[nosferalatu123]

A lot of the myth that "branches are slow on GPUs" is because, way back on the PlayStation 3, they were quite slow. NVIDIA's RSX GPU was on the PS3; it was documented that it was six cycles IIRC, but it always measured slower than that to me. That was for even a completely coherent branch, where all threads in the warp took the same path. Incoherent branches were slower because the IFEH instruction took six cycles, and the GPU would have to execute both sides of the branch. I believe that was the origin of the "branches are slow on GPUs" myth that continues to this day. Nowadays GPU branching is quite cheap especially coherent branches.

[dahart]

If someone says branching without qualification, I have to assume it’s incoherent. The branching mechanics might have lower overhead today, but the basic physics of the situation is that throughput on each side of the branch is reduced to the percentage of active threads. If both sides of a branch are taken, and both sides are the same instruction length, the average perf over both sides is at least cut in half. This is why the belief that branches are slow on GPUs is both persistent and true. And this is why it’s worth trying harder to reformulate the problem without branching, if possible.

[nice_byte]

coherent branches are "free" but the extra instructions increase register pressure. that's the main reason why dynamic branches are avoided, not that they are inherently "slow".