[Mesa-dev] [PATCH] glsl: Transform fb buffers are only active if a variable uses them

[Neil Roberts]

The GL spec will soon be revised to clarify that a buffer binding for
a transform feedback buffer is only required if a variable is actually
defined to use the buffer binding point. Previously a declaration for
the default transform buffer would make it require a binding even if
nothing was declared to use the default buffer.

Affects:
KHR-GL44/45.enhanced_layouts.xfb_stride_of_empty_list
KHR-GL44/45.enhanced_layouts.xfb_stride_of_empty_list_and_api
---
 src/compiler/glsl/link_varyings.cpp | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/src/compiler/glsl/link_varyings.cpp b/src/compiler/glsl/link_varyings.cpp
index 66a20a2..e663930 100644
--- a/src/compiler/glsl/link_varyings.cpp
+++ b/src/compiler/glsl/link_varyings.cpp
@@ -1364,7 +1364,6 @@ store_tfeedback_info(struct gl_context *ctx, struct gl_shader_program *prog,
       if (has_xfb_qualifiers) {
          for (unsigned j = 0; j < MAX_FEEDBACK_BUFFERS; j++) {
             if (prog->TransformFeedback.BufferStride[j]) {
-               buffers |= 1 << j;
                explicit_stride[j] = true;
                xfb_prog->sh.LinkedTransformFeedback->Buffers[j].Stride =
                   prog->TransformFeedback.BufferStride[j] / 4;
@@ -1389,10 +1388,24 @@ store_tfeedback_info(struct gl_context *ctx, struct gl_shader_program *prog,
             num_buffers++;
             buffer_stream_id = -1;
             continue;
-         } else if (tfeedback_decls[i].is_varying()) {
+         }
+
+         if (has_xfb_qualifiers) {
+            buffer = tfeedback_decls[i].get_buffer();
+         } else {
+            buffer = num_buffers;
+         }
+
+         if (tfeedback_decls[i].is_varying()) {
             if (buffer_stream_id == -1)  {
                /* First varying writing to this buffer: remember its stream */
                buffer_stream_id = (int) tfeedback_decls[i].get_stream_id();
+
+               /* Only mark a buffer as active when there is a varying
+                * attached to it. This behaviour is based on a revised version
+                * of section 13.2.2 of the GL 4.6 spec.
+                */
+               buffers |= 1 << buffer;
             } else if (buffer_stream_id !=
                        (int) tfeedback_decls[i].get_stream_id()) {
                /* Varying writes to the same buffer from a different stream */
@@ -1408,13 +1421,6 @@ store_tfeedback_info(struct gl_context *ctx, struct gl_shader_program *prog,
             }
          }
 
-         if (has_xfb_qualifiers) {
-            buffer = tfeedback_decls[i].get_buffer();
-         } else {
-            buffer = num_buffers;
-         }
-         buffers |= 1 << buffer;
-
          if (!tfeedback_decls[i].store(ctx, prog,
                                        xfb_prog->sh.LinkedTransformFeedback,
                                        buffer, num_buffers, num_outputs,
-- 
2.9.5