WavePrefixSum function

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

See also

Overview of Shader Model 6

Shader Model 6