Acceleration structure alignment issue

[mergmann]

Problem

I wanted to run a raytracer I have written quite some time ago, however it didn't run anymore (possibly due to a toolchain update).
It doesn't work in 0.34.1 as well as 0.35.1
The issue has to do with memory alignment of acceleration structures.

Error

I get the error

called `Result::unwrap()` on an `Err` value: info.geometries.data: is `AccelerationStructureGeometryInstancesDataType::Values`, and the buffer's device address is not a multiple of 16

Vulkan VUIDs:
    VUID-vkCmdBuildAccelerationStructuresKHR-pInfos-03715

vulkano/vulkano/src/command_buffer/commands/acceleration_structure.rs

Line 1378 in c9af075

return Err(Box::new(ValidationError {

Used code

This code creates a buffer of acceleration structures (taken from #2314)

let values = Buffer::from_iter(
        memory_allocator.clone(),
        BufferCreateInfo {
            usage: BufferUsage::ACCELERATION_STRUCTURE_BUILD_INPUT_READ_ONLY
                | BufferUsage::SHADER_DEVICE_ADDRESS,
            ..Default::default()
        },
        AllocationCreateInfo {
            memory_type_filter: MemoryTypeFilter::PREFER_DEVICE
                | MemoryTypeFilter::HOST_SEQUENTIAL_WRITE,
            ..Default::default()
        },
        instances,
    )
    .unwrap();

The layout of AccelerationStructureInstance is

BufferContentsLayout(
    Sized(
        DeviceLayout {
            size: 64,
            alignment: 8 (1 << 3),
        },
    ),
)

The alignment is incorrect (according Vulkan Docs)

geometry.instances.data.deviceAddress must be aligned to 16 bytes

I guess the buffer had been over-aligned previously, thus never triggering any errors.

My quick fix without having to patch vulkano is to wrap the instances when allocating and then casting the buffer

#[derive(BufferContents)]
#[repr(C, align(16))]
struct InstanceWrapper(AccelerationStructureInstance);

[Rua]

DeviceAlignment::of doesn't currently appear to have any way to return an alignment different from the host alignment. It just calls std::mem::align_of internally, which is incorrect here. So Vulkano needs a way to override this for some types.

[marc0246]

The way to fix this would be to just add the align(16) repr to AccelerationStructureInstance. This also doesn't have anything to do with DeviceAlignment. There also is a way to create an over-aligned buffer: Buffer::new.