streambuf performance 200x

[Martin Sebor]

Nathan Myers wrote:
> It's time to talk about fixing core streambuf performance.  As it 
> is, when last I measured, IIRC, it was over two orders of magnitude 
> slower than stdio.   We need to do something about that before we 
> commit to a long period of frozen ABI.
> 
> To accomplish this, we have to have absolutely minimal, inline
> implementations of basic_streambuf::sputc and sgetc.  All the
> complications have to go, so that, e.g., sputc expands to the machine 
> equivalent of something very like
> 
>    (this->pp != this->ep) ? (*this->pp++ = c) : this->overflow(c);
> 
> and correspondingly for sgetc.
>

Note that this is complicated by the requirements on strstreambuf.
This program must still return 0 (it currently fails with a non-0
exit status with libstdc++):

     #include <cstring>
     #include <strstream>

     int main ()
     {
         char buf[] = "abc";

         std::strstreambuf sb (buf, sizeof buf, buf);

         sb.freeze ();

         sb.sputc ('x');

         return std::strcmp (buf, "abc");
     }

Regards
Martin