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hrydgard / ppsspp

A PSP emulator for Android, Windows, Mac and Linux, written in C++. Want to contribute? Join us on Discord at https://discord.gg/5NJB6dD or just send pull requests / issues. For discussion use the forums at forums.ppsspp.org.
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Ridge Racers (USJS00001) - CPU autodrive was Algorithm buggy #2990

Open triglav1024 opened 11 years ago

triglav1024 commented 11 years ago

Options -> AV player mode https://www.youtube.com/watch?v=hRrVBM2-OWc https://www.youtube.com/watch?v=XJkM729PeeE https://www.youtube.com/watch?v=3eQ7BlocmUo

Ridge Racers - JP 1.01 / USA 1.00 / EUR 1.00 / HK 1.00 / Asia 1.00

ghost commented 4 years ago

When i played this on Android, I noticed that JIT, IR Interpreter, and Interpreter executes the CPU Autodrive differently, causing different algorithms to happen... Try and list out the differences when using those 3 CPU Cores, and you might find that one mathematical error...

ghost commented 4 years ago

The fact that it desyncs makes me sad because I kept watching those replays on my real PSP when the Wi-Fi dies, brown out, or if im getting bored after I finished the game... The desyncing kinda represents how this game series is getting forgotten because Ridge Racer 8 never got released and the fact that the bug is still here represents that the game got left unfinished and forgotten....

unknownbrackets commented 4 years ago

The xbox 360 and PSP have different CPUs, which is why they have different problems.

The heart of this problem is math. Games use what are called "vector" or "simd" instructions to calculate math in speed critical situations and 3D formulas. If you look here, the Xbox 360 CPU had special modifications to do dot products on the CPU faster:

https://en.wikipedia.org/wiki/Xbox_360_technical_specifications

To help you understand, let's say I was adding up these two numbers:

6628451234 984726456

Google says the result is 7613177690, which is probably accurate. But what if someone did it by hand, and got it wrong? What if they thought it was 7613177609? It's a small difference, but the small differences add up - like a "hyperspace jump" in slightly the wrong direction.

Some (but not all) of the PSP CPU's calculations were wrong - bad math. Crucially, unless we get the math wrong and get it wrong in exactly the same way - these replays won't play correctly.

Xenia probably doesn't have this problem because the Xbox 360 got high marks on its maths. Just like a modern PC or a phone, it can add, multiply, divide, and subtract correctly. So there's no need to simulate the errors.

Notably, it's probably the same reason again for RPCS3. The 7 SPEs also use inaccurate maths.

The reason these calculations were wrong? Most likely speed, power, or cost. Doing math correctly might've required more silicon, more battery juice, or might've made games run slower. These errors are at the hardware level and we don't fully understand them. We don't know exactly how it calculates square roots, and what shortcuts it's using to get a close, but wrong, value.

It's not that anyone doesn't care or wants to see the series dwindle by any means. Several people have spent hours debugging, working on, and trying to fix this very issue.

-[Unknown]

Back2Life888 commented 3 years ago

PPSSPP 1.11.3. Issue still persists.

Back2Life888 commented 2 years ago

I discovered something odd with the desyncing replays. It wouldn't just bug out pre-recorded replays, it could also bug out your own replays. If you save a replay and then update or downgrade PPSSPP to another version, that replay may bug out and desync like the pre-recorded ones. I have some replays saved on an old PPSSPP version and the car just desyncs. Backtracking to an older version fixes the bug on some replays and some of them get fixed somehow.

unknownbrackets commented 2 years ago

This is because we've made some updates to improve accuracy in some CPU instructions. It hasn't been enough to make the pre-recorded missions play correctly, but it means that recordings from previous versions no longer play the way they used to.

This issue basically relies on specific and very accurate mathematical results, matching the same mathematical errors that the PSP CPU makes. Or at least, so we think.

-[Unknown]

unknownbrackets commented 1 year ago

Just an idea that I thought of just now but have not pursued on this:

It could be that it isn't just accuracy, but that there's some actual bug in the math equation, but things work out as long as the replay replicates it because it's small. Specifically, I don't think anyone has ever checked if there is any suspicious vector overlap cases. There's been evidence to suggest that unlike PPSSPP's code, the actual VFPU doesn't guarantee overlap safety in all cases (and when it does, it seems to do so by performing operations in reverse order.)

Probably not likely, but I have already tried flushing everything to zero, adjusting rounding modes, forcing things to the decently accurate vdot, etc.

-[Unknown]