Before, the interrupt would remain enabled during reception, which would
re-set the PCINT flag because of the level changes inside the received
byte. Because interrupts are globally disabled, this would not
immediately trigger an interrupt, but the flag would be remembered to
trigger another PCINT interrupt immediately after the first one is
processed.
Typically this was not a problem, because the second interrupt would see
the stop bit, or an idle line, and decide that the interrupt triggered
for someone else. However, at high baud rates, this could cause the
next interrupt for the real start bit to be delayed so much that the
byte got corrupted.
By clearing the interrupt mask bit for just the RX pin (as opposed to
the PCINT mask bit for the entire port), any PCINT events on other bits
can still set the PCINT flag and be processed as normal. In this case,
it's likely that there will be corruption, but that's inevitable when
(other) interrupts happen during SoftwareSerial reception.
This precalculates the mask register and value, making setRxIntMask
considerably less complicated. Right now, this is not a big deal, but
simplifying it allows using it inside the ISR next.
Since those functions are only called once now, it makes sense to inline
them. This saves a few bytes of program space, but also saves a few
cycles in the critical RX path.
Somehow gcc 4.8 doesn't inline this function, even though it is always
called with constant arguments and can be reduced to just a few
instructions when inlined. Adding the always_inline attribute makes gcc
inline it, saving 46 bytes on the Arduino uno.
gcc 4.3 already inlined this function, so there are no space
savings there.
If an interrupt causing overflow would occur between reading
_buffer_overflow and clearing it, this overflow condition would be
immediately cleared and never be returned by overflow().
By only clearing the overflow flag if an overflow actually occurred,
this problem goes away (worst case overflow() returns false even though
an overflow _just_ occurred, but then the next call to overflow() will
return true).
