DXIL Resource Handling — LLVM 21.1.2 documentation

# DXIL Resource Handling

## Introduction

Resources in DXIL are represented via `TargetExtType` in LLVM IR and eventually lowered by the DirectX backend into metadata in DXIL.

In DXC and DXIL, static resources are represented as lists of SRVs (Shader Resource Views), UAVs (Uniform Access Views), CBVs (Constant Bffer Views), and Samplers. This metadata consists of a “resource record ID” which uniquely identifies a resource and type information. As of shader model 6.6, there are also dynamic resources, which forgo the metadata and are described via `annotateHandle` operations in the instruction stream instead.

In LLVM we attempt to unify some of the alternative representations that are present in DXC, with the aim of making handling of resources in the middle end of the compiler simpler and more consistent.

## Resource Type Information and Properties

There are a number of properties associated with a resource in DXIL.

Resource ID

  An arbitrary ID that must be unique per resource type (SRV, UAV, etc).

  In LLVM we don’t bother representing this, instead opting to generate it at DXIL lowering time.

Binding information

  Information about where the resource comes from. This is either (a) a register space, lower bound in that space, and size of the binding, or (b) an index into a dynamic resource heap.

  In LLVM we represent binding information in the arguments of the handle creation intrinsics. When generating DXIL we transform these calls to metadata, `dx.op.createHandle`, `dx.op.createHandleFromBinding`, `dx.op.createHandleFromHeap`, and `dx.op.createHandleForLib` as needed.

Type information

  The type of data that’s accessible via the resource. For buffers and textures this can be a simple type like `float` or `float4`, a struct, or raw bytes. For constant buffers this is just a size. For samplers this is the kind of sampler.

  In LLVM we embed this information as a parameter on the `target()` type of the resource. See Types of Resource.

Resource kind information

  The kind of resource. In HLSL we have things like `ByteAddressBuffer`, `RWTexture2D`, and `RasterizerOrderedStructuredBuffer`. These map to a set of DXIL kinds like `RawBuffer` and `Texture2D` with fields for certain properties such as `IsUAV` and `IsROV`.

  In LLVM we represent this in the `target()` type. We omit information that’s deriveable from the type information, but we do have fields to encode `IsWriteable`, `IsROV`, and `SampleCount` when needed.

Note

TODO: There are two fields in the DXIL metadata that are not represented as part of the target type: `IsGloballyCoherent` and `HasCounter`.

Since these are derived from analysis, storing them on the type would mean we need to change the type during the compiler pipeline. That just isn’t practical. It isn’t entirely clear to me that we need to serialize this info into the IR during the compiler pipeline anyway - we can probably get away with an analysis pass that can calculate the information when we need it.

If analysis is insufficient we’ll need something akin to `annotateHandle` (but limited to these two properties) or to encode these in the handle creation.

## Types of Resource

We define a set of `TargetExtTypes` that is similar to the HLSL representations for the various resources, albeit with a few things parameterized. This is different than DXIL, as simplifying the types to something like “dx.srv” and “dx.uav” types would mean the operations on these types would have to be overly generic.

### Buffers

    target("dx.TypedBuffer", ElementType, IsWriteable, IsROV, IsSigned)
    target("dx.RawBuffer", ElementType, IsWriteable, IsROV)

We need two separate buffer types to account for the differences between the 16-byte bufferLoad / bufferStore operations that work on DXIL’s TypedBuffers and the rawBufferLoad / rawBufferStore operations that are used for DXIL’s RawBuffers and StructuredBuffers. We call the latter “RawBuffer” to match the naming of the operations, but it can represent both the Raw and Structured variants.

HLSL’s Buffer and RWBuffer are represented as a TypedBuffer with an element type that is a scalar integer or floating point type, or a vector of at most 4 such types. HLSL’s ByteAddressBuffer is a RawBuffer with an i8 element type. HLSL’s StructuredBuffers are RawBuffer with a struct, vector, or scalar type.

One unfortunate necessity here is that TypedBuffer needs an extra parameter to differentiate signed vs unsigned ints. The is because in LLVM IR int types don’t have a sign, so to keep this information we need a side channel.

These types are generally used by BufferLoad and BufferStore operations, as well as atomics.

