LZ10 decompression is builtin to the GBA's bios, so we don't need ZX0. It's also significantly faster
(618 usec instead of 2311 usec in my personal benchmark code for decompression of the same data)
And it seems like by doing so, we saved 1 KB as well!
So, seems like replacing ZX0 is the right move.
The reason I didn't initially is because I misunderstood the documentation. I assumed LZ77UnCompWram could only uncompress into EWRAM, not IWRAM.
But it turns out it can do both.
And using standardized tools is usually better than using a custom implementation.
The only downside of this right now, is that we can no longer stream text tables through a smaller buffer than the entire decompressed size.
Anyway, things seem to work fine, so bye bye ZX0. It's been fun.
This commit moves payload_builder and the z80_asm code to the data-generator subproject in order to generate the gameboy payloads
at compile time instead of at runtime.
In addition, we select a couple of base payloads (more than 1 for compressibility's sake) and generate binary patches to transform them into
other payloads. We then generate a binary file with both the base payload and binary patches and compress these files with zx0.
This reduces the rom size by about 8 KB.
There was a crash happening with ptgb::vector when you'd press A on the CONFIRM button of the box screen. It only occurred on actual gba hardware and
was a real heisenbug: as soon as you'd add code to display logs on screen, the problem would disappear. So it was very difficult to figure this one
out. We're not even entirely sure why, but it looks like the malloc/realloc/free use in ptgb::vector would cause issues.
Maybe it was alignment, but after messing with the code we also saw a warning appear in the terminal telling us that realloc wouldn't properly
deal with non-POD types. It complained about this very thing while referring to the add_track() function, which stores ptgb::vectors inside another
ptgb::vector. We also didn't have a custom copy constructor yet to actually copy the buffer instead of its pointer.
All of these could potentially have led to the crash. But debugging during the link cable flow was difficult, so we were never able to confirm it in
a debugger, log or dump.
Because I suspected the high IWRAM consumption (especially now with ZX0 decompression) for a while, I also did an optimization in mystery_gift_builder
to pass global_memory_buffer as its section_30_data buffer instead. This reduces IWRAM consumption by 4 KB.
There was another problem I discovered during my crash hunt: the out_array (now payload_buffer) was allocated as a 672 byte array, but the payloads
were actually 707 bytes. Therefore writing this to the buffer caused a buffer overflow, thereby corrupting the global variables appearing after it in
IWRAM. It turned out eventually that none of these variables were really critical, but it could explain some minor bugs GearsProgress has seen.
I also did a few performance optimizations:
- At various stages in the code, for loops were used to copy data from one buffer into another byte-by-byte. This was far from optimal because the gba
cpu can load/copy 4 bytes at a time if you ask it to. So I replaced those with memcpy(), which is a hand-optimized assembly function to copy data
using this principle.
- generate_payload was being called twice: once at start_link and once at continue_link, giving the exact same result, even though it was already
being stored in a global buffer allocated in IWRAM. This was also a fairly heavy function. So I optimized the code to only initialize it once in
the script chain and then just retrieve the buffer.
- generate_payload was constructing the eventual payload twice even within the same call. That's because it first merged z80_rng_seed, z80_payload
and z80_patchlist into a full_data ptgb::vector, after which it then copied the data again to out_array (now called payload_buffer). I eliminated the
full_data vector now.
