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https://issues.apache.org/jira/browse/STDCXX-491?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12517675
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Martin Sebor commented on STDCXX-491:
-------------------------------------
Here are the timings for gcc on the same machine for comparison. Interesting
that the gcc-generated code is faster than Intel's own (although we're still
more than 30% faster)...
$ gcc --version && g++ -m64 -O2 -D_REENTRANT -pthread t.cpp && time ./a.out
0xffffffff
gcc (GCC) 3.4.6 20060404 (Red Hat 3.4.6-3)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
real 0m48.376s
user 0m45.259s
sys 0m3.104s
> string::push_back() slow
> ------------------------
>
> Key: STDCXX-491
> URL: https://issues.apache.org/jira/browse/STDCXX-491
> Project: C++ Standard Library
> Issue Type: Bug
> Components: 21. Strings
> Affects Versions: 4.1.3
> Environment: gcc 4.1.2, Linux/x86_64
> Reporter: Mark Brown
> Assignee: Martin Sebor
> Fix For: 4.2
>
>
> According to my timings string::push_back() in stdcxx 4.1.3 is more than
> twice as slow than the same function in gcc 4.1.2 on Linux x86_64:
> $ time ./push_back-stdcxx 100000000
> real 0m2.175s
> user 0m2.004s
> sys 0m0.172s
> $ time ./push_back-gcc 100000000
> real 0m0.924s
> user 0m0.760s
> sys 0m0.164s
> #include <cassert>
> #include <cstdlib>
> #include <string>
> int main (int argc, char *argv[])
> {
> const int N = argc < 2 ? 1 : std::atoi (argv [1]);
> std::string str;
> for (int i = 0; i < N; ++i)
> str.push_back ('x');
> assert (str.size () == std::size_t (N));
> }
> Comparing the generated assembly, the gcc push_back() is mostly inline but
> the stdcxx push_back() is not:
> stdcxx:
> _Z8pushbackRSsc:
> .LFB449:
> movq (%rdi), %rax
> movl %esi, %edx
> movl $1, %ecx
> movsbl %dl,%r8d
> xorl %edx, %edx
> movq -8(%rax), %rsi
> jmp _ZNSs7replaceEmmmc
> gcc:
> _Z8pushbackRSsc:
> .LFB904:
> movq %rbp, -16(%rsp)
> .LCFI0:
> movq %r12, -8(%rsp)
> .LCFI1:
> movq %rdi, %rbp
> movq %rbx, -24(%rsp)
> .LCFI2:
> subq $24, %rsp
> .LCFI3:
> movq (%rdi), %rax
> movl %esi, %r12d
> subq $24, %rax
> movq (%rax), %rbx
> addq $1, %rbx
> cmpq 8(%rax), %rbx
> ja .L2
> movl 16(%rax), %eax
> testl %eax, %eax
> jg .L2
> .L4:
> movq (%rbp), %rdx
> movq -24(%rdx), %rax
> movb %r12b, (%rdx,%rax)
> movq (%rbp), %rax
> subq $24, %rax
> movl $0, 16(%rax)
> movq %rbx, (%rax)
> movb $0, 24(%rax,%rbx)
> movq (%rsp), %rbx
> movq 8(%rsp), %rbp
> movq 16(%rsp), %r12
> addq $24, %rsp
> ret
> .p2align 4,,7
> .L2:
> movq %rbx, %rsi
> movq %rbp, %rdi
> call _ZNSs7reserveEm
> jmp .L4
--
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