Re: Help forging a function_decl

[Rodolfo Guilherme Wottrich <rgwott at gmail dot com>]

Hello,

2013/7/23 Martin Jambor <mjambor@suse.cz>:
> Hi,
>
> But you do call cgraph_add_new_function on it as well, right?  If not,
> how is its symbol table node (also called and serving as the call
> graph node) created?

I call finish_function for my decl. Inside that function, there's this
one call to cgraph_add_new_function, but the condition it is inside is
bypassed:

/* ??? Objc emits functions after finalizing the compilation unit.
  This should be cleaned up later and this conditional removed.  */
  if (cgraph_global_info_ready)
  {
    cgraph_add_new_function (fndecl, false);
    return;
  }

Right after that, there's a call to cgraph_finalize_function, which I
guess works the way it should, creating its cgraph node. I took a look
at the comment in cgraph_add_new_function and it states that "this
function is intended to be used by middle end and allows insertion of
new function at arbitrary point of compilation.  The function can be
either in high, low or SSA form GIMPLE." As I'm doing it in parsing
time and there was no gimplification passes yet, I guess it is ok not
to use it at this point.

> (And BTW, you are hacking on trunk, right?  Older versions can be
> quite a bit different here.)

I understand that, but I intend to use Dragonegg to obtain LLVM IR
after the gcc front-end has done its work, so I'm hacking version
4.6.4, which is said to work better with Dragonegg.

> What do you mean by "50% of the time?"  That you get different results
> even when you do not change your compiler?  That should not happen and
> means you invoke undefined behavior, most likely depending on some
> uninitialized stuff (assuming your HW is OK) so you are probably not
> clearing some allocated structure or something.  (Do you know why
> DECL_EXTERNAL was set?  That looks weird).

Yeah, exactly: different results even not changing the compiler.
About DECL_EXTERNAL: it was my fault. At first I was trying to
reproduce the creation of a function_decl as in
create_omp_child_function, in omp-low.c, and I eventually forgot that
I set that attribute when playing with the possibilities.

> Anyway, my best guess is that your function is removed by
> symtab_remove_unreachable_nodes in ipa.c.  (And now I also see that
> analyze_functions in cgraphunit.c is also doing its own unreachable
> node removal, but hopefully runs early enough this should not be your
> problem.)  If your function is static and is not called or referenced
> from anywhere else, gcc will try to get rid of it.

I traced the execution path in gdb, and it happens that the function
is really being removed in cgraph_remove_unreachable_nodes in ipa.c
(that's basically the same function you pointed out, just another gcc
version). I don't know why that happens anyway, neither why it happens
intermittently when not debugging and never when debugging. My
function is not called or referenced yet, but neither is another
function in my source code which I put just in order to test this
issue, and yet it is not removed like my forged one. So it is in
respect to being or not static. A doubt: I tested setting TREE_STATIC
for my decl, but does that mean my function is static? The
documentation on the TREE_STATIC macro is: "In a FUNCTION_DECL,
nonzero if function has been defined".

> Try setting DECL_PRESERVE_P of your decl (or calling
> cgraph_mark_force_output_node on its call graph node which is cleaner
> but should be equivalent for debugging, I suppose).  If that helps,
> this is most likely your problem.

I tried DECL_PRESERVE_P and it didn't work. In this version of gcc
there's no cgraph_mark_force_output_node. I tried
cgraph_mark_needed_node instead, didn't work either.

> What do you mean?  That your function is not output or that you cannot
> use gdb at all?

That my function is never output when using gdb.

> Add -fdump-ipa-all switch and look whether the function appears
> there.  Especially the cgraph dump can usually tell you that a
> function was removed as unreachable.  Grep all dumps for "Reclaiming
> functions:" and see whether your function is listed.

You were right, but this is a direct consequence of my function being
removed like discussed before.

I'll keep investigating the problem and will inform you in case
something changes.
Thank you,

---
Rodolfo Guilherme Wottrich
Universidade Estadual de Campinas - Unicamp