gl_ClipVertex binding in vertex program

In Shading Language, the vertex shader can write to a built-in varying gl_ClipVertex, the distance between the vertex and the user-clipping plane.

According to ARB_vertex_program (ARB Extension #26), user clipping planes are not supported in standard vertex program.

(20) How should user-defined clipping be supported in this specification?

RESOLVED: User-defined clipping is not supported in standard vertex program mode. User-defined clipping support will be provided for programs that use the “position invariant” option, where all vertex transformation operations are performed by the fixed-function pipeline.

But I can successfully compile a vertex shader written in OpenGL shading language. Which ouput is bound to gl_ClipVertex? (I don’t know how to compile a high-level code to vertex program assembly.)



void main(void)
	vec4 a = gl_Vertex;
	a.x = a.x * 0.5;
	a.y = a.y * 0.5;
	gl_Position = gl_ModelViewProjectionMatrix * a;
	gl_ClipVertex = 1.0;

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