[Bug c/107010] New: iterate section("mysection") struct mystruct fails if first node is const and compiling with O2 or greater

[Bug c/107010] New: iterate section("mysection") struct mystruct fails if first node is const and compiling with O2 or greater

[stian.skjelstad at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107010

            Bug ID: 107010
           Summary: iterate section("mysection") struct mystruct fails if
                    first node is const and compiling with O2 or greater
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stian.skjelstad at gmail dot com
  Target Milestone: ---

Created attachment 53612
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53612&action=edit
example setup compiled with both working and non-working flags

I have a use-case where plugins can be be compiled built in, and they are
detected by having some meta-data added to a section, and in runtime they can
be iterated.


#define SECTION __attribute__ ((section ("plugin_list")))
#define USED    __attribute__ ((used))


Core code:

SECTION const struct plugindata first = {"dummy", 0, 0};


Each plugin (many copies):

SECTION USED static const struct plugindata = {"pluginname", foo, bar};


Last file linked contains a terminator

SECTION USED static const struct plugindata end = {0, 0, 0};


During linking they will gathered and can iterated using code like this:


void show_plugin_names(void)
{
        const struct plugindata *iterator = &first;

        while (iterator->name)
        {
                fprintf (stderr, "name=%s\n", iterator->name);
                iterator++;
        }
}

This works as long as you do not combine "first" being const and using gcc -O2

Tested with:
gcc-9 (Ubuntu 9.4.0-5ubuntu1) 9.4.0
gcc-10 (Ubuntu 10.3.0-15ubuntu1) 10.3.0
gcc-11 (Ubuntu 11.2.0-19ubuntu1) 11.2.0
gcc-12 (Ubuntu 12.1.0-2ubuntu1~22.04) 12.1.0
(using clang works)

I have also tested -fno-strict-aliasing -fwrapv and it didn't change the
behaviour.


I do accept that the use-case is probably a bit unorthodox and that it might be
dismissed as compiler-abuse.

[Bug c/107010] iterate section("mysection") struct mystruct fails if first node is const and compiling with O2 or greater

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107010

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=77964
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
That is because GCC knows that first+1 does not point to a valid pointer
really.

You need to hide the assignment of iterator from GCC.

This will fix the issue:
        const struct linkinfostruct *iterator = &first;
        asm("":"+r"(iterator));

        while (iterator->name)
        {
                fprintf (stderr, "name=%s\n", iterator->name);
                iterator++;
                fprintf (stderr, "    next iteration should be (iterator=%p,
&iterator->name=%p name=%p \"%s\"\n", iterator, &iterator->name,
iterator->name, iterator->name); 
        }


See PR 77964 and PR 106939 also.

[Bug c/107010] iterate section("mysection") struct mystruct fails if first node is const and compiling with O2 or greater

[stian.skjelstad at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107010

--- Comment #2 from Stian Skjelstad <stian.skjelstad at gmail dot com> ---
Thank you for an amazing fast usefull reply.