There are a few fields to describe variants of all of these types:

Table 105 Buffer Fields
Field | Description
--- | ---
ElementType | Type for a single element, such as `i8`, `v4f32`, or a structure type.
IsWriteable | Whether or not the field is writeable. This distinguishes SRVs (not writeable) and UAVs (writeable).
IsROV | Whether the UAV is a rasterizer ordered view. Always `0` for SRVs.
IsSigned | Whether an int element type is signed (“dx.TypedBuffer” only)

## Resource Operations

### Resource Handles

We provide a few different ways to instantiate resources in the IR via the `llvm.dx.handle.*` intrinsics. These intrinsics are overloaded on return type, returning an appropriate handle for the resource, and represent binding information in the arguments to the intrinsic.

The three operations we need are `llvm.dx.resource.handlefrombinding`, `llvm.dx.handle.fromHeap`, and `llvm.dx.handle.fromPointer`. These are rougly equivalent to the DXIL operations `dx.op.createHandleFromBinding`, `dx.op.createHandleFromHeap`, and `dx.op.createHandleForLib`, but they fold the subsequent `dx.op.annotateHandle` operation in. Note that we don’t have an analogue for dx.op.createHandle, since `dx.op.createHandleFromBinding` subsumes it.

We diverge from DXIL and index from the beginning of the binding rather than indexing from the beginning of the binding space. This matches the semantics more clearly and avoids a non-obvious invariant in what constitutes valid arguments.

Table 106 `@llvm.dx.resource.handlefrombinding`
Argument |  | Type | Description
--- | --- | --- | ---
Return value |  | A `target()` type | A handle which can be operated on
`%reg_space` | 1 | `i32` | Register space ID in the root signature for this resource.
`%lower_bound` | 2 | `i32` | Lower bound of the binding in its register space.
`%range_size` | 3 | `i32` | Range size of the binding.
`%index` | 4 | `i32` | Index from the beginning of the binding.
`%non-uniform` | 5 | i1 | Must be `true` if the resource index may be non-uniform.

Note

TODO: Can we drop the uniformity bit? I suspect we can derive it from uniformity analysis…

Examples:

    ; RWBuffer<float4> Buf : register(u5, space3)
    %buf = call target("dx.TypedBuffer", <4 x float>, 1, 0, 0)
         @llvm.dx.resource.handlefrombinding.tdx.TypedBuffer_f32_1_0(
             i32 3, i32 5, i32 1, i32 0, i1 false)

    ; RWBuffer<int> Buf : register(u7, space2)
    %buf = call target("dx.TypedBuffer", i32, 1, 0, 1)
         @llvm.dx.resource.handlefrombinding.tdx.TypedBuffer_i32_1_0t(
             i32 2, i32 7, i32 1, i32 0, i1 false)

    ; Buffer<uint4> Buf[24] : register(t3, space5)
    %buf = call target("dx.TypedBuffer", <4 x i32>, 0, 0, 0)
         @llvm.dx.resource.handlefrombinding.tdx.TypedBuffer_i32_0_0t(
             i32 2, i32 7, i32 24, i32 0, i1 false)

    ; struct S { float4 a; uint4 b; };
    ; StructuredBuffer<S> Buf : register(t2, space4)
    %buf = call target("dx.RawBuffer", {<4 x float>, <4 x i32>}, 0, 0)
        @llvm.dx.resource.handlefrombinding.tdx.RawBuffer_sl_v4f32v4i32s_0_0t(
            i32 4, i32 2, i32 1, i32 0, i1 false)

    ; ByteAddressBuffer Buf : register(t8, space1)
    %buf = call target("dx.RawBuffer", i8, 0, 0)
        @llvm.dx.resource.handlefrombinding.tdx.RawBuffer_i8_0_0t(
            i32 1, i32 8, i32 1, i32 0, i1 false)

    ; RWBuffer<float4> Global[3] : register(u6, space5)
    ; RWBuffer<float4> Buf = Global[2];
    %buf = call target("dx.TypedBuffer", <4 x float>, 1, 0, 0)
        @llvm.dx.resource.handlefrombinding.tdx.TypedBuffer_f32_1_0(
            i32 5, i32 6, i32 3, i32 2, i1 false)

Table 107 `@llvm.dx.handle.fromHeap`
Argument |  | Type | Description
--- | --- | --- | ---
Return value |  | A `target()` type | A handle which can be operated on
`%index` | 0 | `i32` | Index of the resource to access.
`%non-uniform` | 1 | i1 | Must be `true` if the resource index may be non-uniform.

