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Martin Sebor commented on STDCXX-491: ------------------------------------- See the following thread for the patch and a discussion: http://www.mail-archive.com/stdcxx-dev@incubator.apache.org/msg03993.html > 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 > > 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 -- This message is automatically generated by JIRA. - You can reply to this email to add a comment to the issue online.