How does [branch] get executed in HLSL?

Question

The [branch] attribute can mark an if statement in HLSL to make it execute only one branch instead of all branches and discarding the results like when using [flatten] .

My question is how can this actually work, when a branch diverges withing a warp/wavefront? As far as I know, in this case all threads must execute all branches taken by any of the threads in the warp (like when using [flatten] ) which is consequence of the fact, that they are all within the same SIMD block and must execute the same instruction.

Answer 1

Since GeForce series 6xx GPUs do actually support branching, though in limited form and with performance cost. The [branch] and [flatten] tags are just hints to the compiler to prefer one or the other if supported and possible . It basically depends on hardware and on the driver, so different hardware or different driver versions might in the end determine a different execution from what you specified with the tag.

You can find more info online, for example check this link