[Bug ipa/101362] New: can_change_signature does not consider 'used' attribute

[Bug ipa/101362] New: can_change_signature does not consider 'used' attribute

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101362

            Bug ID: 101362
           Summary: can_change_signature does not consider 'used'
                    attribute
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ipa
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

For

static int __attribute__((used)) fn (int x)
{
  return x;
}

int main() { return fn (0); }

compute_fn_summary will end up with ->can_change_signature = true; but I think
'used' implies the function will be used in ways not visible to GCC.

[Bug ipa/101362] can_change_signature does not consider 'used' attribute

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101362

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hubicka at gcc dot gnu.org
             Target|                            |i?86-*-*

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
In particular can_change_signature is one of the keys for ix86_function_regparm
to use local calling conventions on i?86

[Bug ipa/101362] can_change_signature does not consider 'used' attribute

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101362

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-07-16

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug ipa/101362] can_change_signature does not consider 'used' attribute

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101362

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> In particular can_change_signature is one of the keys for
> ix86_function_regparm to use local calling conventions on i?86

So looking through the code on i386 side we check ->local and
->can_change_signature.

But ->local is set to false for node->externally_visible which gets set via
cgraph_externally_visible_p for DECL_PRESERVE_P which gets set via
handle_used_attribute (in c-family/c-attribs.cc).

So can_change_signature might not be worried about here.

Basically as far as I understand is that can_change_signature says if we can
change the signature; though if it is not local, we can't change that version
of it because it might called else where outside of the TU.