HLSL Shader Model 6.5

v1.0 2019-10-15



This doc covers the new shader model 6.5 for Vibranium release (20H1). With the exception of the WaveMatch and WaveMultiPrefix intrinsics, the new features are defined in separate documents.

DXR 1.1

DirectX Raytracing (DXR) Tier 1.1 adds:

Feature support is indicated by the Raytracing Tier.

See the DirectX Raytracing (DXR) Functional spec for details.

Sampler Feedback

Sampler Feedback is an optional feature available in shader model 6.5 that adds 2 new Texture resource types: FeedbackTexture2D<type> and FeedbackTexture2DArray<type> to HLSL. These resource types have a template argument specifying the format of the feedback map.

See the HLSL section of the Sampler Feedback spec for details.

Mesh and Amplification Shaders

Two new shader types are added to HLSL for the new mesh shader graphics pipeline. These are Mesh Shaders ms_6_5 and Amplification Shaders as_6_5.

See the Mesh Shader spec for details.

New Wave Intrinsics

WaveMatch(), and a number of WaveMultiPrefix*() intrinsics, are available starting with shader model 6.5, when the optional Wave Intrinsics feature is supported.

All shader stages, except for Raytracing shaders, support wave intrinsics as of shader model 6.5.


Shader model 6 introduces a set of data parallel wave intrinsics, which implement fundamental computational primitives such as voting, reductions and prefix operations among the lanes in a wave. These intrinsics are valuable tools for many compute algorithms, exploiting efficiency of SIMD execution model of modern GPUs.

Shader model 6.5 adds two new classes of wave intrinsics. They are useful for many data parallel algorithms involving deduplication of data streams, coalescing of memory operations, and implementing efficient concurrent data structures such as hash tables and maps. In particular, these new intrinsics are important building blocks texture-space shading, and provide support for index buffer deduplication in the context of the meshlet programming model.

We believe these data parallel primitives will grow in importance as applications embrace more compute-centric algorithms.

As with the other wave intrinsics, when executed in pixel shaders, these intrinsics operate as if helper lanes are not active when executing the wave intrinsic. This means that the value returned by a wave intrinsic is only defined on active, non-helper lanes. Similarly, the inputs to a wave intrinsic on helper lanes do not impact the result returned from that wave intrinsic on active, non-helper lanes.

WaveMatch() Function

uint4 WaveMatch( <type> val )

The WaveMatch() intrinsic compares the value of the expression in the current lane to its value in all other active lanes in the current wave and returns a bitmask representing the set of lanes matching current lane’s value.

val can be any expression which evaluates to any of the currently supported primitive data types (e.g. float4, uint2, etc.).

The return value is a uint4 representing a 128b bit mask which identifies lanes in the current wave matching current lane’s value of val. Bits in the mask corresponding to inactive lanes, or at positions beyond current implementation’s wave width, will contribute 0’s. Bits in the mask corresponding to active lanes which match the value of val in the current lane will be set to 1. The bit in the mask corresponding to the current lane will always be set to 1.

In pixel shaders, bits corresponding to helper lanes are set to 0, and on helper lanes, the resulting bitmask is undefined.

WaveMatch() Illustration

The following table demonstrates the action of WaveMatch() assuming an implementation with a wave width of 8 lanes. Lanes 0 and 4 are inactive or helper lanes, indicated by “-”. Bits in the mask beyond bit position 7 are guaranteed to be cleared (effective mask width is 8).

uint4 mask = WaveMatch(input);
laneID 7 6 5 4 3 2 1 0
input 15 -1 -1 - 123 0 123 -
mask.x 0x80 0x60 0x60 - 0x0a 0x04 0x0a -

WaveMultiPrefix*() Functions

WaveMultiPrefix*() is a set of functions which implement multi-prefix operations among the set of active lanes in the current wave.

A multi-prefix operation comprises a set of prefix operations, executed in parallel within subsets of lanes identified with the provided bitmasks. These bitmasks represent partitioning of the set of active lanes in the current wave into N groups (where N is the number of unique masks across all lanes in the wave). N prefix operations are then performed each within its corresponding group. The groups are assumed to be non-intersecting (that is, a given lane can be a member of one and only one group), and bitmasks in all lanes belonging to the same group are required to be the same.

The following operations evaluates multiple prefix operations within groups of threads identified by mask:

<type> can be any of the currently supported integer or floating point primitive types.

