[Bug middle-end/45307] New: Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[Bug middle-end/45307] New: Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[hubicka at gcc dot gnu dot org]

Compilling:
 ./g++ -O2 ~/mozalloc.ii  -S -m32
gives me:
        .type   _GLOBAL__I_moz_free, @function
_GLOBAL__I_moz_free:
.LFB58:
        .cfi_startproc
        rep
        ret
        .cfi_endproc
.LFE58:
        .size   _GLOBAL__I_moz_free, .-_GLOBAL__I_moz_free
        .section        .ctors,"aw",@progbits
        .align 4
        .long   _GLOBAL__I_moz_free
        .local  _ZN7mozillaL8fallibleE
        .comm   _ZN7mozillaL8fallibleE,1,1

I.e. an empty constructor function that can be eliminated and also 
local var   _ZN7mozillaL8fallibleE that is not used at all.
The reason is that constructor in .optimized stage looks like:
(static initializers for /mozilla-central/memory/mozalloc/mozalloc.cpp) ()
{
<bb 2>:
  fallible = {};
  return;

}

that later expands to nothing.  If we optimized this away during early opts I
already have patch that makes it easy to eliminate all global ctors/dtors that
are known to be pure.

Honza


-- 
           Summary: Stores expanding to no RTL not removed by tree
                    optimizers, Empty ctors/dtors not eliminated
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hubicka at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[hubicka at gcc dot gnu dot org]

------- Comment #1 from hubicka at gcc dot gnu dot org  2010-08-17 12:38 -------
Created an attachment (id=21503)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=21503&action=view)
testcase.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug c++/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2010-08-17 12:49 -------
It's the FEs job really.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu dot org
          Component|middle-end                  |c++


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[hubicka at gcc dot gnu dot org]

------- Comment #3 from hubicka at gcc dot gnu dot org  2010-08-17 13:34 -------
Adding Mark and Jason to CC then, but Jakub seems right about -Wuninitialized
warnings.


-- 

hubicka at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at redhat dot com,
                   |                            |mark at codesourcery dot com
          Component|c++                         |middle-end


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[mmitchel at gcc dot gnu dot org]

------- Comment #4 from mmitchel at gcc dot gnu dot org  2010-08-17 14:41 -------
I have no object to the FE removing trivial code if it can do so -- but I also
think that the middle-end should be able to deduce that a function is pure
later in the process and eliminate it then.

I don't understand Richard's comment about -Wuninitialized; I don't see any
comments from Jakub in this PR.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2010-08-17 15:57 -------
I also wonder why we can't remove such stores based on ipa-reference analysis.

Reduced testcase:

struct __attribute__ ((visibility ("default"))) fallible_t { };
const fallible_t fallible = fallible_t();


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|rguenther at suse dot de    |rguenth at gcc dot gnu dot
                   |                            |org
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2010-08-17 15:57:59
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[hubicka at gcc dot gnu dot org]

------- Comment #6 from hubicka at gcc dot gnu dot org  2010-08-17 16:42 -------
We are not removing write only vars because richi told me that there is no
convenient way to tell aliasing that variable is write only.  It is easy to
detect for me, but bit harder to get rid of them, since at IPA pass I can't
modify body so I need to pass it down to local optimization and remove only
then.
I guess I will add the feature (detecting them and removing the statements in
transform pass). It just seemed not worth the hassle when I was looking into it
originally.

Concerning Mark's question, we briefly discussed this on IRC

So we now have

1) C++ frontend probably should not produce static constructor for empty struct
at all
2) We should be able to get rid of writes to empty structs in backend
3) Middle end should take away pure constructors
4) We should be able to optimize away write only variables

I will work on 3) and probably 4) later.

Mark, why C++ is not doing 1)? 


-- 

hubicka at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|enhancement                 |normal
           Keywords|missed-optimization         |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[hubicka at gcc dot gnu dot org]

------- Comment #7 from hubicka at gcc dot gnu dot org  2010-08-17 16:45 -------
oops, I've left IRC sentence unfinished.  At IRC Richi told that FEs should not
ever produce stores to empty structs at first place, so we don't need middle
end logic of taking them away.  Jakub thought that those stores are left to
keep -Wuninitialized safe.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[jakub at gcc dot gnu dot org]

------- Comment #8 from jakub at gcc dot gnu dot org  2010-08-17 16:53 -------
Perhaps related to PR43075, before that last commit there genericization was
removing empty struct assignments.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[jason at gcc dot gnu dot org]

------- Comment #9 from jason at gcc dot gnu dot org  2010-08-17 17:41 -------
But that change was largely reversed by the fix for PR 43787.


