GNU logs - #79751, boring messages

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Subject: bug#79751: [PATCH] Fix fix_hash_table, empty_hash_index_vector
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--=-=-=
Content-Type: text/plain

Tags: patch

This is something I noticed when randomly reading the code. Alas it's
not fixing the build problems I have, it just changes where and how it
fails, please see below.

Could you please check, Helmut and Pip? I have added it to my Emacs.

  ELC+ELN  ../lisp/emacs-lisp/cl-generic.elc
Backtrace:
0   bootstrap-emacs                     0x0000000102bb7514 emacs_backtrace + 180
1   bootstrap-emacs                     0x0000000102e8c124 terminate_due_to_signal + 352
2   bootstrap-emacs                     0x0000000102bba570 deliver_thread_signal + 0
3   bootstrap-emacs                     0x0000000102bba604 deliver_thread_signal + 148
4   bootstrap-emacs                     0x0000000102bb70dc deliver_fatal_thread_signal + 32
5   bootstrap-emacs                     0x0000000102bba6b8 handle_sigsegv + 148
6   libsystem_platform.dylib            0x000000019fc12744 _sigtramp + 56
7   bootstrap-emacs                     0x0000000102cc2ff4 SYMBOL_NAME + 24
8   bootstrap-emacs                     0x0000000102cc6e6c print_object + 2716
...
gmake[3]: *** [Makefile:286: ../lisp/emacs-lisp/cl-generic.elc] Segmentation fault: 11


--=-=-=
Content-Type: text/x-patch
Content-Disposition: attachment;
 filename=0001-src-igc.c-fix_hash_table-Don-t-fix-empty_hash_index_.patch

From a7e4133de8bc68770915e8a2fde993aacab8a419 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Gerd=20M=C3=B6llmann?= <gerd.moellmann@HIDDEN>
Date: Sat, 1 Nov 2025 14:42:04 +0100
Subject: [PATCH] * src/igc.c (fix_hash_table): Don't fix
 empty_hash_index_vector

---
 src/igc.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/igc.c b/src/igc.c
index 5bfb8b2dc8c..c54732165ce 100644
--- a/src/igc.c
+++ b/src/igc.c
@@ -2463,7 +2463,10 @@ fix_hash_table (mps_ss_t ss, struct Lisp_Hash_Table *h)
     IGC_FIX12_PVEC (ss, &h->value);
     IGC_FIX12_WRAPPED_BYTES (ss, &h->hash);
     IGC_FIX12_WRAPPED_BYTES (ss, &h->next);
-    IGC_FIX12_WRAPPED_BYTES (ss, &h->index);
+    /* If h->table_size == 0, h->index is empty_hash_index_vector which
+       is not allocated from MPS.  */
+    if (h->table_size > 0)
+      IGC_FIX12_WRAPPED_BYTES (ss, &h->index);
   }
   MPS_SCAN_END (ss);
   return MPS_RES_OK;
-- 
2.51.2


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To: Gerd =?UTF-8?Q?M=C3=B6llmann?= <gerd.moellmann@HIDDEN>
Subject: bug#79751: Acknowledgement ([PATCH] Fix fix_hash_table,
 empty_hash_index_vector)
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Reply-To: 79751 <at> debbugs.gnu.org
Date: Sat, 01 Nov 2025 13:54:02 +0000

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Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#79751: [PATCH] Fix fix_hash_table, empty_hash_index_vector
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Gerd M=C3=B6llmann <gerd.moellmann@HIDDEN> writes:

> Tags: patch
>
> This is something I noticed when randomly reading the code. Alas it's
> not fixing the build problems I have, it just changes where and how it
> fails, please see below.
>
> Could you please check, Helmut and Pip? I have added it to my Emacs.

I'm a bit confused. Fixing a non-MPS object is fine as long as it is
known to be valid (i.e. definitely not in MPS memory), but I have no
objections to avoiding it where we can (if the pointer points into
random MPS memory, we will crash). However, shouldn't we avoid it for
->hash and and ->next, too? Only the Lisp objects are guaranteed to be
valid for empty hash tables, right?

Pip

Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#79751: [PATCH] Fix fix_hash_table, empty_hash_index_vector
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Pip Cet <pipcet@HIDDEN> writes:

> Gerd M=C3=B6llmann <gerd.moellmann@HIDDEN> writes:
>
>> Tags: patch
>>
>> This is something I noticed when randomly reading the code. Alas it's
>> not fixing the build problems I have, it just changes where and how it
>> fails, please see below.
>>
>> Could you please check, Helmut and Pip? I have added it to my Emacs.
>
> I'm a bit confused. Fixing a non-MPS object is fine as long as it is
> known to be valid (i.e. definitely not in MPS memory), but I have no
> objections to avoiding it where we can (if the pointer points into
> random MPS memory, we will crash). However, shouldn't we avoid it for
> ->hash and and ->next, too? Only the Lisp objects are guaranteed to be
> valid for empty hash tables, right?

I'm not sure that fix_wrapped_bytes DTRT for empty_hash_index_vector. Is
is_aligned guaranteed?. I don't think it is on macOS.

hash and next are NULL which fix_wrapped_bytes it handles. I believe
index was also NULL before Matthias changed the hash table
implementation, but I'm not sure.

make_hash_table:

fns.c<cl-packages>:
 4875   if (size =3D=3D 0)
 4876     {
 4877       h->key =3D NULL;
 4878       h->value =3D NULL;
 4879       h->hash =3D NULL;
 4880       h->next =3D NULL;
 4881       h->index_bits =3D 0;
 4882       h->index =3D (hash_idx_t *)empty_hash_index_vector;
 4883       h->next_free =3D -1;
 4884     }

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