[gpderetta]

> The non-exception version manages to avoid the conditional branch by using a conditional move, which would avoid a pipeline stall on a branch mis-prediction.

branches are usually superior to conditional moves for predictable conditions as they break dependency chains. In case the exceptional code path is taken, the cost of the misprediciton is dwarfed by the cost of unwinding the stack.

This is interesting actually, the fact that the compiler uses a conditional move in the error checking case could mean that the compiler has no useful branch probability model for that branch in the error checking case, but even when using __builtin_expect, the compiler still prefers the conditional move.

[abainbridge]

> branches are usually superior to conditional moves for predictable conditions as they break dependency chains.

Interesting, not heard that before. Do you know of somewhere I can read about this?

[gpderetta]

Agner Fog is the usual go-to reference. For this specific case, you can also google any of Linus rants on conditional moves (they used to be very high latency, although today they are not so much of an issue). This one for example: https://yarchive.net/comp/linux/cmov.html

[ncmncm]

It is complicated to describe when cmov is slow and when it is fast. As a rule of thumb, if the next loop iteration data operations depend on a cmov in this one, and around, cmov will be slow. If not, it is very, very fast. Use of cmov can make quicksort 2x as fast.

Gcc absolutely won't generate two cmov instructions in a basic block. Clang, for its part, abandons practically all optimization of loops that could conceivably generate a throw.

[abainbridge]

Nice. Like every other topic, there's more complexity if you keep looking harder.