[Bug c++/106502] New: Three calls to __attribute__((const)) function

[Bug c++/106502] New: Three calls to __attribute__((const)) function

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106502

            Bug ID: 106502
           Summary: Three calls to __attribute__((const)) function
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

#include <system_error>

int main()
{
  __builtin_puts(std::system_category().name());
}

With gcc 12 the resulting assembly is:

        .file   "cat.C"
        .text
        .globl  main
        .type   main, @function
main:
.LFB1309:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        pushq   %rbx
        subq    $8, %rsp
        .cfi_offset 3, -24
        call    _ZNSt3_V215system_categoryEv
        call    _ZNSt3_V215system_categoryEv
        movq    (%rax), %rax
        addq    $16, %rax
        movq    (%rax), %rbx
        call    _ZNSt3_V215system_categoryEv
        movq    %rax, %rdi
        call    *%rbx
        movq    %rax, %rdi
        call    puts
        movl    $0, %eax
        movq    -8(%rbp), %rbx
        leave
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE1309:
        .size   main, .-main
        .ident  "GCC: (GNU) 12.1.1 20220507 (Red Hat 12.1.1-1)"
        .section        .note.GNU-stack,"",@progbits


Why is the std::system_category() function called three times?

[Bug c++/106502] Three calls to __attribute__((const)) function

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106502

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-08-01

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
  (void) __builtin_puts (OBJ_TYPE_REF(*(((const struct error_category *)
std::_V2::system_category ())->_vptr.error_category + 16);(const struct
error_category)(const struct error_category *) std::_V2::system_category
()->2B) ((const struct error_category *) std::_V2::system_category ())) >>>>>;


Confirmed. with the const, the front-end didn't wrap the function call with a
SAVE_EXPR so it was duplicated three times with the OBJ_TYPE_REF call.

[Bug c++/106502] Three calls to __attribute__((const)) function

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106502

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
As you can see by the gimplification:
  _1 = std::_V2::system_category ();
  _2 = std::_V2::system_category ();
  _3 = _2->_vptr.error_category;
  _4 = _3 + 16;
  _5 = *_4;
  _6 = std::_V2::system_category ();
  _7 = OBJ_TYPE_REF(_5;(const struct error_category)_1->2B) (_6);

[Bug c++/106502] Three calls to __attribute__((const)) function

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106502

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It doesn't happen unless the name() function is virtual. Reduced:


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

struct Cat
{
  virtual const char* name();
};

__attribute__((const)) Cat& cat();

int main()
{
  puts(cat().name());
}


Produces this gimple:

int main ()
{
  int D.2388;

  _1 = cat ();
  _2 = cat ();
  _3 = _2->_vptr.Cat;
  _4 = *_3;
  _5 = cat ();
  _6 = OBJ_TYPE_REF(_4;(struct Cat)_1->0B) (_5);
  puts (_6);
  D.2388 = 0;
  return D.2388;
}

[Bug c++/106502] Three calls to __attribute__((const)) function

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106502

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Once you enable optimization a single cat () call remains (as expected).  The
FE does

9931          if (TREE_SIDE_EFFECTS (argarray[0]))
9932            argarray[0] = save_expr (argarray[0]);
9933          t = build_pointer_type (TREE_TYPE (fn));
9934          fn = build_vfn_ref (argarray[0], DECL_VINDEX (fn));

but argarray[0] doesn't have TREE_SIDE_EFFECTS set, it even has TREE_READONLY
set on the CONVERT_EXPR wrapping the call.  Note that save_expr would do
nothing on this tree as well (because TREE_READONLY and !TREE_SIDE_EFFECTS).

[Bug c++/106502] Three calls to __attribute__((const)) function

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106502

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I noticed this by adding a printf statement to the const function for temporary
debugging purposes, which is obviously incorrect and not a problem for real
code. It's not observable that it gets called more than once, it just makes
unoptimized code a bit slower.

[Bug c++/106502] Three calls to __attribute__((const)) function

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106502

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=18487
                 CC|                            |egallager at gcc dot gnu.org

--- Comment #6 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #5)
> I noticed this by adding a printf statement to the const function for
> temporary debugging purposes, which is obviously incorrect

Seems related to bug 18487 IMO.

[Bug c++/106502] Three calls to __attribute__((const)) function

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106502

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The way I found the bug might be, but the bug itself is nothing to do with
that.