bogus -Wnull-dereference warning

Paul Eggert wrote in
<https://lists.gnu.org/archive/html/bug-gnulib/2024-08/msg00088.html>:
> Also m4 still has a bogus -Wnull-dereference diagnostic, unrelated to the
> verror.h changes, that somebody needs to investigate when they find the time.

The warning can be seen with gcc 14.1 or 14.2. How to reproduce:
$ ./configure --enable-gcc-warnings
$ make V=1

The result with gcc 14.x:

gcc    -I../lib -I../lib  -fanalyzer -fstrict-flex-arrays -Wall -Warith-conversion -Wbad-function-cast -Wcast-align=strict -Wdate-time -Wdisabled-optimization -Wdouble-promotion -Wduplicated-branches -Wduplicated-cond -Wextra -Wformat-signedness -Wflex-array-member-not-at-end -Winit-self -Winline -Winvalid-pch -Wlogical-op -Wmissing-declarations -Wmissing-include-dirs -Wmissing-prototypes -Wmissing-variable-declarations -Wnested-externs -Wnull-dereference -Wold-style-definition -Wopenmp-simd -Woverlength-strings -Wpacked -Wpointer-arith -Wshadow -Wstack-protector -Wstrict-flex-arrays -Wstrict-overflow -Wstrict-prototypes -Wsuggest-attribute=cold -Wsuggest-attribute=const -Wsuggest-attribute=format -Wsuggest-attribute=malloc -Wsuggest-attribute=noreturn -Wsuggest-attribute=pure -Wsuggest-final-methods -Wsuggest-final-types -Wsync-nand -Wtrampolines -Wuninitialized -Wunknown-pragmas -Wunsafe-loop-optimizations -Wunused-macros -Wvariadic-macros -Wvector-operation-performance -Wwrite-strings -Warray-bounds=2 -Wattribute-alias=2 -Wbidi-chars=any,ucn -Wformat-overflow=2 -Wformat=2 -Wformat-truncation=2 -Wimplicit-fallthrough=5 -Wshift-overflow=2 -Wuse-after-free=3 -Wunused-const-variable=2 -Wvla-larger-than=4031 -Wno-analyzer-malloc-leak  -g -O2 -c output.c
output.c: In function 'make_room_for':
output.c:491:40: warning: potential null pointer dereference [-Wnull-dereference]
  491 |       output_file = output_diversion->u.file;
      |                     ~~~~~~~~~~~~~~~~~~~^~~~~

I submitted this as a GCC bug:
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116426>

A consequence of this warning is that with --enable-gcc-warnings, all
gcc versions from 11 to 14 produce warnings on glibc systems:

gcc 11.5.0:
error: function might be candidate for attribute 'cold'

gcc 12.4.0:
error: function might be candidate for attribute 'cold'

gcc 13.3.0:
error: function might be candidate for attribute 'cold'
error: potential null pointer dereference

gcc 14.2.0:
error: potential null pointer dereference

Reminder about the other warning:

  CC       macro.o
In function 'call_macro.part.0':
../../../src/macro.c:272:1: error: function might be candidate for attribute 'cold' [-Werror=suggest-attribute=cold]
  272 | call_macro (symbol *sym, int argc, token_data **argv,
      | ^~~~~~~~~~

Which is a bogus warning as well. I cannot find it reported in the GCC bug
tracker. But at least, it's fixed in GCC 14.


What can we do about it? I don't find it adequate to change the logic of
the function 'make_room_for'. Usually we
  - disable a warning option completely if it generates nearly only false
    alarms,
  - disable a warning option for a certain compilation unit, to work around
    specific false alarms.

Here we are in the second case. Here's a proposed patch.

From d891c52c1ca8ffae0aa9cebea572dccba1186355 Mon Sep 17 00:00:00 2001
From: Bruno Haible <[email protected]>
Date: Tue, 20 Aug 2024 03:27:43 +0200
Subject: [PATCH] maint: Avoid a gcc 14 warning that makes
 --enable-gcc-warnings break.

* src/output.c: Disable -Wnull-dereference warnings in this file.
---
 src/output.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/output.c b/src/output.c
index 6dc8280a..65f183dd 100644
--- a/src/output.c
+++ b/src/output.c
@@ -27,6 +27,12 @@
 #include "gl_avltree_oset.h"
 #include "gl_xoset.h"
 
+/* Work around a bogus GCC warning
+   <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116426>.  */
+#if __GNUC__ >= 6
+# pragma GCC diagnostic ignored "-Wnull-dereference"
+#endif
+
 /* Size of initial in-memory buffer size for diversions.  Small diversions
    would usually fit in.  */
 #define INITIAL_BUFFER_SIZE 512
-- 
2.34.1