2 for data in rodata (C string constants)
> 	 -3 for data that must be immovable (used for bytecode)  */
>       ptrdiff_t size_byte;
>
> Try this:
>
> diff --git a/src/lisp.h b/src/lisp.h
> index 1276285e2f2..80bbb047824 100644
> --- a/src/lisp.h
> +++ b/src/lisp.h
> @@ -1685,6 +1685,8 @@ SREF (Lisp_Object string, ptrdiff_t index)
>  INLINE void
>  SSET (Lisp_Object string, ptrdiff_t index, unsigned char new)
>  {
> +  if (XSTRING (string)->u.s.size_byte == -2)
> +    Fsignal (Qsetting_constant, string);
>    SDATA (string)[index] = new;
>  }
>  INLINE ptrdiff_t
>
> Robert

Why does this have to be in SSET and not in PURE_P?