u-c: cabal: add no-msse4.2 flag, enable -msse4.2 as default on x86 architectures

[doyougnu]

closes #464.

I didn't test this locally so we'll see what CI says.

Also I assumed this would be in a new minor release so I added that to the CHANGE.md file but did not change the versioning in the cabal file

[konsumlamm]

I think it would make more sense to call the flag sse42 (the m isn't part of the name, it's just a prefix for machine-specific options) and enable it by default.

[phadej]

I'd suggest OP use some global version of ghc-options: -msse4.2, rather than adding it to individual packages. Adding to packages doesn't scale. I opened a GHC issue https://gitlab.haskell.org/ghc/ghc/-/issues/24073

[TeofilC]

unordered-containers is quite inlining heavy, eg, I often see lookups inlined quite heavily when looking at Core. If the popcount call ends up being inlined when compiling another package (that isn't passed -msse4.2) will this flag still apply?

[phadej]

@TeofilC

will this flag still apply?

Good question. I think it wont, as at Core level (i.e. in unfoldings to be inlined) we only have magic popCount# primop, and the code generation happens later.