Re: ostream::operator<<() and sputn()

["Dietmar Kühl" <dietmar_kuehl@yahoo.com>]

Hello,

Of course, you could detect whether the user did specialize basic_streambuf: the default implementation can use a special member name, let’s say, __not_specialized, which the user cannot write in a portable program and, hence, in the user’s specialization. Detecting that could opt into an enhanced code path. I’m not recommending to use such an implementation. When I implemented my own version of IOStreams I used a few special approaches, notably a segmented iterator optimization for std::[io]streambuf_iterator with a bit more direct access to the streambuf’s buffer. For actually buffered streams these things can yield a substantial performance boost. There is some penalty for non-buffered (well, single character buffered) stream buffers, though.

Making changes to the streams while maintaining ABI compatibility is probably not quite worth the trouble, though.

Thanks,
  Dietmar

> On 14 Jul 2021, at 22:31, Jonathan Wakely via Libstdc++ <libstdc++@gcc.gnu.org> wrote:
> 
> On Wed, 14 Jul 2021 at 22:26, Lewis Hyatt via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
>> 
>> Hello-
>> 
>> I noticed that libstdc++'s implementation of ostream::operator<<() prefers
>> to call sputn() on the underlying streambuf for all char, char*, and string
>> output operations, including single characters, rather than manipulate the
>> buffer directly. I am curious why it works this way, it feels perhaps
>> suboptimal to me because sputn() is mandated to call the virtual function
>> xsputn() on every call, while e.g. sputc() simply manipulates the buffer and
>> only needs a virtual call when the buffer is full. I always thought that the
>> buffer abstraction and the resulting avoidance of virtual calls for the
>> majority of operations was the main point of streambuf's design, and that
>> sputn() was meant for cases when the output would be large enough to
>> overflow the buffer anyway, if it may be possible to skip the buffer and
>> flush directly instead?
>> 
>> It seems to me that for most typical use cases, xsputn() is still going to
>> want to use the buffer if the output fits into it; libstdc++ does this in
>> basic_filebuf, for example. So then it would seem to be beneficial to try
>> the buffer prior to making the virtual function call, instead of after --
>> especially because the typical char instantiation of __ostream_insert that
>> makes this call for operator<<() is hidden inside the .so, and is not
>> inlined or eligible for devirtualization optimizations.
>> 
>> FWIW, here is a small test case.
>> 
>> ---------
>> #include <ostream>
>> #include <iostream>
>> #include <fstream>
>> #include <sstream>
>> #include <chrono>
>> #include <random>
>> using namespace std;
>> 
>> int main() {
>>    constexpr size_t N = 500000000;
>>    string s(N, 'x');
>> 
>>    ofstream of{"/dev/null"};
>>    ostringstream os;
>>    ostream* streams[] = {&of, &os};
>>    mt19937 rng{random_device{}()};
>> 
>>    const auto timed_run = [&](const char* label, auto&& callback) {
>>        const auto t1 = chrono::steady_clock::now();
>>        for(char c: s) callback(*streams[rng() % 2], c);
>>        const auto t2 = chrono::steady_clock::now();
>>        cout << label << " took: "
>>             << chrono::duration<double>(t2-t1).count()
>>             << " seconds" << endl;
>>    };
>> 
>>    timed_run("insert with put()", [](ostream& o, char c) {o.put(c);});
>>    timed_run("insert with op<< ", [](ostream& o, char c) {o << c;});
>> }
>> ---------
>> 
>> This is what I get with the current trunk:
>> ---------
>> insert with put() took: 6.12152 seconds
>> insert with op<<  took: 13.4437 seconds
>> ---------
>> 
>> And this is what I get with the attached patch:
>> ---------
>> insert with put() took: 6.08313 seconds
>> insert with op<<  took: 8.24565 seconds
>> ---------
>> 
>> So the overhead of calling operator<< vs calling put() was reduced by more
>> than 3X.
>> 
>> The prototype patch calls an internal alternate to sputn(), which tries the
>> buffer prior to calling xsputn().
> 
> This won't work if a user provides an explicit specialization of
> basic_streambuf<char, MyTraits>. std::basic_ostream<char, MyTraits>
> will still try to call your new function, but it won't be present in
> the user's specialization, so will fail to compile. The basic_ostream
> primary template can only use the standard API of basic_streambuf. The
> std::basic_ostream<char> specialization can use non-standard members
> of std::basic_streambuf<char> because we know users can't specialize
> that.