A few months ago, I wrote a previous implementation of the _PyBytesWriter API which embedded the "current pointer" inside _PyBytesWriter API. The problem was that GCC produced less efficient code than expect for the hotspot of the encoder.

In the new implementation (attached patch), the "current pointer" is unchanged: it's still a variable local to the encoder function. Instead, the current pointer became a *parameter* to all _PyBytesWriter *functions*.

I expect to not touch performances of encoders for valid encoded strings (when the code calling error handlers is not used), which is important since we have very good performance here.

_PyBytesWriter is not restricted to the code to allocate the buffer.

--

bytes_writer.patch:

+    char stackbuf[256];

Oh, I forgot to mention this other small optimization. I also added a small buffer allocated on the C stack for the UCS1 encoder (ASCII, Latin1). It may optimize a little bit encoding when the output string is smaller than 256 bytes when the error handler is used.

The optimization comes from the very efficient UTF-8 encoder.