Geometry shader with transform feedbacks

[Giels]

Geometry shaders seem to work.
Transform feedbacks seem to work.
But the combination of the two does not seem to generate viable code.
Consider the following:

   (defparameter vert-stream [...])
    (defparameter tf-arr
	  (make-gpu-array nil :element-type :vec3 :dimensions 3))
    (defparameter tf
	  (make-transform-feedback-stream tf-arr)))

With gpu programs:

(defun-g vert ((data :vec3))
 (values (v! data 1) data))

(defun-g geom ((data (:vec3 1)))
	 (declare (output-primitive :kind :triangle-strip :max-vertices 3))
 (emit () (v! (aref data 0) 1) (:feedback (aref data 0)))
 (emit () (v! (aref data 0) 1) (:feedback (aref data 0)))
 (emit () (v! (aref data 0) 1) (:feedback (aref data 0)))
 (end-primitive))

(defpipeline-g pp
 :vertex (vert :vec3)
 :geometry (geom (:vec3 1)))

During linking around something like

(with-transform-feedbacks (tf)
 (map-g #'pp vert-stream))

An error such as

Error Linking Program
error: Transform feedback varying _GEOMETRY_STAGE_OUT_0 undeclared.

Compiled-stages:

 // vertex-stage
#version 450

layout(location = 0)  in vec3 VERT;

out _FROM_VERTEX_STAGE_
{
     out vec3 _VERTEX_STAGE_OUT_1;
} v_out;

void main()
{
    v_out._VERTEX_STAGE_OUT_1 = VERT;
    gl_Position = vec4(VERT,1.0f);
    return;
}

 // geometry-stage
#version 450

layout (points) in;

in _FROM_VERTEX_STAGE_
{
     in vec3 _VERTEX_STAGE_OUT_1;
} v_in[1];

layout (triangle_strip, max_vertices = 3) out;

out _FROM_GEOMETRY_STAGE_
{
     out vec3 _GEOMETRY_STAGE_OUT_0;
} v_out;

void main()
{
                (gl_Position = v_in[0]._VERTEX_STAGE_OUT_1);
                v_out._GEOMETRY_STAGE_OUT_0 = vec4(v_in[0]._VERTEX_STAGE_OUT_1,1);
                EmitVertex();
    (gl_Position = v_in[0]._VERTEX_STAGE_OUT_1);
                v_out._GEOMETRY_STAGE_OUT_0 = vec4(v_in[0]._VERTEX_STAGE_OUT_1,1);
                EmitVertex();
    (gl_Position = v_in[0]._VERTEX_STAGE_OUT_1);
                v_out._GEOMETRY_STAGE_OUT_0 = vec4(v_in[0]._VERTEX_STAGE_OUT_1,1);
                EmitVertex();
            EndPrimitive();
    }
    return;
}

Appears. Modifying the program minimally to disable the feedbacks compiles and links correctly.
Caveat emptor: this is edited based on the real program which is too long. If needed, a real mwe will follow.
There's a high chance I mixed up the invocations somewhere or forgot a declaration to make this work, but there seems to be no example of the combination of geometry shaders and transform feedbacks with cepl anywhere...

[Giels]

Here's the fully-functional MWE.

(cepl:repl)

(defvar *quad* nil)
(defvar *quad-stream* nil)
(defvar *running* nil)
(defvar *tf-arr*
  (cepl:make-gpu-array nil :element-type 'cepl:g-pnt :dimensions 6))
(defvar *tf*
  (cepl:make-transform-feedback-stream *tf-arr*))

(cepl:defun-g dummy-vert ((vert :vec3))
  (values (rtg-math:v! vert 1) vert))

(cepl:defun-g dummy-geom ((vert (:vec3 3)))
	 (declare (vari:output-primitive :kind :triangle-strip :max-vertices 3))
	 (vari:emit ()
		    (rtg-math:v! (aref vert 0) 1)
		    (:feedback (aref vert 0))
		    (:feedback (rtg-math:v! 0 1 0))
		    (:feedback (rtg-math:v! 0 0)))
	 (vari:emit ()
		    (rtg-math:v! (aref vert 1) 1)
		    (:feedback (aref vert 1))
		    (:feedback (rtg-math:v! 0 1 0))
		    (:feedback (rtg-math:v! 0 0)))
	 (vari:emit ()
		    (rtg-math:v! (aref vert 2) 1)
		    (:feedback (aref vert 2))
		    (:feedback (rtg-math:v! 0 1 0))
		    (:feedback (rtg-math:v! 0 0)))
	 (vari:end-primitive))

