[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

--- Comment #10 from Antony Polukhin <antoshkka at gmail dot com> ---
Any progress?

Multiple compilers already eliminate the atexit call. Moreover, some of the
compilers even eliminate the guard variable after that 
https://godbolt.org/z/dbdfMrroa

Note that the atexit elimination would benefit the libstdc++, as the latter now
uses a bunch of constant_init instances that have empty destructor
in libstdc++-v3/src/c++17/memory_resource.cc and
libstdc++-v3/src/c++11/system_error.cc . It would be possible to eliminate the
atexit calls for those cases and speedup startup times
https://godbolt.org/z/MKaWKevzq

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rdiezmail-gcc at yahoo dot de

--- Comment #11 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 107500 has been marked as a duplicate of this bug. ***

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2006-02-05 21:10:26         |2022-11-2

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced from the code in libstdc++:

extern "C" int puts(const char*);

struct A
{
  constexpr A()  { }
  ~A() { puts("bye"); }
};

namespace
{
  struct constant_init
  {
    union {
      A obj;
    };
    constexpr constant_init() : obj() { }

    ~constant_init() { /* do nothing, union member is not destroyed */ }
  };


  // Single-threaded fallback buffer.
  constinit constant_init global;
}

extern "C" A* get() { return &global.obj; }

With -std=c++20 -Os GCC emits a call to atexit:

(anonymous namespace)::constant_init::~constant_init() [base object
destructor]:
        ret
get:
        mov     eax, OFFSET FLAT:_ZN12_GLOBAL__N_16globalE
        ret
_GLOBAL__sub_I_get:
        mov     edx, OFFSET FLAT:__dso_handle
        mov     esi, OFFSET FLAT:_ZN12_GLOBAL__N_16globalE
        mov     edi, OFFSET FLAT:_ZN12_GLOBAL__N_113constant_initD1Ev
        jmp     __cxa_atexit


With the same options, Clang doesn't:

get:                                    # @get
        lea     rax, [rip + (anonymous namespace)::global]
        ret

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> ---
*** Bug 107500 has been marked as a duplicate of this bug. ***

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |pinskia at gcc dot gnu.org

--- Comment #14 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I am testing a (small) patch for this. Basically DCE can remove the calls to
__cxa_atexit .

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

--- Comment #15 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The patch is able to handle all 3 testcases here even:
For the libstdc++ one:
t3.cc.123t.dce2:Deleting : __cxxabiv1::__cxa_atexit (__dt_comp , &global,
&__dso_handle);

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

--- Comment #16 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So what is interesting is the way LLVM implements this as an IPO pass as how I
described in comment #8, 6 years after I wrote that. Well they used "empty"
rather than const . Note using it as an IPO pass I think is not wrong just
having a specialized pass here where this is the same as DCE so just have DCE
handle it instead seems better; plus with the availability of having pure/const
flags with LTO and not having to load in the function seems much better really.

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[i at maskray dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

Fangrui Song <i at maskray dot me> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |i at maskray dot me

--- Comment #17 from Fangrui Song <i at maskray dot me> ---
(In reply to Andrew Pinski from comment #16)
> So what is interesting is the way LLVM implements this as an IPO pass as how
> I described in comment #8, 6 years after I wrote that. Well they used
> "empty" rather than const . Note using it as an IPO pass I think is not
> wrong just having a specialized pass here where this is the same as DCE so
> just have DCE handle it instead seems better; plus with the availability of
> having pure/const flags with LTO and not having to load in the function
> seems much better really.

Yes, LLVM recognizes __cxa_atexit as a library function and removes it in
GlobalOpt (part of IPO) since 2011
(https://github.com/llvm/llvm-project/commit/ee6bc70d2f1c2434ca9ca8092216bdeab322c7e5),
likely because
GlobalOpt is already doing global variable optimizations (e.g. removal if dead,
constant folding). Technically __cxa_atexit removal can be moved elsewhere.

There are two GlobalOpt passes in the optimization pipeline (see -mllvm
-print-changed=cdiff output), one before the inliner and one after.

  SROA
  ...
  GlobalOpt
  ...
  buildInlinerPipeline
  ...
  GlobalOpt

For empty functions like `~constant_init() {}`, SROA deletes unneeded IR
instructions (spill of the "this" pointer) and actually makes the IR function
empty. Then GlobalOpt removes __cxa_atexit call sites.

For `static void empty() {} ~constant_init() { empty(); }`, the inliner removes
the `empty()` and makes the IR function empty. Then GlobalOpt removes
__cxa_atexit call sites.

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

--- Comment #18 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 57987
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57987&action=edit
Patch which I will be submitting once GCC 15 stage 1 opens up

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

--- Comment #19 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #18)
> Created attachment 57987 [details]
> Patch which I will be submitting once GCC 15 stage 1 opens up

I forgot to add the testcase from comment #12. the patch which I am going to
submit will have it.

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

--- Comment #20 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Andrew Pinski <pinskia@gcc.gnu.org>:

https://gcc.gnu.org/g:c9dd853680b12d9c9def5de61abde5d057c526ba

commit r15-308-gc9dd853680b12d9c9def5de61abde5d057c526ba
Author: Andrew Pinski <quic_apinski@quicinc.com>
Date:   Fri Mar 15 16:34:22 2024 -0700

    DCE __cxa_atexit calls where the function is pure/const [PR19661]

    In C++ sometimes you have a deconstructor function which is "empty", like
for an
    example with unions or with arrays.  The front-end might not know it is
empty either
    so this should be done on during optimization.o
    To implement it I added it to DCE where we mark if a statement is necessary
or not.

    Bootstrapped and tested on x86_64-linux-gnu with no regressions.

    Changes since v1:
      * v2: Add support for __aeabi_atexit for arm-*eabi. Add extra comments.
            Add cxa_atexit-5.C testcase for -fPIC case.
      * v3: Fix testcases for the __aeabi_atexit (forgot to do in the v2).

            PR tree-optimization/19661

    gcc/ChangeLog:

            * tree-ssa-dce.cc (is_cxa_atexit): New function.
            (is_removable_cxa_atexit_call): New function.
            (mark_stmt_if_obviously_necessary): Don't mark removable
            cxa_at_exit calls.
            (mark_all_reaching_defs_necessary_1): Likewise.
            (propagate_necessity): Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/tree-ssa/cxa_atexit-1.C: New test.
            * g++.dg/tree-ssa/cxa_atexit-2.C: New test.
            * g++.dg/tree-ssa/cxa_atexit-3.C: New test.
            * g++.dg/tree-ssa/cxa_atexit-4.C: New test.
            * g++.dg/tree-ssa/cxa_atexit-5.C: New test.
            * g++.dg/tree-ssa/cxa_atexit-6.C: New test.

    Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
   Target Milestone|---                         |15.0
         Resolution|---                         |FIXED

--- Comment #21 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed.

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu dot org]

------- Comment #9 from pinskia at gcc dot gnu dot org  2008-02-20 21:52 -------
(In reply to comment #7)
> Why do you think the front-end doesn't know that the destructor is empty? 

Because it is non trivial.  Try it with a more complex case where you have
multiple layer destructors and inlining, you will see that the front-end does
not know if it is empty until way after inlining.

-- Pinski


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661

[Bug tree-optimization/19661] unnecessary atexit calls emitted for static objects with empty destructors

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-09-24 17:42 -------
Hmm, the easy way to to have a pass which finds all the atexit calls and sees if the function which is 
passed is a const function and if it is, then just remove the atexit call.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |tree-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19661