Push constant limit in geometry shader?

Question

I have a geometry shader with the following push constant block:

layout(push_constant) uniform Instance {
    mat4 VP;
    vec3 posCam;
    float radius;
    float curvature;
} u_instance;

The push constants are defined in the pipeline layout like this:

uint32_t offset = 0;
uint32_t size = 21 *sizeof(float);
vk::PushConstantRange range {vk::ShaderStageFlagBits::eGeometry,offset,size};

However, the Vulkan validation layers throw this error:

Push constant range covering variable starting at offset 0 not accessible from stage VK_SHADER_STAGE_GEOMETRY_BIT

What does 'not accessible' mean here? Why wouldn't they be accessible? If I move the push constants to a different stage (e.g. fragment or vertex shader), no error occurs.

Additionally, I only get this error on a Nvidia GeForce GTX 650 Ti. I've also tried it on an AMD card, which worked fine.

Is there some kind of limitation on push constants for geometry shaders? I've checked the limitations for my Nvidia GPU, the total max push constant size is 256 bytes, and geometry shaders are supported. I also can't find anything in the Vulkan specification either.

Sascha Willems Sascha Willems · Accepted Answer · 2016-06-29T17:55:41

Can you please add some more code (or upload it to somewhere)? I just tested this with your push constant block on a GTX 980 with validation layers compiled from source and don't get any validation warnings.

Additionally, I only get this error on a Nvidia GeForce GTX 650 Ti. I've also tried it on an AMD card, which worked fine.

That's very odd, as the validation messages are not generated by the drivers and as such should not differ between implementations (unless it's a validation related to device limits).

Is there some kind of limitation on push constants for geometry shaders? I've checked the limitations for my Nvidia GPU, the total max push constant size is 256 bytes, and geometry shaders are supported. I also can't find anything in the Vulkan specification either.

There is no push constant limit specific to the geometry shader. If you exceed the push constant size limit the validation layers will throw an error.

I think that std430 packing rules (or 14.5.4. Offset and Stride Assignment of the Vulkan spec) can mess up the sizes. E.g. the vec3 would be laid out as vec4 (so maybe 22*sizeof(float)? - sorry not exactly confident in this myself).

Not sure if packing would be a problem here, but basically this should work without a validation layer message. If stride would be a problem the validation layers should still not trigger anything if you start with offset 0.

Push constant limit in geometry shader?

2 Answers