Cranelift: implement bmask instruction

[bjorn3]

There is currently neither an encoding, nor a legalization for this instruction.

[bjorn3]

I think it should be used by translate_vector_icmp and translate_vector_fcmp after an encoding is added.

[abrown]

Not sure I follow: I do plan to eventually add a SIMD bitmask using PMOVMSKB like what is described in the SIMD spec issue but I don't see how that would change translate_vector_icmp and translate_vector_fcmp--it would be a separate instruction.

[bjorn3]

According to the docs the instruction would for every lane return an all ones integer for a true bool and an all zeros integer for a false bool. Eg bmask.i8x2 on [false, true] would return [0x00, 0xff].

translate_vector_{icmp,fcmp} currently assume that icmp and fcmp on vectors represent the resulting bool using all zeros / all ones. Otherwise the raw_bitcast would fail. This is not guaranteed anywhere and for non vectors this assumption is wrong. Using eg bmask.i8x16 to convert a b8x16 instead of using raw_bitcast.i8x16 would avoid making this assumption.

[abrown]

As I understand it, on x86 and ARM the output of vector icmp and fcmp is another vector with lanes of all zeroes or all ones. Are you suggesting using bmask.i8x16 in translate_vector_{icmp,fcmp} to make things look more type-correct but actually encode it as a noop on those platforms?

[bjorn3]

Yes

[github-actions]

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[cfallin]

Closing this in favor of #3205, which encompasses the general topic of our vector-bool representation.