[Too]

Now I see what you mean by under-specified in the case of branch hints. But does the solution need to involve giving more direct machine access? Similar benefits can be given with higher layer abstractions as well, without adding more UB, in fact the examples below remove UB. Take restrict keyword in C, which theoretically could be automatically inferred in Rust (not sure if they actually do nowadays). Iterators, or as you say better addition operators, can hide the details of array indexing and overflows. Hinting the probability of a certain branch certainly sounds like a higher layer construct as well, not something to expose a direct machine instruction for.

[dundarious]

I’m not proposing machine specific additions to C. One of my complaints is that the C language doesn’t have enough features, so I end up using machine specific assembly or compiler specific builtins. And I’m complaining about C pushing developers onto the knife edge of UB or performant code — I don’t want more UB either.

I want higher level constructs in the C language (restrict isn’t a great feature, but it’s high level, so like that) that can map to the actual feature set found on actual machines since 2004 (and compiler writers can do that mapping per machine). But C is stuck with a simplified model of the machine that ignores what almost all hardware can do these days.

I think I’ll always have to dip into assembly/intrinsics sometimes, so I’m not looking for super advanced/rich features. Actually, I think the real benefit would be giving compiler writers ways to improve performance without pushing devs onto the UB knife edge.