Examples:

    ; RWStructuredBuffer<float4> Buf = ResourceDescriptorHeap[2];
    declare
      target("dx.RawBuffer", <4 x float>, 1, 0)
      @llvm.dx.handle.fromHeap.tdx.RawBuffer_v4f32_1_0(
          i32 %index, i1 %non_uniform)
    ; ...
    %buf = call target("dx.RawBuffer", <4 x f32>, 1, 0)
                @llvm.dx.handle.fromHeap.tdx.RawBuffer_v4f32_1_0(
                    i32 2, i1 false)

### Accessing Resources as Memory

relevant types: Buffers and Textures

Loading and storing from resources is generally represented in LLVM using operations on memory that is only accessible via a handle object. Given a handle, llvm.dx.resource.getpointer gives a pointer that can be used to read and (depending on type) write to the resource.

Accesses using llvm.dx.resource.getpointer are replaced with direct load and store operations in the DXILResourceAccess pass. These direct loads and stores are described later in this document.

Note

Currently the pointers returned by dx.resource.getpointer are in the default address space, but that will likely change in the future.

Table 108 `@llvm.dx.resource.getpointer`
Argument |  | Type | Description
--- | --- | --- | ---
Return value |  | Pointer | A pointer to an object in the buffer
`%buffer` | 0 | `target(dx.TypedBuffer, ...)` | The buffer to access
`%index` | 1 | `i32` | Index into the buffer

Examples:

    %ptr = call ptr @llvm.dx.resource.getpointer.p0.tdx.TypedBuffer_v4f32_0_0_0t(
        target("dx.TypedBuffer", <4 x float>, 0, 0, 0) %buffer, i32 %index)

### Loads, Samples, and Gathers

relevant types: Buffers and Textures

All load, sample, and gather operations in DXIL return a ResRet type. These types are structs containing 4 elements of some basic type, and a 5th element that is used by the CheckAccessFullyMapped operation. Some of these operations, like RawBufferLoad include a mask and/or alignment that tell us some information about how to interpret those four values.

In the LLVM IR representations of these operations we instead return scalars or vectors, but we keep the requirement that we only return up to 4 elements of a basic type. This avoids some unnecessary casting and structure manipulation in the intermediate format while also keeping lowering to DXIL straightforward.

LLVM intrinsics that map to operations returning ResRet return an anonymous struct with element-0 being the scalar or vector type, and element-1 being the `i1` result of a `CheckAccessFullyMapped` call. We don’t have a separate call to `CheckAccessFullyMapped` at all, since that’s the only operation that can possibly be done on this value. In practice this may mean we insert a DXIL operation for the check when this was missing in the HLSL source, but this actually matches DXC’s behaviour in practice.

For TypedBuffer and Texture, we map directly from the contained type of the resource to the return value of the intrinsic. Since these resources are constrained to contain only scalars and vectors of up to 4 elements, the lowering to DXIL ops is generally straightforward. The one exception we have here is that double types in the elements are special - these are allowed in the LLVM intrinsics, but are lowered to pairs of i32 followed by `MakeDouble` operations for DXIL.

Table 109 `@llvm.dx.resource.load.typedbuffer`
Argument |  | Type | Description
--- | --- | --- | ---
Return value |  | A structure of the contained type and the check bit | The data loaded from the buffer and the check bit
`%buffer` | 0 | `target(dx.TypedBuffer, ...)` | The buffer to load from
`%index` | 1 | `i32` | Index into the buffer

Examples:

