LLVM assertion in simdnoise crate

[pietroalbini]

The simdnoise crate is failing to build with an LLVM assertion:

rustc: /checkout/src/llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp:118: static uint8_t (anonymous namespace)::X86MCCodeEmitter::ModRMByte(unsigned int, unsigned int, unsigned int): Assertion `Mod < 4 && RegOpcode < 8 && RM < 8 && "ModRM Fields out of range!"' failed.

This is not a regression, it's confirmed to also fail on 1.28.0 (alt builds are not available past that).

[jonas-schievink]

Wow, this one's amazing.

[nikic]

This one still reproduces on master.

[nikic]

Unfortunately, I wasn't able to get this to assert with --emit=llvm-bc and a separate llc invocation.

[FrankSD]

Can an absolute beginner take this one?Let me know if someone is working on this already

[jonas-schievink]

@FrankSD This is an assertion failure in the very depths of LLVM's machine code backend, you can try to find a minimal test case if you want, but I wouldn't recommend finding or fixing the actual bug to a beginner (but feel free to try if you feel very ambitious!). You can also try your hand at one of the E-Easy issues, which are specifically meant for beginners.

[mati865]

I can reproduce it with some old nightlies but rustc 1.33.0-nightly (03acbd71c 2019-01-14) works just fine.

[nikic]

@mati865 Did you use an assertion-enabled build? I tried again just now and am still seeing the assertion:

nikic@helios:~/rust-simd-noise$ rustc +ae1ba150a280ebe428c760fbbbd58ae758ba9ea7-alt -V
rustc 1.34.0-nightly (ae1ba150a 2019-01-29)
nikic@helios:~/rust-simd-noise$ cargo +ae1ba150a280ebe428c760fbbbd58ae758ba9ea7-alt build
   Compiling simdeez v0.4.2
   Compiling simdnoise v2.3.6 (/home/nikic/rust-simd-noise)
rustc: /checkout/src/llvm-project/llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp:118: static uint8_t (anonymous namespace)::X86MCCodeEmitter::ModRMByte(unsigned int, unsigned int, unsigned int): Assertion `Mod < 4 && RegOpcode < 8 && RM < 8 && "ModRM Fields out of range!"' failed.
error: Could not compile `simdnoise`.

[mati865]

@nikic my bad, new nightly builds don't have llvm assertions enabled.