public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug libstdc++/63466] New: sstream is very slow
@ 2014-10-06 16:56 andi-gcc at firstfloor dot org
2014-10-06 17:59 ` [Bug libstdc++/63466] " glisse at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: andi-gcc at firstfloor dot org @ 2014-10-06 16:56 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63466
Bug ID: 63466
Summary: sstream is very slow
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: andi-gcc at firstfloor dot org
sstream is very slow. Comparing two simple programs that parse a stream with C
and with sstream. The sstream version is an order of magnitude slower.
gcc version 4.9.1 20140423 (prerelease) (GCC)
# C++
% time ./a.out < testfile
real 0m0.893s
user 0m0.888s
sys 0m0.004s
# C
time ./tstream-c < testfile
real 0m0.032s
user 0m0.030s
sys 0m0.001s
Here's a profile.
16.13% a.out libc-2.18.so [.] _IO_getc
10.39% a.out libc-2.18.so [.] _IO_ungetc
9.15% a.out libstdc++.so.6.0.20 [.] std::basic_istream<char,
std::char_traits<char> >& std::getline<char, std::char_traits<char>,
std::allocator<char> >(std::basic_istream<char, std::char_traits<char> >&,
std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, char)
7.87% a.out libstdc++.so.6.0.20 [.] __dynamic_cast
4.99% a.out libc-2.18.so [.] __GI___strcmp_ssse3
3.95% a.out libstdc++.so.6.0.20 [.] std::basic_istream<char,
std::char_traits<char> >& std::operator>><char, std::char_traits<char>,
std::allocator<char> >(std::basic_istream<char, std::char_traits<char> >&,
std::basic_string<char, std::char_traits<char>, std::allocator<char> >&)
3.89% a.out libc-2.18.so [.] _int_free
2.79% a.out libstdc++.so.6.0.20 [.]
__cxxabiv1::__vmi_class_type_info::__do_dyncast(long,
__cxxabiv1::__class_type_info::__sub_kind, __cxxabiv1::__class_type_info
const*, void const*, __cxxabiv1::__class_type_info const*, void const*,
__cxxabiv1::__class_type_info::__dyncast_result&) const
2.65% a.out a.out [.] main
2.58% a.out libc-2.18.so [.] malloc
2.30% a.out libstdc++.so.6.0.20 [.]
__cxxabiv1::__si_class_type_info::__do_dyncast(long,
__cxxabiv1::__class_type_info::__sub_kind, __cxxabiv1::__class_type_info
const*, void const*, __cxxabiv1::__class_type_info const*, void const*,
__cxxabiv1::__class_type_info::__dyncast_result&) const
1.96% a.out libc-2.18.so [.] _int_malloc
1.86% a.out libstdc++.so.6.0.20 [.]
std::istream::sentry::sentry(std::istream&, bool)
1.55% a.out libc-2.18.so [.] _IO_sputbackc
1.51% a.out libstdc++.so.6.0.20 [.]
__gnu_cxx::stdio_sync_filebuf<char, std::char_traits<char> >::underflow()
Test case:
Generate test file:
perl -e 'for($i=0;$i<1000000;$i++) { printf("%d %d\n", $i, $i); } ' > testfile
C++ version:
#include <iostream>
#include <string>
#include <sstream>
using namespace std;
void __attribute__((noinline, noclone)) func(string &, string &)
{
}
int main()
{
string line;
while (getline(cin, line)) {
istringstream iss(line);
string index, s;
if (!(iss >> index >> s))
continue;
func(index, s);
}
return 0;
}
C version:
#define _GNU_SOURCE 1
#include <stdio.h>
#include <string.h>
void __attribute__((noinline, noclone)) func(char *a, char *b)
{
}
int main()
{
char *line = NULL;
size_t linelen = 0;
while (getline(&line, &linelen, stdin) > 0) {
char *p = line;
char *a = strsep(&p, " \t\n");
char *b = strsep(&p, " \t\n");
func(a, b);
}
return 0;
}
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug libstdc++/63466] sstream is very slow
2014-10-06 16:56 [Bug libstdc++/63466] New: sstream is very slow andi-gcc at firstfloor dot org
@ 2014-10-06 17:59 ` glisse at gcc dot gnu.org
2014-10-06 18:13 ` andi-gcc at firstfloor dot org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: glisse at gcc dot gnu.org @ 2014-10-06 17:59 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63466
--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
To be a bit less unfair, you could pull the declarations of the 3 variables out
of the loop. Even if optimizations are possible, I doubt we can go anywhere
near the C perf...