    %ret = call {<4 x float>, i1}
        @llvm.dx.resource.load.typedbuffer.v4f32.tdx.TypedBuffer_v4f32_0_0_0t(
            target("dx.TypedBuffer", <4 x float>, 0, 0, 0) %buffer, i32 %index)
    %ret = call {float, i1}
        @llvm.dx.resource.load.typedbuffer.f32.tdx.TypedBuffer_f32_0_0_0t(
            target("dx.TypedBuffer", float, 0, 0, 0) %buffer, i32 %index)
    %ret = call {<4 x i32>, i1}
        @llvm.dx.resource.load.typedbuffer.v4i32.tdx.TypedBuffer_v4i32_0_0_0t(
            target("dx.TypedBuffer", <4 x i32>, 0, 0, 0) %buffer, i32 %index)
    %ret = call {<4 x half>, i1}
        @llvm.dx.resource.load.typedbuffer.v4f16.tdx.TypedBuffer_v4f16_0_0_0t(
            target("dx.TypedBuffer", <4 x half>, 0, 0, 0) %buffer, i32 %index)
    %ret = call {<2 x double>, i1}
        @llvm.dx.resource.load.typedbuffer.v2f64.tdx.TypedBuffer_v2f64_0_0t(
            target("dx.TypedBuffer", <2 x double>, 0, 0, 0) %buffer, i32 %index)

For RawBuffer, an HLSL load operation may return an arbitrarily sized result, but we still constrain the LLVM intrinsic to return only up to 4 elements of a basic type. This means that larger loads are represented as a series of loads, which matches DXIL. Unlike in the RawBufferLoad operation, we do not need arguments for the mask/type size and alignment, since we can calculate these from the return type of the load during lowering.

Table 110 `@llvm.dx.resource.load.rawbuffer`
Argument |  | Type | Description
--- | --- | --- | ---
Return value |  | A structure of a scalar or vector and the check bit | The data loaded from the buffer and the check bit
`%buffer` | 0 | `target(dx.RawBuffer, ...)` | The buffer to load from
`%index` | 1 | `i32` | Index into the buffer
`%offset` | 2 | `i32` | Offset into the structure at the given index

Examples:

    ; float
    %ret = call {float, i1}
        @llvm.dx.resource.load.rawbuffer.f32.tdx.RawBuffer_f32_0_0_0t(
            target("dx.RawBuffer", float, 0, 0, 0) %buffer,
            i32 %index,
            i32 0)
    %ret = call {float, i1}
        @llvm.dx.resource.load.rawbuffer.f32.tdx.RawBuffer_i8_0_0_0t(
            target("dx.RawBuffer", i8, 0, 0, 0) %buffer,
            i32 %byte_offset,
            i32 0)

    ; float4
    %ret = call {<4 x float>, i1}
        @llvm.dx.resource.load.rawbuffer.v4f32.tdx.RawBuffer_v4f32_0_0_0t(
            target("dx.RawBuffer", float, 0, 0, 0) %buffer,
            i32 %index,
            i32 0)
    %ret = call {float, i1}
        @llvm.dx.resource.load.rawbuffer.v4f32.tdx.RawBuffer_i8_0_0_0t(
            target("dx.RawBuffer", i8, 0, 0, 0) %buffer,
            i32 %byte_offset,
            i32 0)

    ; struct S0 { float4 f; int4 i; };
    %ret = call {<4 x float>, i1}
        @llvm.dx.resource.load.rawbuffer.v4f32.tdx.RawBuffer_sl_v4f32v4i32s_0_0t(
            target("dx.RawBuffer", {<4 x float>, <4 x i32>}, 0, 0, 0) %buffer,
            i32 %index,
            i32 0)
    %ret = call {<4 x i32>, i1}
        @llvm.dx.resource.load.rawbuffer.v4i32.tdx.RawBuffer_sl_v4f32v4i32s_0_0t(
            target("dx.RawBuffer", {<4 x float>, <4 x i32>}, 0, 0, 0) %buffer,
            i32 %index,
            i32 1)

    ; struct Q { float4 f; int3 i; }
    ; struct R { int z; S x; }
    %ret = call {i32, i1}
        @llvm.dx.resource.load.rawbuffer.i32(
            target("dx.RawBuffer", {i32, {<4 x float>, <3 x i32>}}, 0, 0, 0)
                %buffer, i32 %index, i32 0)
    %ret = call {<4 x float>, i1}
        @llvm.dx.resource.load.rawbuffer.i32(
            target("dx.RawBuffer", {i32, {<4 x float>, <3 x i32>}}, 0, 0, 0)
                %buffer, i32 %index, i32 4)
    %ret = call {<3 x i32>, i1}
        @llvm.dx.resource.load.rawbuffer.i32(
            target("dx.RawBuffer", {i32, {<4 x float>, <3 x i32>}}, 0, 0, 0)
                %buffer, i32 %index, i32 20)

