Niche in Arc<..> variant of enum not used by Option<..> about rust HOT 2 OPEN

Unfortunately, wrapping Foo in Option<_> does not make use of the remaining niche: It could set both niches to null (basically selecting the Foo::B variant and nulling the Arc's NotNull pointer) to indicate the None variant.

That would require two-usize comparison (or arbitrarily-wide comparison if this scheme were extended to more fields) which is more expensive than single-register operation.

#119507 could be related, but it's about structs. Not sure if that qualifies as dup.

Enum variant payload and struct layout share the same code. So yes, it's quite similar. Though that issue talks about 2x NonZeroU8, which could still be merged into a single register.

from rust.

Unfortunately, wrapping Foo in Option<_> does not make use of the remaining niche: It could set both niches to null (basically selecting the Foo::B variant and nulling the Arc's NotNull pointer) to indicate the None variant.

That would require two-usize comparison (or arbitrarily-wide comparison if this scheme were extended to more fields) which is more expensive than single-register operation.

Aha, indeed. I had noticed that two reads are needed for the Some(_) variants, which is the same in the non-inlined case. I did not see that two reads would then also needed for the None variant, which is of course a drawback.

from rust.