Slang provides preliminary support for Shader Execution Reordering (SER). The API hasn't been finalized and may change in the future.
The feature is available on D3D12 via NVAPI and on Vulkan through the GL_NV_shader_invocation_reorder extension.
SER as implemented on Vulkan has extra limitations on usage. On D3D via NvAPI HitObject variables are like regular variables. They can be assigned, passed to functions and so forth. Using GL_NV_shader_invocation_reorder on Vulkan, this isn't the case and HitObject variables are special and act is if their introduction allocates a single unique entry. One implication of this is there are limitations on Vulkan around HitObject with flow control, and assignment to HitObject variables.
TODO: Examples and discussion around these limitation.
The API is preliminary and based on the NvAPI SER interface. It may change with future Slang versions.
Immutable data type representing a ray hit or a miss. Can be used to invoke hit or miss shading, or as a key in ReorderThread. Created by one of several methods described below. HitObject and its related functions are available in raytracing shader types only.
- TraceRay
- TraceMotionRay
- MakeMiss
- MakeHit
- MakeMotionHit
- MakeMotionMiss
- MakeNop
- Invoke
- IsMiss
- IsHit
- IsNop
- GetRayDesc
- GetShaderTableIndex
- GetInstanceIndex
- GetInstanceID
- GetGeometryIndex
- GetPrimitiveIndex
- GetHitKind
- LoadLocalRootTableConstant
Executes ray traversal (including anyhit and intersection shaders) like TraceRay, but returns the resulting hit information as a HitObject and does not trigger closesthit or miss shaders.
static HitObject HitObject.TraceRay<payload_t>(
RaytracingAccelerationStructure AccelerationStructure,
uint RayFlags,
uint InstanceInclusionMask,
uint RayContributionToHitGroupIndex,
uint MultiplierForGeometryContributionToHitGroupIndex,
uint MissShaderIndex,
RayDesc Ray,
inout payload_t Payload);
Executes motion ray traversal (including anyhit and intersection shaders) like TraceRay, but returns the resulting hit information as a HitObject and does not trigger closesthit or miss shaders.
static HitObject HitObject.TraceMotionRay<payload_t>(
RaytracingAccelerationStructure AccelerationStructure,
uint RayFlags,
uint InstanceInclusionMask,
uint RayContributionToHitGroupIndex,
uint MultiplierForGeometryContributionToHitGroupIndex,
uint MissShaderIndex,
RayDesc Ray,
float CurrentTime,
inout payload_t Payload);
Creates a HitObject representing a hit based on values explicitly passed as arguments, without tracing a ray. The primitive specified by AccelerationStructure, InstanceIndex, GeometryIndex, and PrimitiveIndex must exist. The shader table index is computed using the formula used with TraceRay. The computed index must reference a valid hit group record in the shader table. The Attributes parameter must either be an attribute struct, such as BuiltInTriangleIntersectionAttributes, or another HitObject to copy the attributes from.
static HitObject HitObject.MakeHit<attr_t>(
RaytracingAccelerationStructure AccelerationStructure,
uint InstanceIndex,
uint GeometryIndex,
uint PrimitiveIndex,
uint HitKind,
uint RayContributionToHitGroupIndex,
uint MultiplierForGeometryContributionToHitGroupIndex,
RayDesc Ray,
attr_t attributes);
static HitObject HitObject.MakeHit<attr_t>(
uint HitGroupRecordIndex,
RaytracingAccelerationStructure AccelerationStructure,
uint InstanceIndex,
uint GeometryIndex,
uint PrimitiveIndex,
uint HitKind,
RayDesc Ray,
attr_t attributes);
See MakeHit but handles Motion Currently only supported on VK
static HitObject HitObject.MakeMotionHit<attr_t>(
RaytracingAccelerationStructure AccelerationStructure,
uint InstanceIndex,
uint GeometryIndex,
uint PrimitiveIndex,
uint HitKind,
uint RayContributionToHitGroupIndex,
uint MultiplierForGeometryContributionToHitGroupIndex,
RayDesc Ray,
float CurrentTime,
attr_t attributes);
static HitObject HitObject.MakeMotionHit<attr_t>(
uint HitGroupRecordIndex,
RaytracingAccelerationStructure AccelerationStructure,
uint InstanceIndex,
uint GeometryIndex,
uint PrimitiveIndex,
uint HitKind,
RayDesc Ray,
float CurrentTime,
attr_t attributes);
Creates a HitObject representing a miss based on values explicitly passed as arguments, without tracing a ray. The provided shader table index must reference a valid miss record in the shader table.