    ; byteaddressbuf.Load<int64_t4>
    %ret = call {<4 x i64>, i1}
        @llvm.dx.resource.load.rawbuffer.v4i64.tdx.RawBuffer_i8_0_0t(
            target("dx.RawBuffer", i8, 0, 0, 0) %buffer,
            i32 %byte_offset,
            i32 0)

### Stores

relevant types: Textures and Buffer

The TextureStore, BufferStore, and RawBufferStore DXIL operations write four components to a texture or a buffer. These include a mask argument that is used when fewer than 4 components are written, but notably this only takes on the contiguous x, xy, xyz, and xyzw values.

We define the LLVM store intrinsics to accept vectors when storing multiple components rather than using undef and a mask, but otherwise match the DXIL ops fairly closely.

For TypedBuffer, we only need one coordinate, and we must always write a vector since partial writes aren’t possible. Similarly to the load operations described above, we handle 64-bit types specially and only handle 2-element vectors rather than 4.

Examples:

Table 111 `@llvm.dx.resource.store.typedbuffer`
Argument |  | Type | Description
--- | --- | --- | ---
Return value |  | `void` |
`%buffer` | 0 | `target(dx.TypedBuffer, ...)` | The buffer to store into
`%index` | 1 | `i32` | Index into the buffer
`%data` | 2 | A 4- or 2-element vector of the type of the buffer | The data to store

Examples:

    call void @llvm.dx.resource.store.typedbuffer.tdx.Buffer_v4f32_1_0_0t(
        target("dx.TypedBuffer", f32, 1, 0) %buf, i32 %index, <4 x f32> %data)
    call void @llvm.dx.resource.store.typedbuffer.tdx.Buffer_v4f16_1_0_0t(
        target("dx.TypedBuffer", f16, 1, 0) %buf, i32 %index, <4 x f16> %data)
    call void @llvm.dx.resource.store.typedbuffer.tdx.Buffer_v2f64_1_0_0t(
        target("dx.TypedBuffer", f64, 1, 0) %buf, i32 %index, <2 x f64> %data)

For RawBuffer, we need two indices and we accept scalars and vectors of 4 or fewer elements. Note that we do allow vectors of 4 64-bit elements here.

Examples:

Table 112 `@llvm.dx.resource.store.rawbuffer`
Argument |  | Type | Description
--- | --- | --- | ---
Return value |  | `void` |
`%buffer` | 0 | `target(dx.RawBuffer, ...)` | The buffer to store into
`%index` | 1 | `i32` | Index into the buffer
`%offset` | 2 | `i32` | Byte offset into structured buffer elements
`%data` | 3 | Scalar or vector | The data to store

Examples:

    ; float
    call void @llvm.dx.resource.store.rawbuffer.tdx.RawBuffer_f32_1_0_0t.f32(
        target("dx.RawBuffer", float, 1, 0, 0) %buffer,
        i32 %index, i32 0, float %data)
    call void @llvm.dx.resource.store.rawbuffer.tdx.RawBuffer_i8_1_0_0t.f32(
        target("dx.RawBuffer", i8, 1, 0, 0) %buffer,
        i32 %index, i32 0, float %data)

    ; float4
    call void @llvm.dx.resource.store.rawbuffer.tdx.RawBuffer_v4f32_1_0_0t.v4f32(
        target("dx.RawBuffer", <4 x float>, 1, 0, 0) %buffer,
        i32 %index, i32 0, <4 x float> %data)
    call void @llvm.dx.resource.store.rawbuffer.tdx.RawBuffer_i8_1_0_0t.v4f32(
        target("dx.RawBuffer", i8, 1, 0, 0) %buffer,
        i32 %index, i32 0, <4 x float> %data)

    ; struct S0 { float4 f; int4 i; }
    call void @llvm.dx.resource.store.rawbuffer.v4f32(
        target("dx.RawBuffer", { <4 x float>, <4 x i32> }, 1, 0, 0) %buffer,
        i32 %index, i32 0, <4 x float> %data0)
    call void @llvm.dx.resource.store.rawbuffer.v4i32(
        target("dx.RawBuffer", { <4 x float>, <4 x i32> }, 1, 0, 0) %buffer,
        i32 %index, i32 16, <4 x i32> %data1)

