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This article is to share with you about the efficiency of if and for in Shader and what the strategy is. The editor thinks it is very practical, so I share it with you to learn. I hope you can get something after reading this article.

Let's come to a conclusion:

If and for do have efficiency issues (compared to the case in CPU), but usually the situation is unavoidable and does not need to be dealt with.

When step is used instead of if, there is no optimization or negative optimization in most cases, and there is only a small improvement in some individual cases.

You can replace some if that are only related to fixed parameters with Shader variants, but this is only to reduce the cost of if itself (if and the expressions executed in it are instructions, after all)

If and for are slower than expected because GPU's "computing units" and "logical units that execute instructions" are not one-to-one, but one-to-many.

When you execute an instruction, you do not operate on a pixel, but on a set of pixels at the same time, and execute one instruction down.

If if appears in your code, there may be conditional expressions that return true or false in this set of pixels. But as long as one returns true, the following code must be executed. For those pixels that do not need to be executed, although they do not need to be calculated this time, there are no other logical units to match them, so they can only wait.

The part of X can only wait.

The same is true for the later part of else, which takes up all the resources even if there is only one pixel to calculate.

Therefore, the efficiency problem of if is that it can cause multiple branches to execute repeatedly.

The problem with for is similar to that of if, which results in loop logic having to be executed according to the maximum number of cycles in this set of pixels.

However, that's it.

If is inefficient because it causes multiple branches to repeat execution, not how slow it is. If you get rid of if, but the logic executed is still the sum of all the branches, there won't be much improvement. This is what step does instead of if.

If you consider the cost of the instruction if itself. Although if itself is not a simple instruction, if you want to use step instead of if, the literal translation goes like this: fixed4 c = lerp (c * Color,Color,step (c * Color,Color,step 0.5))

After all, these are two instructions, which are much better than if itself. But if you can omit that lerp in some specific cases, it is possible to gain some advantages (for example: if (x > = 1) x + = 1-> x + = step (1memx))

However, the use of if does not always cause branches to repeat execution.

When within that set of pixels, if all c. A > 0. 5 judgments return false, it means that all the results calculated by the following instruction will be discarded. At times like this, GPU will skip this statement normally.

All branches have no choice but to do so. As long as it can be avoided, it will be avoided naturally.

In the actual operating environment, all c.an in a batch of pixels

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