glsl: remove unused varyings before packing them

Previously we would pack varyings before trying to remove them, this
relied on the packing pass not packing varyings with a location of -1
to avoid packing varyings that should be removed.
However this meant unused varyings with an explicit location would be
packed before they could be removed when we enable packing of them in a
later patch.

V2: fix regression in V1 removing unused varyings in multi-stage SSO,
fix regression with single stage programs.

Reviewed-by: Anuj Phogat <[email protected]>
Reviewed-by: Edward O'Callaghan <[email protected]>

3 files changed, 54 insertions, 48 deletions

diff --git a/src/glsl/link_varyings.cpp b/src/glsl/link_varyings.cpp
index c43abbcc6a5..a80a1b5660d 100644
--- a/src/glsl/link_varyings.cpp
+++ b/src/glsl/link_varyings.cpp
@@ -309,6 +309,41 @@ cross_validate_outputs_to_inputs(struct gl_shader_program *prog,
    }
 }
 
+/**
+ * Demote shader inputs and outputs that are not used in other stages, and
+ * remove them via dead code elimination.
+ */
+void
+remove_unused_shader_inputs_and_outputs(bool is_separate_shader_object,
+                                        gl_shader *sh,
+                                        enum ir_variable_mode mode)
+{
+   if (is_separate_shader_object)
+      return;
+
+   foreach_in_list(ir_instruction, node, sh->ir) {
+      ir_variable *const var = node->as_variable();
+
+      if ((var == NULL) || (var->data.mode != int(mode)))
+	 continue;
+
+      /* A shader 'in' or 'out' variable is only really an input or output if
+       * its value is used by other shader stages. This will cause the
+       * variable to have a location assigned.
+       */
+      if (var->data.is_unmatched_generic_inout) {
+         assert(var->data.mode != ir_var_temporary);
+	 var->data.mode = ir_var_auto;
+      }
+   }
+
+   /* Eliminate code that is now dead due to unused inputs/outputs being
+    * demoted.
+    */
+   while (do_dead_code(sh->ir, false))
+      ;
+
+}
 
 /**
  * Initialize this object based on a string that was passed to
@@ -1671,6 +1706,16 @@ assign_varying_locations(struct gl_context *ctx,
             }
          }
       }
+
+      /* Now that validation is done its safe to remove unused varyings. As
+       * we have both a producer and consumer its safe to remove unused
+       * varyings even if the program is a SSO because the stages are being
+       * linked together i.e. we have a multi-stage SSO.
+       */
+      remove_unused_shader_inputs_and_outputs(false, producer,
+                                              ir_var_shader_out);
+      remove_unused_shader_inputs_and_outputs(false, consumer,
+                                              ir_var_shader_in);
    }
 
    if (!disable_varying_packing) {
diff --git a/src/glsl/link_varyings.h b/src/glsl/link_varyings.h
index 1d12978fa30..b2812614ecc 100644
--- a/src/glsl/link_varyings.h
+++ b/src/glsl/link_varyings.h
@@ -268,6 +268,11 @@ parse_tfeedback_decls(struct gl_context *ctx, struct gl_shader_program *prog,
                       const void *mem_ctx, unsigned num_names,
                       char **varying_names, tfeedback_decl *decls);
 
+void
+remove_unused_shader_inputs_and_outputs(bool is_separate_shader_object,
+                                        gl_shader *sh,
+                                        enum ir_variable_mode mode);
+
 bool
 store_tfeedback_info(struct gl_context *ctx, struct gl_shader_program *prog,
                      unsigned num_tfeedback_decls,
diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
index acc63ae8cf6..7a18523fe23 100644
--- a/src/glsl/linker.cpp
+++ b/src/glsl/linker.cpp
@@ -2723,30 +2723,6 @@ match_explicit_outputs_to_inputs(struct gl_shader_program *prog,
 }
 
 /**
- * Demote shader inputs and outputs that are not used in other stages
- */
-void
-demote_shader_inputs_and_outputs(gl_shader *sh, enum ir_variable_mode mode)
-{
-   foreach_in_list(ir_instruction, node, sh->ir) {
-      ir_variable *const var = node->as_variable();
-
-      if ((var == NULL) || (var->data.mode != int(mode)))
-	 continue;
-
-      /* A shader 'in' or 'out' variable is only really an input or output if
-       * its value is used by other shader stages.  This will cause the variable
-       * to have a location assigned.
-       */
-      if (var->data.is_unmatched_generic_inout) {
-         assert(var->data.mode != ir_var_temporary);
-	 var->data.mode = ir_var_auto;
-      }
-   }
-}
-
-
-/**
  * Store the gl_FragDepth layout in the gl_shader_program struct.
  */
 static void
@@ -4446,14 +4422,8 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
       do_dead_builtin_varyings(ctx, sh, NULL,
                                num_tfeedback_decls, tfeedback_decls);
 
-      if (!prog->SeparateShader) {
-         demote_shader_inputs_and_outputs(sh, ir_var_shader_out);
-         /* Eliminate code that is now dead due to unused outputs being
-          * demoted.
-          */
-         while (do_dead_code(sh->ir, false))
-            ;
-      }
+      remove_unused_shader_inputs_and_outputs(prog->SeparateShader, sh,
+                                              ir_var_shader_out);
    }
    else if (first == MESA_SHADER_FRAGMENT) {
       /* If the program only contains a fragment shader...
@@ -4471,12 +4441,8 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
                                        NULL /* tfeedback_decls */))
             goto done;
       } else {
-         demote_shader_inputs_and_outputs(sh, ir_var_shader_in);
-         /* Eliminate code that is now dead due to unused inputs being
-          * demoted.
-          */
-         while (do_dead_code(sh->ir, false))
-            ;
+         remove_unused_shader_inputs_and_outputs(false, sh,
+                                                 ir_var_shader_in);
       }
    }
 
@@ -4497,16 +4463,6 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
                 next == MESA_SHADER_FRAGMENT ? num_tfeedback_decls : 0,
                 tfeedback_decls);
 
-      demote_shader_inputs_and_outputs(sh_i, ir_var_shader_out);
-      demote_shader_inputs_and_outputs(sh_next, ir_var_shader_in);
-
-      /* Eliminate code that is now dead due to unused outputs being demoted.
-       */
-      while (do_dead_code(sh_i->ir, false))
-         ;
-      while (do_dead_code(sh_next->ir, false))
-         ;
-
       /* This must be done after all dead varyings are eliminated. */
       if (!check_against_output_limit(ctx, prog, sh_i))
          goto done;