3 ms·
Sorry. My bad. But looking at ARM64 https://godbolt.org/z/YEjGKce1Y https://godbolt.org/z/YEjGKce1Y The difference is CSEL and BLT. The question still stands. D
by zhzy0077 3y ago
Sorry. My bad. But looking at ARM64 https://godbolt.org/z/YEjGKce1Y https://godbolt.org/z/YEjGKce1Y
The difference is CSEL and BLT. The question still stands. Does CSEL have no branch predication?
- ludiludi 3y agoIt appears so. https://developer.arm.com/documentation/102374/0101/Program-flow---conditional-select-instructions https://developer.arm.com/documentation/102374/0101/Program-... "So far, we have seen examples that use branches to handle decisions. The A64 instruction set also provides conditional select instructions. In many cases, these instructions can be used as an alternative to branches." Seems CSEL is not a branch.
- masklinn 3y agoCSEL is a conditional move, they’re usually used to avoid branches (and branch predication) entirely. Here it turns out to be a very bad choice, because it creates unnecessary data dependencies. But a cmov would be a fine choice if the branch was impossible to predict (e.g. if the input was random, well even then I’d expect the limit to mostly creep up so it should be predicated as mostly cold).