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* [Bug c++/110879] New: Unnecessary reread from memory in a loop
@ 2023-08-02 19:26 palevichva at gmail dot com
  2023-08-03 11:42 ` [Bug libstdc++/110879] [14 Regression] Unnecessary reread from memory in a loop with std::vector rguenth at gcc dot gnu.org
                   ` (5 more replies)
  0 siblings, 6 replies; 7+ messages in thread
From: palevichva at gmail dot com @ 2023-08-02 19:26 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110879

            Bug ID: 110879
           Summary: Unnecessary reread from memory in a loop
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: palevichva at gmail dot com
  Target Milestone: ---

Created attachment 55678
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55678&action=edit
preprocessed file by g++ from revision dd2eb972a

I've found a strange regression in optimization. Trunk version of g++ produces
less optimal assembly. It rereads same memory location in every iteration of a
loop. More specifically, it rereads fields _M_finish and _M_end_of_storage of a
vector from memory every push_back call, although it is not necessary.
Released version 13.2 doesn't do that, and just uses values from registers.

I'm compiling following code:

#include <vector>

std::vector<int> f(std::size_t n) {
    std::vector<int> res;
    res.reserve(n);
    for (std::size_t i = 0; i < n; ++i) {
        res.push_back(i*i);
    }
    return res;
}

The main body of a loop looks like this:
~/.local/gcc/bin/g++ -S -fverbose-asm -O3 -std=c++20 pb.cpp

>.L41:
># /home/scaiper/.local/gcc/include/c++/14.0.0/bits/stl_construct.h:97:     { return ::new((void*)__location) _Tp(std::forward<_Args>(__args)...); }
>        movl    %r15d, (%rbx)   # _3, *prephitmp_51
># /home/scaiper/.local/gcc/include/c++/14.0.0/bits/vector.tcc:119:          ++this->_M_impl._M_finish;
>        addq    $4, %rbx        #, tmp135
>        movq    %rbx, 8(%rbp)   # tmp135, res_8(D)->D.35756._M_impl.D.35067._M_finish
>.L8:
># pb.cpp:6:     for (std::size_t i = 0; i < n; ++i) {
>        addq    $1, %r13        #, i
># pb.cpp:6:     for (std::size_t i = 0; i < n; ++i) {
>        cmpq    %r13, %r12      # i, n
>        je      .L1     #,
># /home/scaiper/.local/gcc/include/c++/14.0.0/bits/vector.tcc:114:      if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
>        movq    8(%rbp), %rbx   # res_8(D)->D.35756._M_impl.D.35067._M_finish, prephitmp_51
># /home/scaiper/.local/gcc/include/c++/14.0.0/bits/vector.tcc:114:      if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
>        movq    16(%rbp), %rax  # res_8(D)->D.35756._M_impl.D.35067._M_end_of_storage, pretmp_52
>.L16:
># pb.cpp:7:         res.push_back(i*i);
>        movl    %r13d, %r15d    # i, _3
>        imull   %r13d, %r15d    # i, _3
># /home/scaiper/.local/gcc/include/c++/14.0.0/bits/vector.tcc:114:      if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
>        cmpq    %rax, %rbx      # pretmp_52, prephitmp_51
>        jne     .L41    #,

Same loop as produced by 13.2:
~/.local/gcc-13.2/bin/g++ -v -S -fverbose-asm -O3 -std=c++20 pb.cpp

>.L43:
># /home/scaiper/.local/gcc-13.2/include/c++/13.2.0/bits/stl_construct.h:97:     { return ::new((void*)__location) _Tp(std::forward<_Args>(__args)...); }
>        movl    %r12d, (%rcx)   # _3, *prephitmp_4
># /home/scaiper/.local/gcc-13.2/include/c++/13.2.0/bits/vector.tcc:119:             ++this->_M_impl._M_finish;
>        addq    $4, %rcx        #, prephitmp_4
>        movq    %rcx, 8(%rbp)   # prephitmp_4, res_8(D)->D.35699._M_impl.D.35010._M_finish
>.L8:
># pb.cpp:6:     for (std::size_t i = 0; i < n; ++i) {
>        addq    $1, %rbx        #, i
># pb.cpp:6:     for (std::size_t i = 0; i < n; ++i) {
>        cmpq    %rbx, %r13      # i, n
>        je      .L1     #,
>.L18:
># pb.cpp:7:         res.push_back(i*i);
>        movl    %ebx, %r12d     # i, _3
>        imull   %ebx, %r12d     # i, _3
># /home/scaiper/.local/gcc-13.2/include/c++/13.2.0/bits/vector.tcc:114:         if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
>        cmpq    %r8, %rcx       # prephitmp_74, prephitmp_4
>        jne     .L43    #,

Notice this extra commands in the first snippet:
movq    8(%rbp), %rbx
movq    16(%rbp), %rax

I've bisected this problem to the commit dd2eb972a (libstdc++: Use RAII in
std::vector::_M_realloc_insert)
(https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=dd2eb972a5b063e10c83878d5c9336a818fa8291).
It doesn't look like commit is the problem. Code looks pretty equivalent. But
for some reason compiler produces different result.

I'm using version built from aforementioned commit dd2eb972a:
Target: x86_64-pc-linux-gnu
Configured with: ../gcc/configure --enable-languages=c++ --disable-multilib
--prefix=/home/scaiper/.local/gcc
gcc version 14.0.0 20230623 (experimental) (GCC)
COLLECT_GCC_OPTIONS='-v' '-S' '-fverbose-asm' '-O3' '-std=c++20'
'-shared-libgcc' '-mtune=generic' '-march=x86-64'

Comparing with 13.2:
Target: x86_64-pc-linux-gnu
Configured with: ../gcc/configure --enable-languages=c++ --disable-multilib
--prefix=/home/scaiper/.local/gcc-13.2
gcc version 13.2.0 (GCC)
COLLECT_GCC_OPTIONS='-v' '-S' '-fverbose-asm' '-O3' '-std=c++20'
'-shared-libgcc' '-mtune=generic' '-march=x86-64'

^ permalink raw reply	[flat|nested] 7+ messages in thread
end of thread, other threads:[~2023-11-23 10:58 UTC | newest]

Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-08-02 19:26 [Bug c++/110879] New: Unnecessary reread from memory in a loop palevichva at gmail dot com
2023-08-03 11:42 ` [Bug libstdc++/110879] [14 Regression] Unnecessary reread from memory in a loop with std::vector rguenth at gcc dot gnu.org
2023-09-01 15:02 ` cvs-commit at gcc dot gnu.org
2023-09-01 16:12 ` redi at gcc dot gnu.org
2023-11-22  8:53 ` jakub at gcc dot gnu.org
2023-11-22 11:27 ` redi at gcc dot gnu.org
2023-11-23 10:58 ` cvs-commit at gcc dot gnu.org

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