[zhzy0077]

I'm a noob. Looking at the disasm: https://godbolt.org/z/766aPTPc3 It turns a CMOVQLT to a JLT. Is the blog saying CMOVQLT don't have branch predication? I don't get it.

[ludiludi]

Your disasm is for x86-64. The benchmarks in the blog were run on an M1 MacBook Pro, which is an ARM64.

[zhzy0077]

Sorry. My bad. But looking at ARM64 https://godbolt.org/z/YEjGKce1Y The difference is CSEL and BLT. The question still stands. Does CSEL have no branch predication?

[ludiludi]

It appears so. 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]

CSEL 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).