fuzz: enable relaxed-simd for the Wasmi oracle

[Robbepop]

This enables relaxed-simd for the Wasmi differential fuzzing oracle.

Wasmi implements all relaxed-simd operators deterministically. Fused multiply-add has only 1 rounding error on all platforms. On no_std we use libm to achieve this. Where possible Wasmi simply uses the non-relaxed counterparts for some of the relaxed-simd operators.

Note that the relaxed-simd proposal support is very new for Wasmi. I have only ran Wasmi's differential fuzzing for ~45 minutes before it crashed with the already known Wasmtime bug.

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[alexcrichton]

Unfortunately I don't think this will do anything at this time due to this line. I'm not sure how we can change that because while wasmi/wasmtime may be able to be differentially fuzzed against each other it assumes certain wasmtime configuration which isn't plumbed yet and differential execution against v8 or even the spec interpreter in the future may not be possible

[Robbepop]

@alexcrichton thanks for the clarifications.

I agree, this won't be very useful for Wasmtime at the moment. I currently do perform relaxed-simd differential fuzzing in Wasmi's fuzzer and it seems to match Wasmtime's semantics. However, I just got the weird feeling that even with config.relaxed_simd_deterministic() enbabled this might change since relaxed-simd instructions can remain deterministic while changing semantics as long as the new semantics are deterministic across platforms.

So maybe ... I should stop fuzzing relaxed-simd in Wasmi as well. 🤔

[alexcrichton]

I think the intention is that the deterministic profile of relaxed-simd is spec'd as one thing and that "thing" won't change. Officially though there aren't profiles at the moment, so it's more of a handshake between folks at this point that the deterministic profile won't be changing AFAIK

[Robbepop]

I think the intention is that the deterministic profile of relaxed-simd is spec'd as one thing and that "thing" won't change. Officially though there aren't profiles at the moment, so it's more of a handshake between folks at this point that the deterministic profile won't be changing AFAIK

Ah I didn't know about the deterministic profile but it seems I got it right by accident.

[Robbepop]

I think this can be closed for now.