-- 

jason at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[mmitchel at gcc dot gnu dot org]

------- Comment #10 from mmitchel at gcc dot gnu dot org  2010-08-17 17:50 -------
The problem with -Wuninitialized might be the same problem I've been
sermonizing about for years -- we're trying to issue sensible warnings from our
optimizers, which means that as the optimizers are perturbed, the set of
warnings will change in various ways.  But, there's no reason for
-Wuninitialized to warn about a use of an empty structure, so I don't see why
removing the stores is unreasonable.

I certainly think that the front-end could safely optimize away such stores,
and that it would be reasonably easy to do so in the trivial cases. 
(Obviously, the optimizers would be responsible for more complex cases where
things are not perhaps immediately obvious to the front-end.)  I don't think
there's any reason we don't do that in the front-end; it's just something we
haven't done.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[jason at gcc dot gnu dot org]

------- Comment #11 from jason at gcc dot gnu dot org  2010-08-17 18:09 -------
There are two issues here:

1) expand_static_init decides whether a variable needs static initialization
before gimplification, and
2) Richard's MEM_REF-related change to cp_gimplify_expr caused us to stop
removing the copy in this example.

It sounds like if we fix #2 then the patch Honza mentions having should do the
trick.  I'll do that.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[jason at gcc dot gnu dot org]

------- Comment #12 from jason at gcc dot gnu dot org  2010-08-17 18:18 -------
Actually, Richard's change didn't affect this; we were already missing it
because of the complex interoperation of cp_gimplify_expr and
gimplify_modify_expr_rhs.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[jason at gcc dot gnu dot org]

------- Comment #13 from jason at gcc dot gnu dot org  2010-08-19 17:01 -------
Subject: Bug 45307

Author: jason
Date: Thu Aug 19 17:00:51 2010
New Revision: 163380

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=163380
Log:
        PR c++/45307
        * gimplify.c (gimplify_init_constructor): Just return GS_UNHANDLED
        if ctor is empty.
        (gimplify_modify_expr_rhs): Adjust.

Added:
    trunk/gcc/testsuite/g++.dg/tree-ssa/empty-2.C
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/cp-gimplify.c
    trunk/gcc/gimplify.c
    trunk/gcc/testsuite/ChangeLog


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[hubicka at gcc dot gnu dot org]

------- Comment #14 from hubicka at gcc dot gnu dot org  2010-08-20 10:23 -------
http://gcc.gnu.org/ml/gcc-patches/2010-08/msg01544.html
has patch for empty constructor removal in middle end.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[hubicka at gcc dot gnu dot org]

------- Comment #15 from hubicka at gcc dot gnu dot org  2010-08-21 09:46 -------
Subject: Bug 45307

Author: hubicka
Date: Sat Aug 21 09:46:15 2010
New Revision: 163439

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=163439
Log:

        PR c++/45307
        PR c++/17736
        * cgraph.h (cgraph_only_called_directly_p,
        cgraph_can_remove_if_no_direct_calls_and_refs_p): Handle
        static cdtors.
        * cgraphunit.c (cgraph_decide_is_function_needed): Static cdtors
        are not needed.
        (cgraph_finalize_function): Static cdtors are reachable.
        (cgraph_mark_functions_to_output): Use cgraph_only_called_directly_p.

        * gcc.dg/ipa/ctor-empty-1.c: Add testcase.
        * g++.dg/tree-ssa/empty-2.C: Check that constructor got optimized out.


Added:
    trunk/gcc/testsuite/gcc.dg/ipa/ctor-empty-1.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/cgraph.c
    trunk/gcc/cgraph.h
    trunk/gcc/cgraphunit.c
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/testsuite/g++.dg/tree-ssa/empty-2.C


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307

[Bug middle-end/45307] Stores expanding to no RTL not removed by tree optimizers, Empty ctors/dtors not eliminated

[hubicka at gcc dot gnu dot org]

------- Comment #16 from hubicka at gcc dot gnu dot org  2010-08-21 09:48 -------
Empty constructors are now optimized away.  We still should get removal of
write only global vars.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45307