Adding Rand crate (or possibly other no std lib ones)

[reinvantveer]

Hey I tried installing the rand crate for obvious game design reasons, but so far compilation fails using the cargo megadrive --verbose build command (see my fork for the megatowerdefense example, one has to start somewhere).

OK so I'm sure I would have done better asking you first for advice on this, but instead I created a StackOverflow issue because the question wasn't asked there yet.

https://stackoverflow.com/questions/67627335/how-do-i-use-the-rand-crate-without-the-standard-library

I have an inkling that the question is gravitating towards the megadrive cargo command, but I'm out of my depth here. The command exits in a flurry of errors all originating with libc conflicts. Any thoughts?

[ricky26]

True enough, there's no libc on the Mega Drive. Judging by your Stack Overflow post, you're running it without default features though, which shouldn't depend on it. I'm happy to take a look if you've got it spitting out some logs!

The cargo megadrive command is purely for building the final Mega Drive image, it largely just does a cargo build --release and then composes the output into the MD image format. Libraries built as part of that (or just building libraries on their own) can just use regular cargo. (I'm guessing that you've added this as a dependency to your game though!)

[reinvantveer]

I believe this is the magic link: rust-embedded/wg#36 (comment)

[reinvantveer]

I tried (in vain) to fix this through some configuration in a .cargo/config file: https://github.com/reinvantveer/rust-mega-drive/blob/main/examples/megatowerdefense/.cargo/config but this results in much confusion for the compiler:

Step 15/17 : RUN cargo build --release
 ---> Running in 32c32c536c06
   Compiling megadrive-sys v0.1.0 (/rust-mega-drive/libs/megadrive-sys)
   Compiling rand_core v0.6.2
'+m' is not a recognized feature for this target (ignoring feature)
'+m' is not a recognized feature for this target (ignoring feature)
'+m' is not a recognized feature for this target (ignoring feature)
'+m' is not a recognized feature for this target (ignoring feature)
'+m' is not a recognized feature for this target (ignoring feature)
'+m' is not a recognized feature for this target (ignoring feature)
error[E0463]: can't find crate for `core`
  |
  = note: the `m68k-unknown-linux-gnu` target may not be installed

[reinvantveer]

This in an attempt to emulate https://github.com/riscv-rust/riscv-crates/blob/38008c4dd7ea2a5712dfd12b6bfc9d00ea2c4b21/.cargo/config#L8 but it isn't quite there

[ricky26]

I'm afraid that the 68k doesn't actually have a 32-bit multiplication unit so that bug is valid. Adding +m won't work.

This is one of the reasons that only release builds work: release builds strip unused code. There are a few functions missing from compiler-builtins for the 68k (like __mulsi3).

You can probably find copies of equivalent functions from the era written in assembly and try and implement them in the entry assembly but that's sadly not going to be in lovely Rust. Otherwise, we need to get those library functions implemented.

I've added a tracking issue: #4

[reinvantveer]

Well, it was worth a shot but I can hardly fault the makers of the rand crate for not supporting m68k targets 😂 . We're definitely not out of options either, I already took a peek at Stephane-D/SGDK to see how they do things.

[reinvantveer]

Oh before I forget, I managed to get this working! Thanks to Stephane-D/SGDK, I ported their code over:

const GFX_HVCOUNTER_PORT: u32 = 0xC00008;

struct PseudoRng {
    current_rand:  u16,
}

impl PseudoRng {
    // Thank you Stephane Dallongeville!
    pub fn from_seed(seed: u16) -> PseudoRng {
        PseudoRng {
            current_rand: seed ^ 0xD94B // XOR with some val to avoid 0
        }
    }

    pub fn random(&mut self) -> u16 {
        unsafe {
            // https://github.com/Stephane-D/SGDK/blob/908926201af8b48227be4dbc8fbb0d5a18ac971b/src/tools.c#L36
            let hv_counter = read_volatile(&GFX_HVCOUNTER_PORT) as u16;
            self.current_rand ^= (self.current_rand >> 1) ^ hv_counter;
            self.current_rand ^= self.current_rand << 1;
            self.current_rand
        }
    }
}

As a rather dubious achievement, it's the first time I've used the unsafe keyword 🙃

[parasyte]

You may want to use a crate like randomize instead.

You can use the HV COUNTER for entropy to initialize the random state, but the algorithm shown above will produce very poor quality random values, especially when advanced in a tight loop.

[reinvantveer]

I appreciate the suggestion @parasyte but to date I haven't found a rng no_std lib that is supported on the m68k's lack of support for 32-bit multiplication. Unfortunately (I checked today), this also holds for the randomize crate. I'd basically have to re-implement it using u16 or i16, probably.

[reinvantveer]

It would be cool to generate a plot of the number distribution for certain time intervals, and see if something like bit rotation or something else helps.