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Returns the sum of all of the values in the active lanes with smaller indices than this one.
Syntax
<type> WavePrefixSum(
<type> value
);
Parameters
value
The value to sum up.
Return value
The sum of the values.
Remarks
The order of operations on this routine cannot be guaranteed. So, effectively, the [precise] flag is ignored within it.
A postfix sum can be computed by adding the prefix sum to the current lane's value.
Note that the active lane with the lowest index will always receive a 0 for its prefix sum.
This function is supported from shader model 6.0 in all shader stages.
Examples
uint numToSum = 2;
uint prefixSum = WavePrefixSum( numToSum );
On a machine with a wave size of 8, and all lanes active except lanes 0 and 4, the following values would be returned from WavePrefixSum.
| lane index | status | prefixSum |
|---|---|---|
| 0 | inactive | n/a |
| 1 | active | = 0 |
| 2 | active | = 0+2 |
| 3 | active | = 0+2+2 |
| 4 | inactive | n/a |
| 5 | active | = 0+2+2+2 |
| 6 | active | = 0+2+2+2+2 |
| 7 | active | = 0+2+2+2+2+2 |