(cepl:defpipeline-g dummy-pipeline ()
	       :vertex (dummy-vert :vec3)
	       :geometry (dummy-geom (:vec3 3)))

(defun step-me ()
  (cepl:step-host)
  (livesupport:update-repl-link)
  (cepl:clear)
  (cepl:with-transform-feedback (*tf*)
				(cepl:map-g #'dummy-pipeline *quad-stream*))
  (cepl:swap))

(defun init ()
  (unless *quad*
    (setf *quad* (cepl:make-gpu-array
                  (list (rtg-math:v! -1.0   1.0 0 0)
                        (rtg-math:v! -1.0  -1.0 0 0)
                        (rtg-math:v!  1.0  -1.0 0 0)
                        (rtg-math:v! -1.0   1.0 0 0)
                        (rtg-math:v!  1.0  -1.0 0 0)
                        (rtg-math:v!  1.0   1.0 0 0))
                  :element-type :vec3
                  :dimensions 6)))
  (unless *quad-stream*
    (setf *quad-stream* (cepl:make-buffer-stream *quad* :retain-arrays t))))

(defun run-loop ()
  (setf *running* t)
  (init)
  (loop :while *running* :do (livesupport:continuable (step-me))))

(defun stop-loop ()
  (setf *running* nil))

(run-loop)

Result:

Error Linking Program
error: Transform feedback varying _GEOMETRY_STAGE_OUT_0 undeclared.

Compiled-stages:

 // vertex-stage
#version 450

layout(location = 0)  in vec3 VERT;

out _FROM_VERTEX_STAGE_
{
     out vec3 _VERTEX_STAGE_OUT_1;
} v_out;

void main()
{
    vec4 g_PROG1_TMP12619 = vec4(VERT,1.0f);
    v_out._VERTEX_STAGE_OUT_1 = VERT;
    vec4 g_GEXPR0_12620 = g_PROG1_TMP12619;
    gl_Position = g_GEXPR0_12620;
    return;
}



 // geometry-stage
#version 450

layout (triangles) in;

in _FROM_VERTEX_STAGE_
{
     in vec3 _VERTEX_STAGE_OUT_1;
} v_in[3];

layout (triangle_strip, max_vertices = 3) out;

out _FROM_GEOMETRY_STAGE_
{
     out vec3 _GEOMETRY_STAGE_OUT_0;
     out vec3 _GEOMETRY_STAGE_OUT_1;
     out vec2 _GEOMETRY_STAGE_OUT_2;
} v_out;

void main()
{
    (gl_Position = vec4(v_in[0]._VERTEX_STAGE_OUT_1,1.0f));
    v_out._GEOMETRY_STAGE_OUT_0 = v_in[0]._VERTEX_STAGE_OUT_1;
    v_out._GEOMETRY_STAGE_OUT_1 = vec3(0.0f,1.0f,0.0f);
    v_out._GEOMETRY_STAGE_OUT_2 = vec2(0.0f,0.0f);
    EmitVertex();
    (gl_Position = vec4(v_in[1]._VERTEX_STAGE_OUT_1,1.0f));
    v_out._GEOMETRY_STAGE_OUT_0 = v_in[1]._VERTEX_STAGE_OUT_1;
    v_out._GEOMETRY_STAGE_OUT_1 = vec3(0.0f,1.0f,0.0f);
    v_out._GEOMETRY_STAGE_OUT_2 = vec2(0.0f,0.0f);
    EmitVertex();
    (gl_Position = vec4(v_in[2]._VERTEX_STAGE_OUT_1,1.0f));
    v_out._GEOMETRY_STAGE_OUT_0 = v_in[2]._VERTEX_STAGE_OUT_1;
    v_out._GEOMETRY_STAGE_OUT_1 = vec3(0.0f,1.0f,0.0f);
    v_out._GEOMETRY_STAGE_OUT_2 = vec2(0.0f,0.0f);
    EmitVertex();
    EndPrimitive();
    return;
}