<int_type> can be any of the currently supported integer primitive types.

val is the value to perform the prefix operation on.

mask is a 128b bitmask, representing the partitioning of the current wave into groups of lanes, as described above. Bits in the masks at positions beyond current implementation’s wave width, or corresponding to inactive or helper lanes, are ignored (assumed to be 0). If the masks do not form non-intersecting subsets of lanes, then the values returned by this intrinsic are undefined. Bitmasks for all lanes belonging to the same group are required to match, otherwise the results returned by this intrinsic are undefined.

Returned <type> is the same type as the input type for val. The result of the prefix operation is computed with values from prior lanes in the same group only; it does not include the value from the current lane. A postfix value would be computed by applying the corresponding operator between the result of the prefix operation and the value passed in to the prefix operation.

<type> WaveMultiPrefixSum( <type> val, uint4 mask )

val0 + val1 + val2 …

<type> WaveMultiPrefixProduct( <type> val, uint4 mask )

val0 * val1 * val2 …

uint WaveMultiPrefixCountBits( bool val, uint4 mask )

(val0 ? 1 : 0) + (val1 ? 1 : 0) + (val2 ? 1 : 0) …

<int_type> WaveMultiPrefixAnd( <int_type> val, uint4 mask )

val0 & val1 & val2 …

<int_type> WaveMultiPrefixOr( <int_type> val, uint4 mask )

val0 | val1 | val2 …

<int_type> WaveMultiPrefixXor( <int_type> val, uint4 mask )

val0 ^ val1 ^ val2 …

WaveMultiPrefixSum() Illustration

The following table demonstrates the action of WaveMultiPrefixSum(), assuming an implementation with wave width of 8 lanes. Lane 1 is an inactive or helper lane, and is indicated by “-”.

output = WaveMultiPrefixSum(value, mask);
laneID 7 6 5 4 3 2 1 0
mask.x 0xe0 0xe0 0xe0 0x14 0x09 0x14 - 0x0b
value 5 4 1 -2 3 0 - 6
output 5 1 0 0 6 0 - 0

Note how subset with mask.x == 0x0b refers to lane 1, which is either inactive or is a helper lane. This doesn’t affect the result since bits in the mask corresponding to inactive or helper lanes are ignored.

Example usage

WaveMatch() and WaveMultiPrefix*() intrinsics are designed to work together. In particular, the masks returned by the WaveMatch() intrinsic can be used directly as group masks in the WaveMultiPrefix*() set of intrinsics.

The following illustrates obtaining an equivalent result to a WaveMultiPrefixSum operation using the WaveReadLaneFirst and WaveActiveSum intrinsics. However, using the new wave intrinsics provides more optimization opportunities for hardware to take advantage of.

// Given:
int sum = 0, value = ...;
int expr = ...;  // Uniform subsets exist for this expr value

// The following:
uint4 mask = WaveMatch(expr);
sum = WaveMultiPrefixSum(value, mask);

// Is equivalent to writing a loop like this:
while (true) {
  if (WaveReadLaneFirst(expr) == expr) {
    sum = WaveActiveSum(value));

The following example demonstrates how to coalesce atomic OR operations to a surface with x,y coordinates computed dynamically per lane.

uint key = computeHash(x, y); // compute a key for matching

uint4 groupMask = WaveMatch( key );

// firstbithigh returns -1 when no bit set, otherwise < 32,
// so OR will add lane index offset without changing -1.
int4 highLanes = (int4)(firstbithigh(groupMask) | uint4(0, 0x20, 0x40, 0x60));
// The signed max should be the highest lane index in the group.
uint highLane = (uint)max(max(max(highLanes.x, highLanes.y), highLanes.z), highLanes.w);
bool leader = WaveGetLaneIndex() == highLane;

unsigned int result = WaveMultiPrefixBitOr( myValueToOr, groupMask );

if (leader)
    InterlockedOr( mem[key], result | myValueToOr );

First, WaveMatch() is used to identify sets of threads which have the same x,y coordinates (that is, will update the same location in memory). Then, a single thread from each set is elected to issue a single atomic operation to memory on behalf of all lanes in the set. The WaveMultiPrefixBitOr() function is used to apply bitwise-OR reduction within multiple sets of colliding lanes concurrently, the results of which are then used in the elected threads to issue atomic operations to memory.