NVIDIA GPU 中的 if 语句是如何执行的？

Question

As much as know GPU cores are very simple and can only execute basic mathematic instructions. If I have a kernel with an if statement, then what does execute that if statement? Fp32, Fp64 and Int32 can only execute operations with floats, doubles and integers, not a COMPARE instruction, am I wrong. What happens if I have printf function in kernel? Who executes that.

Answer 1

Compare instructions are arithmetic instructions, you can implement a comparison with subtraction and a flag register, and GPGPUs have them.
But they are often not advertised as much as the number-crunching capability of the whole GPU.

NVIDIA doesn't publish the machine code documentation for their GPUs nor the ISA of the respective assembly (called SASS).
Instead, NVIDIA maintains the PTX language which is designed to be more portable across different generations while still being very close to the actual machine code.

PTX is a predicated architecture . The setp instruction (which again, is just a subtraction with a few caveats) sets the value of the defined predicate registers and these are used to conditionally execute other instructions. Including the bra instruction which is a branch, making it possible to execute conditional branches.

One could argue that PTX is not SASS but it seems the predicate architecture is what NVIDIA GPUs, at least, used to do .

AMD GPUs seem to use the traditional approach to branching : there are comparison instructions (eg S_CMP_EQ_U64 ) and conditional branches (eg S_CBRANCH_SCCZ ).

Intel GPUs also rely on predication but have different instructions for divergent vs non-divergent branches.

So GPGPUs do have branch instructions, in fact, their SIMT model has to deal with the branch divergence problem .

Before c. 2006 GPUs were not fully programmable and programmers had to rely on other tricks (like data masking or branchless code) to implement their kernel.
Keep in mind that at the time it was not widely accepted that one could execute arbitrary programs or make arbitrary shading effects with GPUs. GPUs relaxed their programming constraints with time.

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Putting a printf in a CUDA kernel won't probably work because there is no C runtime on the GPU (remember the GPU is an entirely different executor from the CPU) and the linking would fail I guess.
You can theoretically force a GPU implementation of the CRT and design a mechanism to call syscalls from the GPU code but that would be unimaginably slow since GPUs are not designed for this kind of work.
EDIT : Apparently NVIDIA actually did implement a printf on the GPU that prints to a buffer shared with host.
The problem here is not the presence of branches but the very nature of printf .