Module storage

Generating accesses to ByteAddressBuffer contents.

Naga IR globals in the Storage address space are rendered as ByteAddressBuffers or RWByteAddressBuffers in HLSL. These buffers don’t have HLSL types (structs, arrays, etc.); instead, they are just raw blocks of bytes, with methods to load and store values of specific types at particular byte offsets. This means that Naga must translate chains of Access and AccessIndex expressions into HLSL expressions that compute byte offsets into the buffer.

To generate code for a Storage access:

• Call Writer::fill_access_chain on the expression referring to the value. This populates Writer::temp_access_chain with the appropriate byte offset calculations, as a vector of SubAccess values.

• Call Writer::write_storage_address to emit an HLSL expression for a given slice of SubAccess values.

Naga IR expressions can operate on composite values of any type, but ByteAddressBuffer and RWByteAddressBuffer have only a fixed set of Load and Store methods, to access one through four consecutive 32-bit values. To synthesize a Naga access, you can initialize temp_access_chain to refer to the composite, and then temporarily push and pop additional steps on Writer::temp_access_chain to generate accesses to the individual elements/members.

The temp_access_chain field is a member of Writer solely to allow re-use of the Vec’s dynamic allocation. Its value is no longer needed once HLSL for the access has been generated.

Note about DXC and Load/Store functions:

DXC’s HLSL has a generic Load and Store function for ByteAddressBuffer and RWByteAddressBuffer. This is not available in FXC’s HLSL, so we use it only for types that are only available in DXC. Notably 64 and 16 bit types.

FXC’s HLSL has functions Load, Load2, Load3, and Load4 and Store, Store2, Store3, Store4. This loads/stores a vector of length 1, 2, 3, or 4. We use that for 32bit types, bitcasting to the correct type if necessary.

Enums

StoreValue 🔒

SubAccess 🔒

One step in accessing a Storage global’s component or element.

Constants

STORE_TEMP_NAME 🔒