Branch prediction hints (i.e. Likely/Unlikely)

[aclysma]

In some performance sensitive code, hinting to the compiler that a certain path is likely or unlikely to be taken can be a helpful optimization:

• Sometimes the unlikely path is extremely unlikely/unintended (for example, a fixed-size object pool being full, trying to normalize a 0-length vector). Sometimes, the path is completely unreachable.
• Sometimes you may want to optimize for the unlikely path - for example a spin loop waiting for an event.

There is a discussion here: https://github.com/rust-lang/rust/issues/26179

The RFC was accepted but the intrinsics are not stabilized, I think mainly because:

• The LLVM team recommends (and clang uses) branch weights instead. (These intrinsics disable certain optimizations in LLVM, which might defeat the purpose of using them.)
• This doesn't support hinting if let or match statements.

Workarounds:

• Compiler intrinsics std::intrinsics::likely and std::intrinsics::unlikely are available in nightly, even though they could be harmful the way they work now
• #[cold] will hint to llvm that a certain path is less likely to be called. This also prevents inlining which can reduce icache pressure
• unreachable!() and unreachable_unchecked!() are worth considering for the unreachable case. It's not clear to me if unreachable!() does any optimization, but unreachable_unchecked does. (not sure, but probably via llvm's assume)
• Profile-guided optimization

[mtak-]

My crate nudge is possibly a good workaround for now. It does not require nightly.