Is there a way (at least in GCC or clang's take on C and C++) to mark (un-)likely situations that doesn't involve conditionals?
Like, for example, in `(x + 1) % y` I want to be able to tell the compiler that x+1 is likely to be less than y so it can (perhaps automagically) do a fast path for that situation 
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You can make up a conditional to do so: https://stackoverflow.com/a/26195434
It's always unclear whether the optimizer will be able to make use of that, but I'm pretty sure that (as long as the conditional expression is obviously side-effect-free) the optimizer will remove the conditional if its body is just assume_unreachable.
@koakuma
Ah, sorry, s/conditional with assume_unreachable inside/conditional with (un)likely around the condition with empty body/