A quick test to check which instructions are available in the x86-64-v2 target arch, for purposes of figuring out whether the feature detection that the crc32fast crate does (and some other tests that other crates to) is happening at runtime or compile-time.

Examine assembly with cargo asm --target-cpu=x86-64-v2 'test' and look for a branch (je or jne).

Results:

1. sse2 is available in x86-64-v1 (default target CPU)
2. sse4.1 is available in x86-64-v2
3. pclmulqdq is not available even in x86-64-v4, even though in practice it seems the pclmulqdq instruction is actually available on all consumer CPUs from 2013 onwards (2010 for Intel).
4. f16c for the half crate is available in x86-64-v3

Points 1 and 2 and 4 are expected based on https://en.wikipedia.org/wiki/X86-64#Microarchitecture_levels - but point 3 I wasn't sure of.

See also:

• there are some instructions for checking target features in this unfinished book on Rust SIMD
• https://doc.rust-lang.org/reference/attributes/codegen.html#the-target_feature-attribute to explicitly enable features for parts of the codebase.