static HitObject HitObject.MakeMiss(
uint MissShaderIndex,
RayDesc Ray);
See MakeMiss but handles Motion Currently only supported on VK
static HitObject HitObject.MakeMotionMiss(
uint MissShaderIndex,
RayDesc Ray,
float CurrentTime);
Creates a HitObject representing “NOP” (no operation) which is neither a hit nor a miss. Invoking a NOP hit object using HitObject::Invoke has no effect. Reordering by hit objects using ReorderThread will group NOP hit objects together. This can be useful in some reordering scenarios where future control flow for some threads is known to process neither a hit nor a miss.
static HitObject HitObject.MakeNop();
Invokes closesthit or miss shading for the specified hit object. In case of a NOP HitObject, no shader is invoked.
static void HitObject.Invoke<payload_t>(
RaytracingAccelerationStructure AccelerationStructure,
HitObject HitOrMiss,
inout payload_t Payload);
Returns true if the HitObject encodes a miss, otherwise returns false.
bool HitObject.IsMiss();
Returns true if the HitObject encodes a hit, otherwise returns false.
bool HitObject.IsHit();
Returns true if the HitObject encodes a nop, otherwise returns false.
bool HitObject.IsNop();
Queries ray properties from HitObject. Valid if the hit object represents a hit or a miss.
RayDesc HitObject.GetRayDesc();
Queries shader table index from HitObject. Valid if the hit object represents a hit or a miss.
uint HitObject.GetShaderTableIndex();
Returns the instance index of a hit. Valid if the hit object represents a hit.
uint HitObject.GetInstanceIndex();
Returns the instance ID of a hit. Valid if the hit object represents a hit.
uint HitObject.GetInstanceID();
Returns the geometry index of a hit. Valid if the hit object represents a hit.
uint HitObject.GetGeometryIndex();
Returns the primitive index of a hit. Valid if the hit object represents a hit.
uint HitObject.GetPrimitiveIndex();
Returns the hit kind. Valid if the hit object represents a hit.
uint HitObject.GetHitKind();
Returns the attributes of a hit. Valid if the hit object represents a hit or a miss.
attr_t HitObject.GetAttributes<attr_t>();
Loads a root constant from the local root table referenced by the hit object. Valid if the hit object represents a hit or a miss. RootConstantOffsetInBytes must be a multiple of 4.
uint HitObject.LoadLocalRootTableConstant(uint RootConstantOffsetInBytes);
Reorders threads based on a coherence hint value. NumCoherenceHintBits indicates how many of the least significant bits of CoherenceHint should be considered during reordering (max: 16). Applications should set this to the lowest value required to represent all possible values in CoherenceHint. For best performance, all threads should provide the same value for NumCoherenceHintBits. Where possible, reordering will also attempt to retain locality in the thread’s launch indices (DispatchRaysIndex in DXR).
ReorderThread(HitOrMiss) is equivalent to
void ReorderThread( HitObject HitOrMiss, uint CoherenceHint, uint NumCoherenceHintBitsFromLSB );
With CoherenceHint and NumCoherenceHintBitsFromLSB as 0, meaning they are ignored.
void ReorderThread(
uint CoherenceHint,
uint NumCoherenceHintBitsFromLSB);
void ReorderThread(
HitObject HitOrMiss,
uint CoherenceHint,
uint NumCoherenceHintBitsFromLSB);
void ReorderThread(HitObject HitOrMiss);