HLSL Dynamic Resources

Introduction

HLSL Dynamic Resources is a feature introduced in Shader Model 6.6 that allows shaders to create resources from descriptors by directly indexing the CBV/SRV/UAV heap or the sampler heap. Dynamic resources are also known as bindless resources or directly indexed resources.

HLSL exposes these heaps through two built-in global objects: ResourceDescriptorHeap and SamplerDescriptorHeap. Indexing either object yields an intermediate value that can be converted to a resource or sampler object. Resources created this way do not require binding locations or root signature descriptor-table mappings.

Implementation Details

The built-in header hlsl/hlsl_resources.h declares the global variables ResourceDescriptorHeap and SamplerDescriptorHeap and is included by the default header hlsl.h. Their types, resource_descriptor_heap_struct and sampler_descriptor_heap_struct, are defined in the hlsl::__detail namespace in hlsl/hlsl_detail.h.

Each global’s indexing operator returns a small internal struct containing the heap index: ResourceDescriptorHeap returns heap_resource_info, while SamplerDescriptorHeap returns heap_sampler_info. HLSLExternalSemaSource defines both structs in the hlsl::__detail namespace. Their distinct types allow overload resolution to reject attempts to construct a resource from the wrong heap: sampler types accept only heap_sampler_info, while CBV/SRV/UAV resource types accept only heap_resource_info.

Each resource class has an implicit constructor that accepts the corresponding heap info struct and passes it to __builtin_hlsl_resource_handlefromheap to create a concrete resource handle. This allows assignments, initializations, and function arguments to use standard C++ implicit conversions without special handling in Sema. For resources with counters, the constructor creates the counter handle with the separate __builtin_hlsl_resource_counterhandlefromheap built-in.

During code generation, each call to the built-in is lowered to target-specific intrinsics for heap resources.