[gaze]

This is so strange! If something is unlikely to execute, it is unlikely to execute due to a condition, which you clue the branch predictor as unlikely and the layout thingy as out of line. Why would something be unconditionally unlikely to execute?! I don't understand what a lambda buys you here.

[nkurz]

In theory, the lambda combined with the noinline attribute forces the compiler to use a call/ret to a function rather than making a local jump. Since the function usually will have alignment requirements, this often will mean the error handling is in a different 64-byte cache line. If this error never occurs, this cache line will never occupy space in the instruction cache (i-cache), and useful instructions can be held there instead.

In practice, I'd be surprised if you can come up with a case where this makes a significant difference. If you are on a hot enough path for this to matter, on a modern processor you probably are running out of the even lower level decoded µop cache, which doesn't cache µops for branches that are not taken. If you aren't in this cache, your efforts are probably better spent making this happen.

Edit: ot's comment about how this affects the size of the parent function and whether it will be inlined is a good point, and might well make a measurable difference in the cases where it is true.

[mattgodbolt]

Author here. I use this trick for a general 'bail out with error code' throw macro. By definition it's the exceptional case and so I'm happy to take the hit of going out of line. It's a macro used all over a very large codebase and so it made sense to do this. And yes, in some cases it helps the compiler inline more aggressively where it does matter.