Per patch varyings : arrays vs vectors

When setting a specific component of an array using gl_InvocationID, I get undefined results from patch varyings set in my tessellation control shaders with vectors while arrays work fine. I don’t see why vectors should give an undefined result.
Here’s some code illustrating the problem (using triangular patches):

// tessellation control shader
#version 410
layout(location = 0) patch out vec3  oData1; 
layout(location = 1) patch out float oData2[3];


void main()
	oData1[gl_InvocationID] = someScalarValue(gl_InvocationID); // undefined in my tessellation evaluation shader 
	oData2[gl_InvocationID] = someScalarValue(gl_InvocationID); // works fine in my tessellation evaluation shader

Either I missed something in the specs, either it’s a driver bug, I’m on Catalyst 11.9.

oData1 is not an array. So how are you accessing it as an array?

It’s perfectly valid to access the components of a vector using the bracket operator (see, for example, page 80 of glsl420 specs: section 5.5).

Tessellation shader invocations were designed to be run in parallel. AMD hardware is vector-based, so I’d guess that what is actually happening in each invocation is:

load vec3 data
insert scalar->vec3
store vec3 data

Because of the fact that the entire vector is being stored, parallel invocations are stomping on the same data location, producing unpredictable results. The float[3] array is very likely being stored in separate 128b-padded addresses, so no data-stomping occurs.

This is in theory, anyway :slight_smile: – I don’t exactly know what’s going on behind the scenes.

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