You've got it the wrong way around. If the branch predictor was 100% accurate, branchless code would have no performance benefit compared to branching code.

> X86 should support no-branch-predict-if

It almost does. You can use __builtin_expect to hint if the branch is predictable - like assertions. Don't annotate conditionals if they're truly unpredictable. The branch predictor will run, but it's not expensive.