Boost C++ Libraries: Ticket #817: Performance problem in iostreams

gmsb — Mon, 29 Jan 2007 16:01:09 GMT

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I used to have exactly the performance problems described. Here is the patch I've been using. Hopefully Jonathan Turkanis can review whether this patch has undesirable side-effects.
template<typename T, typename Tr, typename Alloc, typename Mode>
typename indirect_streambuf<T, Tr, Alloc, Mode>::pos_type
indirect_streambuf<T, Tr, Alloc, Mode>::seek_impl
    (stream_offset off, BOOST_IOS::seekdir way, BOOST_IOS::openmode which)
{
    //XX+ GMSB 20-Feb-2006 Performance fix for tellg() and small seekg()
    if ( gptr() != 0                                          // when currently input buffered
      && way == BOOST_IOS::cur && which == BOOST_IOS::in      // for a relative seek
      && eback() - gptr() <= off && off <= egptr() - gptr() ) // within the buffer
    {
      gbump( off );
      off = 0;
      const pos_type pos_at_egptr = obj().seek(off, way, which, next_);
      return offset_to_position( position_to_offset( pos_at_egptr ) - static_cast<stream_offset>(egptr() - gptr()) );
    }
    //XX- GMSB 20-Feb-2006 Performance fix for tellg() and small seekg()
    if (pptr() != 0)
        this->BOOST_IOSTREAMS_PUBSYNC(); // sync() confuses VisualAge 6.
    if (way == BOOST_IOS::cur && gptr())
        off -= static_cast<off_type>(egptr() - gptr());
    setg(0, 0, 0);
    setp(0, 0);
    return obj().seek(off, way, which, next_);
}

component changed; severity set

Daryle Walker — Fri, 03 Aug 2007 12:20:58 GMT

component None → iostreams
severity → Problem

status, resolution changed

Jonathan Turkanis — Sun, 30 Dec 2007 09:57:49 GMT

status assigned → closed
resolution None → fixed

I applied the patch from GMSB in [42367] and [42638] in branches/iostreams_dev, to be merged with trunk shortly.

I changed the code slightly, to reflect the coding style of the library and to remove two unnecessary conversions. Table 88 of the C++ standard shows that a value of type off_type can be subtracted from a value of type pos_type to yield a value of type pos_type; since in the case at hand we are dealing with small offsets it is safe to convert egptr() - gptr() to off_type.