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jdub

Can someone explain [loop] and [unroll] to me?

4 posts in this topic

So I am having a hard time understanding the difference between [loop] and [unroll].  From what I understand, [unroll] causes the compiler to take the contents inside a loop and repeatedly place it's machine code in the output for as many times as the loop executes.  I am unsure of how to use [loop] and what the tradeoff's are between the two?  I have looked at the MSDN explanation but it seems to be brief and doesn't really give me enough information to understand it.

 

 

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If you know that your loop often quits early (well before the max iterations), you can hint this to the compiler with the [loop] attribute. The early exits you potentially gain this way can amortize the cost of dynamic looping.

 

Unrolling, on the other hand, has a static cost for the given number of iterations, but it tends to perform more efficiently because the system can determine your memory access pattern in advance. So if you know that most of your loop iterations are going to run to the finish, you likely want to unroll. Also, if you logically have a fixed number of iterations in your loops, then unrolling is almost always more efficient (unless the number of iterations is very high, in which case the length of the unrolled program itself would become the bottleneck.)

 

The reason you have to hint this yourself is that the compiler cannot guess anything about how the shader actually gets used. If the loop counter is based on some dynamic data, there is no way for the compiler to determine which one out of unrolling or actual looping is more effective - because that data is not available during the compile time.

 

Finally, some shader profiles do not even support actual looping; the hint can still be specified, but it is ignored if it is incompatible with the profile. This way, you don't have to write multiple versions of the code for multiple shader profiles. 

Edited by Nik02
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The reason you can't use some sampling function inside a dynamic-counter loop is that during sampling, the system needs to calculate the derivatives of the interpolants for the current pixel group (2x2) in order to determine the correct mip level for the current pixel. The derivatives are impossible to calculate if the pixels of the current group each potentially execute a different logic (as is the nature of dynamic looping).

 

You can circumvent this by calculating the needed derivatives outside the loop (so that the basis of the derivatives effectively become non-varying across the pixel group), and then use those derivatives within the loop by calling purpose-built versions of the sampling functions that take them as parameters. The compiler will also try to do this automatically, but if the surrounding logic is complex it may fail to do so. 

Edited by Nik02
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