If I use vertex shader to do all operations on object, then constant buffer can be empty?

Question

The program cycle is

Update();
UpdatePipeline();

In Update() constant buffer for each object, that after transformations, has this object world matrix is copied to GPU upload heap. And in UpdatePipeline() , among other things, installed shaders are called. Because we do all matrix transformation using CPU, vertex shader just returns position, right? If yes - is it true that performance will increase?

Now I want to do all transformations using GPU, ie via vertex shader. It means that in Update() I just should call memcpy() with an empty constant buffer as a source?

Answer 1

1. A constant buffer is just a buffer to move data from the CPU to the GPU. Whether you use one, or how many you use, and what you are using them for, is up to you.
2. The most common and most simple use-case for a constant buffer is to move a transformation matrix to the GPU. That matrix is indeed calculated by the CPU, and the vertex shader uses that matrix to transform the positions in the vertex buffer from local to screen space. This allows the CPU to move an object around without the need to update the (usually quite big) vertex buffer.
3. Whether performance increases depends on your hardware, your code, and - most importantly - what you are comparing the performance with. Since I don't know your current code, nor what exact changes you intent to do, I can't even guess if it will increase or not.
4. Also, even though I don't know your code, just by the way you phrased your question I would assume that you defenitly don't want to use a constant buffer as a source for any operation on the CPU.