glsl/linker: link shaders in the opposite order (from fragment to vertex)

This ensures that inter-shader outputs and inputs are properly eliminated
across 3 or more shader stages. The behavior is unchanged with 2 or less
shader stages.

For example, elimination of unused FS inputs causes elimination of matching
GS outputs, which causes elimination of the GS inputs that were needed for
evaluation of the eliminated GS outputs, which causes elimination of
matching VS outputs. An unused FS input is all that's needed to trigger
this chain reaction.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

1 files changed, 58 insertions, 50 deletions

diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
index 61405ea..3adcddd 100644
--- a/src/glsl/linker.cpp
+++ b/src/glsl/linker.cpp
@@ -1836,9 +1836,9 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
       goto done;
    }
 
-   unsigned prev;
-   for (prev = 0; prev < MESA_SHADER_TYPES; prev++) {
-      if (prog->_LinkedShaders[prev] != NULL)
+   unsigned first;
+   for (first = 0; first < MESA_SHADER_TYPES; first++) {
+      if (prog->_LinkedShaders[first] != NULL)
 	 break;
    }
 
@@ -1850,7 +1850,7 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
        *     non-zero, but the program object has no vertex or geometry
        *     shader;
        */
-      if (prev >= MESA_SHADER_FRAGMENT) {
+      if (first >= MESA_SHADER_FRAGMENT) {
          linker_error(prog, "Transform feedback varyings specified, but "
                       "no vertex or geometry shader is present.");
          goto done;
@@ -1864,69 +1864,77 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
          goto done;
    }
 
-   for (unsigned i = prev + 1; i < MESA_SHADER_TYPES; i++) {
-      if (prog->_LinkedShaders[i] == NULL)
-	 continue;
-
-      if (!assign_varying_locations(
-				    ctx, mem_ctx, prog, prog->_LinkedShaders[prev], prog->_LinkedShaders[i],
-             i == MESA_SHADER_FRAGMENT ? num_tfeedback_decls : 0,
-             tfeedback_decls))
-	 goto done;
-
-      prev = i;
+   /* Linking the stages in the opposite order (from fragment to vertex)
+    * ensures that inter-shader outputs written to in an earlier stage are
+    * eliminated if they are (transitively) not used in a later stage.
+    */
+   int last, next;
+   for (last = MESA_SHADER_TYPES-1; last >= 0; last--) {
+      if (prog->_LinkedShaders[last] != NULL)
+         break;
    }
 
-   if (prev != MESA_SHADER_FRAGMENT && num_tfeedback_decls != 0) {
-      /* There was no fragment shader, but we still have to assign varying
-       * locations for use by transform feedback.
-       */
-      if (!assign_varying_locations(
-				    ctx, mem_ctx, prog, prog->_LinkedShaders[prev], NULL, num_tfeedback_decls,
-             tfeedback_decls))
-         goto done;
-   }
+   if (last >= 0 && last < MESA_SHADER_FRAGMENT) {
+      gl_shader *const sh = prog->_LinkedShaders[last];
 
-   if (!store_tfeedback_info(ctx, prog, num_tfeedback_decls, tfeedback_decls))
-      goto done;
+      if (num_tfeedback_decls != 0) {
+         /* There was no fragment shader, but we still have to assign varying
+          * locations for use by transform feedback.
+          */
+         if (!assign_varying_locations(ctx, mem_ctx, prog,
+                                       sh, NULL,
+                                       num_tfeedback_decls, tfeedback_decls))
+            goto done;
+      }
 
-   if (prog->_LinkedShaders[MESA_SHADER_VERTEX] != NULL) {
-      demote_shader_inputs_and_outputs(prog->_LinkedShaders[MESA_SHADER_VERTEX],
-				       ir_var_shader_out);
+      demote_shader_inputs_and_outputs(sh, ir_var_shader_out);
 
-      /* Eliminate code that is now dead due to unused vertex outputs being
-       * demoted.
+      /* Eliminate code that is now dead due to unused outputs being demoted.
        */
-      while (do_dead_code(prog->_LinkedShaders[MESA_SHADER_VERTEX]->ir, false))
-	 ;
+      while (do_dead_code(sh->ir, false))
+         ;
    }
-
-   if (prog->_LinkedShaders[MESA_SHADER_GEOMETRY] != NULL) {
-      gl_shader *const sh = prog->_LinkedShaders[MESA_SHADER_GEOMETRY];
+   else if (first == MESA_SHADER_FRAGMENT) {
+      /* If the program only contains a fragment shader, just demote
+       * user-defined varyings.
+       */
+      gl_shader *const sh = prog->_LinkedShaders[first];
 
       demote_shader_inputs_and_outputs(sh, ir_var_shader_in);
-      demote_shader_inputs_and_outputs(sh, ir_var_shader_out);
 
-      /* Eliminate code that is now dead due to unused geometry outputs being
-       * demoted.
-       */
-      while (do_dead_code(prog->_LinkedShaders[MESA_SHADER_GEOMETRY]->ir, false))
-	 ;
+      while (do_dead_code(sh->ir, false))
+         ;
    }
 
-   if (prog->_LinkedShaders[MESA_SHADER_FRAGMENT] != NULL) {
-      gl_shader *const sh = prog->_LinkedShaders[MESA_SHADER_FRAGMENT];
+   next = last;
+   for (int i = next - 1; i >= 0; i--) {
+      if (prog->_LinkedShaders[i] == NULL)
+         continue;
 
-      demote_shader_inputs_and_outputs(sh, ir_var_shader_in);
+      gl_shader *const sh_i = prog->_LinkedShaders[i];
+      gl_shader *const sh_next = prog->_LinkedShaders[next];
 
-      /* Eliminate code that is now dead due to unused fragment inputs being
-       * demoted.  This shouldn't actually do anything other than remove
-       * declarations of the (now unused) global variables.
+      if (!assign_varying_locations(ctx, mem_ctx, prog, sh_i, sh_next,
+                next == MESA_SHADER_FRAGMENT ? num_tfeedback_decls : 0,
+                tfeedback_decls))
+         goto done;
+
+      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(prog->_LinkedShaders[MESA_SHADER_FRAGMENT]->ir, false))
-	 ;
+      while (do_dead_code(sh_i->ir, false))
+         ;
+      while (do_dead_code(sh_next->ir, false))
+         ;
+
+      next = i;
    }
 
+   if (!store_tfeedback_info(ctx, prog, num_tfeedback_decls, tfeedback_decls))
+      goto done;
+
    update_array_sizes(prog);
    link_assign_uniform_locations(prog);
    store_fragdepth_layout(prog);