Is there a way to use clz() in a Vulkan compute shader?

Question

I'm interested in implementing a particular algorithm in a set of Vulkan compute shaders. The algorithm uses a clz() function at one point. I expect that my NVIDIA GPU probably offers hardware support for this function; CUDA uses a clz instruction apparently, and clz() is in OpenCL 1.2 as well. So I don't want to write my own clz(). Is there any way for me to call the function in the way CUDA or OpenCL would do?

I suppose I could try compiling an OpenCL kernel to SPIR-V and using that in Vulkan, but I don't suppose Vulkan would be very happy about that...?

Another thought I've had is that maybe I could translate a very simple OpenCL kernel containing a clz() call to SPIR-V assembly, do the same with my GLSL shader, and then manually hack the clz() call, as it appears in the kernel assembly code, into the shader's assembly code. But I don't really know anything about the details of SPIR-V, or about any limits Vulkan may place on what sorts of SPIR-V instructions a compute shader may use, so I have hardly any idea about whether that could actually work.

Answer 1

Vulkan-bound SPIR-V has access to the GLSL extended instruction set, which includes the function FindUMSB, which finds the most-significant bit. You can use that to emulate clz by doing 31 - FindUMSB. It's possible, if the hardware has an explicit clz instruction, that the compiler can factor out the subtraction and replace the expression with the internal clz.