Apparently sometimes, a proper "frame info" block is not found after
a "sync word". Keep looking for one as needed, instead of giving up
after one try.
This was one reason that the "bartlebeats" mp3s would not play.
In conversion I missed these arguments were being passed, but noticed
it when an implausible value for audio_channel was sent to mp3's
reset_buffer method.
It's not clear whether there was any negative impact to this, but it
should be fixed!
These s16-s21 registers are used by gcc so need to be saved. Future
versions of gcc (beyond v9.1.0), or other compilers, may eventually need
additional registers saved/restored.
See issue #4844.
Recent versions of gcc perform optimisations which can lead to the
following code from the MP_NLR_JUMP_HEAD macro being omitted:
top->ret_val = val; \
MP_NLR_RESTORE_PYSTACK(top); \
*_top_ptr = top->prev; \
This is noticeable (at least) in the unix coverage on x86-64 built with gcc
9.1.0. This is because the nlr_jump function is marked as no-return, so
gcc deduces that the above code has no effect.
Adding MP_UNREACHABLE tells the compiler that the asm code may branch
elsewhere, and so it cannot optimise away the code.
There were several problems with the way this worked -- the read_count
approach was too complicated and I made a mistake "simplifying" it from
WaveFile. And when the right channel was returned, it was off by 1 byte,
making it into static.
Instead, directly track which is the "other" channel that has data
available, and by using the right data type make the "+ channel"
arithmetic give the right result.
This requires a double cast (int16_t*)(void*) due to an alignment warning;
the alignment is now ensured manually, but the compiler doesn't make the
necessary inference that the low address bit must be clear.
When a playing mp3 is deinitted, it's possible to reach get_buffer,
but all the internal pointers are NULL. This would lead to a hard fault.
Avoid it by returning GET_BUFFER_ERROR instead.
