[Mesa-dev] [RFC 4/4] glsl: don't sort varying in separate shader mode

[Gregory Hainaut]

Current issue is the addition of FLAT qualifier on varying_matches::record()
which break the varying expected order

Future issue is the removal of the interpolation qualifier matching constrain

In my humble opinion, it is the responsability of the GL developer to optimize
their slots assignment in SSO with the help of GL_ARB_enhanced_layouts

Signed-off-by: Gregory Hainaut <gregory.hainaut at gmail.com>
---
 src/glsl/link_varyings.cpp | 36 ++++++++++++++++++++++++++++++------
 1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/src/glsl/link_varyings.cpp b/src/glsl/link_varyings.cpp
index 6d6e9b5..31efee7 100644
--- a/src/glsl/link_varyings.cpp
+++ b/src/glsl/link_varyings.cpp
@@ -766,7 +766,7 @@ public:
                    gl_shader_stage consumer_stage);
    ~varying_matches();
    void record(ir_variable *producer_var, ir_variable *consumer_var);
-   unsigned assign_locations(uint64_t reserved_slots);
+   unsigned assign_locations(uint64_t reserved_slots, bool separate_shader);
    void store_locations() const;
 
 private:
@@ -986,11 +986,34 @@ varying_matches::record(ir_variable *producer_var, ir_variable *consumer_var)
  * passed to varying_matches::record().
  */
 unsigned
-varying_matches::assign_locations(uint64_t reserved_slots)
+varying_matches::assign_locations(uint64_t reserved_slots, bool separate_shader)
 {
-   /* Sort varying matches into an order that makes them easy to pack. */
-   qsort(this->matches, this->num_matches, sizeof(*this->matches),
-         &varying_matches::match_comparator);
+   /* Disable the varying sorting for separate shader program
+    * 1/ All programs must sort the code in the same order to guarantee the
+    *    interface matching. However varying_matches::record() will change the
+    *    interpolation qualifier of some stages.
+    *
+    * 2/ GLSL version 4.50 removes the matching constrain on the interpolation
+    *    qualifier.
+    *
+    * Chapter 4.5 of GLSL 4.40:
+    *    "The type and presence of interpolation qualifiers of variables with
+    *    the same name declared in all linked shaders for the same cross-stage
+    *    interface must match, otherwise the link command will fail.
+    *
+    *    When comparing an output from one stage to an input of a subsequent
+    *    stage, the input and output don't match if their interpolation
+    *    qualifiers (or lack thereof) are not the same."
+    *
+    * Chapter 4.5 of GLSL 4.50:
+    *    "It is a link-time error if, within the same stage, the interpolation
+    *    qualifiers of variables of the same name do not match."
+    */
+   if (!separate_shader) {
+      /* Sort varying matches into an order that makes them easy to pack. */
+      qsort(this->matches, this->num_matches, sizeof(*this->matches),
+            &varying_matches::match_comparator);
+   }
 
    unsigned generic_location = 0;
    unsigned generic_patch_location = MAX_VARYING*4;
@@ -1590,7 +1613,8 @@ assign_varying_locations(struct gl_context *ctx,
       reserved_varying_slot(producer, ir_var_shader_out) |
       reserved_varying_slot(consumer, ir_var_shader_in);
 
-   const unsigned slots_used = matches.assign_locations(reserved_slots);
+   const unsigned slots_used = matches.assign_locations(reserved_slots,
+                                                        prog->SeparateShader);
    matches.store_locations();
 
    for (unsigned i = 0; i < num_tfeedback_decls; ++i) {
-- 
2.1.4