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug libstdc++/63466] sstream is very slow
2014-10-06 16:56 [Bug libstdc++/63466] New: sstream is very slow andi-gcc at firstfloor dot org
2014-10-06 17:59 ` [Bug libstdc++/63466] " glisse at gcc dot gnu.org
@ 2014-10-06 18:13 ` andi-gcc at firstfloor dot org
2014-10-07 10:31 ` redi at gcc dot gnu.org
2014-10-07 10:39 ` redi at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: andi-gcc at firstfloor dot org @ 2014-10-06 18:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63466
--- Comment #2 from Andi Kleen <andi-gcc at firstfloor dot org> ---
Looking at the profile there's plenty of room for optimization. e.g. not using
getc/ungetc, but directly accessing the buffer, or maybe even some kind of
template specialization.
With the variables pulled out it's faster, but still a lot slower than C:
% time ./a.out < testfile
real 0m0.400s
user 0m0.397s
sys 0m0.002s
% time ./tstream-c < testfile
real 0m0.033s
user 0m0.028s
sys 0m0.004s
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug libstdc++/63466] sstream is very slow
2014-10-06 16:56 [Bug libstdc++/63466] New: sstream is very slow andi-gcc at firstfloor dot org
2014-10-06 17:59 ` [Bug libstdc++/63466] " glisse at gcc dot gnu.org
2014-10-06 18:13 ` andi-gcc at firstfloor dot org
@ 2014-10-07 10:31 ` redi at gcc dot gnu.org
2014-10-07 10:39 ` redi at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: redi at gcc dot gnu.org @ 2014-10-07 10:31 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63466
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
You're comparing apples and oranges.
This function forces you to do additional allocations for the arguments, which
has nothing to do with iostream performance:
void __attribute__((noinline, noclone)) func(string &, string &)
This allocates even more memory:
istringstream iss(line);
The expression (iss >> index >> s) is far more flexible than your C version,
handling adjacent whitespace and being able to extract arbitrary types from the
stream.
If you don't need those extra allocations and flexibility then don't use them;
your C code is a valid C++ program too. But rewriting your C++ code to be
equivalent to the C code (e.g. by using std::string::find_first_of) would
remove any use of stringstream, leaving only the performance of std::getline as
a limiting factor, which is not the topic of this bug report.
So if your point is simply "iostreams are slower than stdio" then yes, we know,
welcome to 1998 ;-)
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug libstdc++/63466] sstream is very slow
2014-10-06 16:56 [Bug libstdc++/63466] New: sstream is very slow andi-gcc at firstfloor dot org
` (2 preceding siblings ...)
2014-10-07 10:31 ` redi at gcc dot gnu.org
@ 2014-10-07 10:39 ` redi at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: redi at gcc dot gnu.org @ 2014-10-07 10:39 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63466
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The calls you see to getc are nothing to do with <sstream>, they're from the
std::getline call reading from stdin, and are required because you didn't tell
the C++ runtime that you don't need it to be synchronised with C stdio. If you
don't need to mix C and C++ I/O then you forgot the most important thing for
iostream performance on standard I/O streams:
std::ios::sync_with_stdio(false);
If you don't need that synchronisation, don't use it.
Something like this is a more accurate equivalent of the C program, and much
closer in performance:
#include <iostream>
#include <string>
#include <sstream>
using namespace std;
void __attribute__((noinline, noclone)) func(char*, char*)
{
}
int main()
{
std::ios::sync_with_stdio(false);
string line;
while (getline(cin, line)) {
char* a = &line[0];
char* b = nullptr;
auto pos1 = line.find_first_of(" \t\n");
if (pos1 != std::string::npos) {
line[pos1] = '\0';
b = a + pos1 + 1;
auto pos2 = line.find_first_of(" \t\n", pos1+1);
if (pos2 != std::string::npos) {
line[pos2] = '\0';
}
}
func(a, b);
}
}
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2014-10-07 10:39 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2014-10-06 16:56 [Bug libstdc++/63466] New: sstream is very slow andi-gcc at firstfloor dot org
2014-10-06 17:59 ` [Bug libstdc++/63466] " glisse at gcc dot gnu.org
2014-10-06 18:13 ` andi-gcc at firstfloor dot org
2014-10-07 10:31 ` redi at gcc dot gnu.org
2014-10-07 10:39 ` redi at gcc dot gnu.org
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).