    ; struct Q { float4 f; int3 i; }
    ; struct R { int z; S x; }
    call void @llvm.dx.resource.store.rawbuffer.i32(
        target("dx.RawBuffer", {i32, {<4 x float>, <3 x half>}}, 1, 0, 0)
            %buffer,
        i32 %index, i32 0, i32 %data0)
    call void @llvm.dx.resource.store.rawbuffer.v4f32(
        target("dx.RawBuffer", {i32, {<4 x float>, <3 x half>}}, 1, 0, 0)
            %buffer,
        i32 %index, i32 4, <4 x float> %data1)
    call void @llvm.dx.resource.store.rawbuffer.v3f16(
        target("dx.RawBuffer", {i32, {<4 x float>, <3 x half>}}, 1, 0, 0)
            %buffer,
        i32 %index, i32 20, <3 x half> %data2)

    ; byteaddressbuf.Store<int64_t4>
    call void @llvm.dx.resource.store.rawbuffer.tdx.RawBuffer_i8_1_0_0t.v4f64(
        target("dx.RawBuffer", i8, 1, 0, 0) %buffer,
        i32 %index, i32 0, <4 x double> %data)

### Constant Buffer Loads

relevant types: CBuffers

The CBufferLoadLegacy operation, which despite the name is the only supported way to load from a cbuffer in any DXIL version, loads a single “row” of a cbuffer, which is exactly 16 bytes. The return value of the operation is represented by a CBufRet type, which has variants for 2 64-bit values, 4 32-bit values, and 8 16-bit values.

We represent these in LLVM IR with 3 separate operations, which return a 2-element, 4-element, or 8-element struct respectively.

Table 113 `@llvm.dx.resource.load.cbufferrow.4`
Argument |  | Type | Description
--- | --- | --- | ---
Return value |  | A struct of 4 32-bit values | A single row of a cbuffer, interpreted as 4 32-bit values
`%buffer` | 0 | `target(dx.CBuffer, ...)` | The buffer to load from
`%index` | 1 | `i32` | Index into the buffer

Examples:

    %ret = call {float, float, float, float}
        @llvm.dx.resource.load.cbufferrow.4(
            target("dx.CBuffer", target("dx.Layout", {float}, 4, 0)) %buffer,
            i32 %index)
    %ret = call {i32, i32, i32, i32}
        @llvm.dx.resource.load.cbufferrow.4(
            target("dx.CBuffer", target("dx.Layout", {i32}, 4, 0)) %buffer,
            i32 %index)

Table 114 `@llvm.dx.resource.load.cbufferrow.2`
Argument |  | Type | Description
--- | --- | --- | ---
Return value |  | A struct of 2 64-bit values | A single row of a cbuffer, interpreted as 2 64-bit values
`%buffer` | 0 | `target(dx.CBuffer, ...)` | The buffer to load from
`%index` | 1 | `i32` | Index into the buffer

Examples:

    %ret = call {double, double}
        @llvm.dx.resource.load.cbufferrow.2(
            target("dx.CBuffer", target("dx.Layout", {double}, 8, 0)) %buffer,
            i32 %index)
    %ret = call {i64, i64}
        @llvm.dx.resource.load.cbufferrow.2(
            target("dx.CBuffer", target("dx.Layout", {i64}, 4, 0)) %buffer,
            i32 %index)

Table 115 `@llvm.dx.resource.load.cbufferrow.8`
Argument |  | Type | Description
--- | --- | --- | ---
Return value |  | A struct of 8 16-bit values | A single row of a cbuffer, interpreted as 8 16-bit values
`%buffer` | 0 | `target(dx.CBuffer, ...)` | The buffer to load from
`%index` | 1 | `i32` | Index into the buffer

Examples:

    %ret = call {half, half, half, half, half, half, half, half}
        @llvm.dx.resource.load.cbufferrow.8(
            target("dx.CBuffer", target("dx.Layout", {half}, 2, 0)) %buffer,
            i32 %index)
    %ret = call {i16, i16, i16, i16, i16, i16, i16, i16}
        @llvm.dx.resource.load.cbufferrow.8(
            target("dx.CBuffer", target("dx.Layout", {i16}, 2, 0)) %buffer,
            i32 %index)