Sun
25
Sep 2022

It will be a brief article. Imagine we implement a postprocessing effect that needs to recalculate all the pixels on the screen, reading one input texture as SRV 0 and writing one output texture as UAV 0. We use a compute shader that has numthreads(8, 8, 1) declared, so each thread group processes 8 x 8 pixels. When looking at various codebases in gamedev, I've seen many times a code similar to this one:

renderer->SetConstantBuffer(0, &computeConstants);
renderer->BindTexture(0, &inputTexture);
renderer->BindUAV(0, &outputTexture);
constexpr uint32_t groupSize = 8;
renderer->Dispatch(
    (screenWidth  + groupSize - 1) / groupSize,
    (screenHeight + groupSize - 1) / groupSize,
    1);

It should work fine. We definitely need to align up the number of groups to dispatch, so we don't skip pixels on the right and the bottom edge in case our screen resolution is not a multiply of 8. The reason I don't like this code is that it uses a "trick" that in my opinion should be encapsulated like this:

uint32_t DivideRoundingUp(uint32_t a, uint32_t b)
{
    return (a + b - 1) / b;
}
...
renderer->Dispatch(
    DivideRoundingUp(screenWidth, groupSize),
    DivideRoundingUp(screenHeight, groupSize),
    1);

Abstraction is the fundamental concept of software engineering. It is hard to work with complex systems without hiding details behind some higher-level abstraction. This idea applies to entire software modules, but also to small, 1-line pieces of code like this one above. For some programmers it might be obvious when seeing (a + b - 1) / b that we do a division with rounding up, but a junior programmer who just joined the team may not know this trick. By moving it out of view and giving it a descriptive name, we make the code cleaner and easier to understand for everyone. Therefore I think that all small arithmetic or bit tricks like this should be enclosed in a library of functions rather than used inline. Same with the popular formula for checking if a number is a power of two:

bool IsPow2(uint32_t x)
{
    return (x & (x - 1)) == 0;
}

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