bug#74966: 31.0.50; Crash report (using igc on macOS)

[Pip Cet via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

"Andrea Corallo" <acorallo@gnu.org> writes:

> Eli Zaretskii <eliz@gnu.org> writes:
>
>>> From: Andrea Corallo <acorallo@gnu.org>
>>> Cc: Pip Cet <pipcet@protonmail.com>,  monnier@iro.umontreal.ca,
>>>   gerd.moellmann@gmail.com,  spd@toadstyle.org,  74966@debbugs.gnu.org
>>> Date: Tue, 31 Dec 2024 05:22:11 -0500
>>>
>>> Eli Zaretskii <eliz@gnu.org> writes:
>>>
>>> >> > I think a much simpler change is to use the sign bit to distinguish indices
>>> >> > into the constant vector from indices into the DOC file.
>>> >>
>>> >> And use one's-complement, I assume, to guard against some future weird
>>> >> nativecomp change resulting in the index -0?  :-)
>>> >>
>>> >> I really have no strong preference here.
>>> >
>>> > And I still want to hear from Andrea.  It's his code, so the solution
>>> > he prefers gets my vote.
>>>
>>> Sorry for behing late here.
>>>
>>> I'm for the sign bit, it saves memory (why not), and we can sanity check
>>> that when the bit is set the function is a non-primitve one and vice
>>> versa.
>>
>> OK, can you post a patch (or even install it)?
>
> Will do.  I'll just need some time, I'm catching up with mails after
> holidays while being sick at the same time.

Sorry to hear it.  Get well soon!

> If someone is motivate to
> jump on the task before me feel free.

Here's a patch (IIRC, C doesn't require ~x = -x - 1, so I used
the more explicit notation to avoid compiler warnings).

commit 9654100f0ecc312b14ba56bf2e13691168342cf9 (HEAD -> signbit)
Author: Pip Cet <pipcet@protonmail.com>
Date:   Tue Jan 7 12:52:16 2025 +0000

    Fix store_function_docstring for native subrs (Bug#74966)
    
    * src/comp.c (native_function_doc):
    (make_subr): Use one's complement of doc index.
    * src/doc.c (store_function_docstring): Add assertion.
    * src/lisp.h (struct Lisp_Subr): Document 'doc' sign bit.

diff --git a/src/comp.c b/src/comp.c
index 70a9a64a714..b96fae4ae95 100644
--- a/src/comp.c
+++ b/src/comp.c
@@ -5488,7 +5488,10 @@ native_function_doc (Lisp_Object function)
   if (!VECTORP (cu->data_fdoc_v))
     xsignal2 (Qnative_lisp_file_inconsistent, cu->file,
 	      build_string ("missing documentation vector"));
-  return AREF (cu->data_fdoc_v, XSUBR (function)->doc);
+  EMACS_INT doc = XSUBR (function)->doc;
+  if (doc < 0)
+    return AREF (cu->data_fdoc_v, -doc - 1);
+  return make_fixnum (doc);
 }
 
 static Lisp_Object
@@ -5529,7 +5532,8 @@ make_subr (Lisp_Object symbol_name, Lisp_Object minarg, Lisp_Object maxarg,
   x->s.symbol_name = xstrdup (SSDATA (symbol_name));
   x->s.intspec.native = intspec;
   x->s.command_modes = command_modes;
-  x->s.doc = XFIXNUM (doc_idx);
+  x->s.doc = -XFIXNUM (doc_idx) - 1;
+  eassert (x->s.doc < 0);
 #ifdef HAVE_NATIVE_COMP
   x->s.native_comp_u = comp_u;
   x->s.native_c_name = xstrdup (SSDATA (c_name));
diff --git a/src/doc.c b/src/doc.c
index 88be9121dab..04afe50d3dd 100644
--- a/src/doc.c
+++ b/src/doc.c
@@ -479,7 +479,10 @@ store_function_docstring (Lisp_Object obj, EMACS_INT offset)
     fun = XCDR (fun);
   /* Lisp_Subrs have a slot for it.  */
   if (SUBRP (fun))
-    XSUBR (fun)->doc = offset;
+    {
+      XSUBR (fun)->doc = offset;
+      eassert (XSUBR (fun)->doc >= 0);
+    }
   else if (CLOSUREP (fun))
     {
       /* This bytecode object must have a slot for the docstring, since
diff --git a/src/lisp.h b/src/lisp.h
index 339ff5e83b0..0979fedd846 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -2228,6 +2228,9 @@ CHAR_TABLE_SET (Lisp_Object ct, int idx, Lisp_Object val)
       Lisp_Object native;
     } intspec;
     Lisp_Object command_modes;
+    /* positive values: offset into etc/DOC.  Negative values: one's
+       complement of index into the native comp unit's constant
+       vector.  */
     EMACS_INT doc;
 #ifdef HAVE_NATIVE_COMP
     Lisp_Object native_comp_u;