[Bug runtime/5154] aux_syscalls lookup tables needlessly included

[Bug runtime/5154] aux_syscalls lookup tables needlessly included

[dsmith at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=5154

David Smith <dsmith at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |dsmith at redhat dot com
         Resolution|---                         |FIXED

--- Comment #1 from David Smith <dsmith at redhat dot com> ---
From looking at tapset/linux/aux_syscalls.stp, it looks like this work has been
done, so I'm closing this one.

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[Bug runtime/5154] aux_syscalls lookup tables needlessly included

[dsmith at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=5154

David Smith <dsmith at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |---

--- Comment #2 from David Smith <dsmith at redhat dot com> ---
From looking at pass 3 output, the lookup tables are still being needlessly
included. The *functions* are marked as pure (and get elided when not used),
but the lookup tables themselves are always included.

Here's what the code looks like:

====
# ...

%{
static const _stp_val_array const _stp_XXXX_list[] = {
    V(XXX_1),
    V(XXX_2),
    {0, NULL}
};
%}

function _XXXX_str:string(val:long)
%{ /* pure */
    _stp_lookup_str(_stp_XXXX_list, STAP_ARG_val, STAP_RETVALUE,
            MAXSTRINGLEN);
%}
====

Here the translator can know that _XXXX_str() isn't used and elide it. However,
it can't really know that that embedded-C block above it is used or unused, so
it always includes it.

One possible solution here would be to move the lookup table inside the
function, but still static. The lookup table would need to be static so it
wouldn't be put on the limited kernel stack. In this scenario, if the
translator elides the function, it would automatically elide the lookup table.

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[Bug runtime/5154] aux_syscalls lookup tables needlessly included

[jistone at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=5154

Josh Stone <jistone at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jistone at redhat dot com

--- Comment #3 from Josh Stone <jistone at redhat dot com> ---
When not used, do the tables actually survive through compilation?  For
unreferenced static const data, I'd expect gcc would just drop them.

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[Bug runtime/5154] aux_syscalls lookup tables needlessly included

[dsmith at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=5154

David Smith <dsmith at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from David Smith <dsmith at redhat dot com> ---
(In reply to Josh Stone from comment #3)
> When not used, do the tables actually survive through compilation?  For
> unreferenced static const data, I'd expect gcc would just drop them.

Good question. It looks like you are correct, gcc is discarding that data. See
the following:

====
# stap -p3 -e 'probe begin { println(_mmap_flags(0)); exit() }' | egrep
_stp_.+_list
static const _stp_val_array const _stp_sockopt_level_list[] = {
static const _stp_val_array const _stp_signal_list[] = {
    const _stp_val_array * const array = _stp_signal_list;
static const _stp_val_array const _stp_semctl_list[] = {
static const _stp_val_array const _stp_fork_list[] = {
static const _stp_val_array const _stp_atflag_list[] = {
static const _stp_val_array const _stp_shmat_list[] = {
static const _stp_val_array const _stp_mprotect_list[] = {
static const _stp_val_array const _stp_mmap_list[] = {
  static const _stp_val_array const _stp_sa_handler_list[] = {
  static const _stp_val_array const _stp_sa_flags_list[] = {
  _stp_lookup_str(_stp_sa_handler_list, (long)act->sa_handler,
      _stp_lookup_or_str(_stp_sa_flags_list, act->sa_flags, ptr, len);
static const _stp_val_array const _stp_irq_list[] = {
    _stp_lookup_or_str(_stp_mmap_list, STAP_ARG_flags, STAP_RETVALUE,
MAXSTRINGLEN);
#  stap -m test -p4 -e 'probe begin { println(_mmap_flags(0)); exit() }'
test.ko
# nm test.ko | egrep _stp_.+_list
0000000000000100 r _stp_mmap_list
====

From the above output, the source (-p3 output) contained ~15 lookup tables. But
the final module only contained the one actually used.

I'll reclose this one.

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