[Bug c++/110819] New: Missed optimization: when vector size is 0 but vector::reserve has been called.

[Bug c++/110819] New: Missed optimization: when vector size is 0 but vector::reserve has been called.

[hiraditya at msn dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110819

            Bug ID: 110819
           Summary: Missed optimization: when vector size is 0 but
                    vector::reserve has been called.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hiraditya at msn dot com
  Target Milestone: ---

#include<vector>

void f(int);

void use_idx_const_size_reserve() {
    std::vector<int> v;
    v.reserve(100000);
    auto s = v.size();
    for (std::vector<int>::size_type i = 0; i < s; i++)
        f(v[i]);
}

$ g++ -O3

use_idx_const_size_reserve():
        sub     rsp, 8
        mov     edi, 400000
        call    operator new(unsigned long)
        mov     esi, 400000
        add     rsp, 8
        mov     rdi, rax
        jmp     operator delete(void*, unsigned long)


$ clang++ -O3 -stdlib=libc++

use_idx_const_size_reserve():        # @use_idx_const_size_reserve()
        ret

[Bug tree-optimization/110819] Missed optimization: when vector's size is 0 but vector::reserve has been called.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110819

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |tree-optimization
           Keywords|                            |missed-optimization
           Severity|normal                      |enhancement

[Bug tree-optimization/110819] Missed optimization: when vector's size is 0 but vector::reserve has been called.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110819

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=110137

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I guess gcc can't optimize this away because it doesn't know if operator new
has side effects. When compiled with clang, libstdc++'s std::vector uses
__builtin_operator_new which always has the -fassume-sane-operator-new
semantics, and so can be optimized.

[Bug tree-optimization/110819] Missed optimization: when vector's size is 0 but vector::reserve has been called.

[hiraditya at msn dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110819

--- Comment #2 from AK <hiraditya at msn dot com> ---
> When compiled with clang, libstdc++'s std::vector uses __builtin_operator_new which always has the -fassume-sane-operator-new semantics, and so can be optimized.

yes clang optimizes with libstdc++ as well. what can be done in gcc for it to
detect that the new+delete pair can be optimized away?

[Bug tree-optimization/110819] Missed optimization: when vector's size is 0 but vector::reserve has been called.

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110819

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |xry111 at gcc dot gnu.org

--- Comment #3 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to AK from comment #2)
> > When compiled with clang, libstdc++'s std::vector uses __builtin_operator_new which always has the -fassume-sane-operator-new semantics, and so can be optimized.
> 
> yes clang optimizes with libstdc++ as well. what can be done in gcc for it
> to detect that the new+delete pair can be optimized away?

Basically we'll need __builtin_operator_new.  See all the "see also" links.