Typically in SIMT, all lanes in a "simd" (aka subgroup in Vulkanspeak) share the same program counter. However, there is an execution mask so not all lanes are active. Thus, to simulate "if A then B else C", you evaluate A for each lane to derive a mask, jump to C if the mask is empty, evaluate B only for the enabled lanes, invert the mask, skip over C if the mask is then empty, and evaluate C only for the enabled lanes. Loops also work, with similar logic (the real loop terminates when the predicate is false for all lanes).
