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by masklinn 1171 days ago
> Why would you want that?

To aid with optimisation, it basically lets you ask the compiler to remove branches, and provide constraints to the same.

An implementation might trap in debug code, but given no context would be provided you'd likely avoid this and would instead use your own wrapper macro to output a message of some sort in that case.
2 comments
properparity 1171 days ago
But why put in unreachable? Doesn't make any sense to me.

If a branch is truly not supposed to ever happen, why have a branch at all? Just remove that code from the source entirely- that helps the optimizer even more, because the most optimal code is of course no code at all.

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masklinn 1171 days ago
> But why put in unreachable? Doesn't make any sense to me.

Because sometimes you don't have a choice e.g. say you have a switch/case, if you don't do anything and none of the cases match, then it's equivalent to having an empty `default`. But you may want a `default: unreachable()` instead, to tell the compiler that it needs no fallback.

> If a branch is truly not supposed to ever happen, why have a branch at all? Just remove that code from the source entirely- that helps the optimizer even more, because the most optimal code is of course no code at all.

Except the compiler may compile code with the assumption that it needs to handle edge cases you "know" are not valid. By providing these branches-which-are-not, you're giving the compiler more data to work with. That extra data might turn out to be useless, but it might not.

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benj111 1171 days ago
But this example isn't adding a constraint. The if statement is getting optimised away???
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masklinn 1171 days ago
It is adding a constraint. The constraint is that argc can’t be smaller than 2. This is a literal “can’t”, as far as the compiler is concerned it’s a logical impossibility.

The branch containing the unreachable() obviously gets removed but the compiler then propagates the constraint (the condition for that illegal branch), and can prune any other path where `argc <= 2` upstream and downstream, as they are dead code per the constraint.

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