glsl: Eliminate unused built-in variables after compilation

After compilation (and before linking) we can eliminate quite a few
built-in variables.  Basically, any uniform or constant (e.g.,
gl_MaxVertexTextureImageUnits) that isn't used (with one exception) can
be eliminated.  System values, vertex shader inputs (with one
exception), and fragment shader outputs that are not used and not
re-declared in the shader text can also be removed.

gl_ModelViewProjectMatrix and gl_Vertex are used by the built-in
function ftransform.  There are some complications with eliminating
these variables (see the comment in the patch), so they are not
eliminated.

Valgrind massif results for a trimmed apitrace of dota2:

                  n        time(i)         total(B)   useful-heap(B) extra-heap(B)    stacks(B)
Before (32-bit): 46 40,661,487,174       75,116,800       68,854,065     6,262,735            0
After  (32-bit): 50 40,564,927,443       69,185,408       63,683,871     5,501,537            0

Before (64-bit): 64 37,200,329,700      104,872,672       96,514,546     8,358,126            0
After  (64-bit): 59 36,822,048,449       96,526,888       89,113,000     7,413,888            0

A real savings of 4.9MiB on 32-bit and 7.0MiB on 64-bit.

v2: Don't remove any built-in with Transpose in the name.

v3: Fix comment typo noticed by Anuj.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Suggested-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Acked-by: Anuj Phogat <anuj.phogat@gmail.com>
Cc: Eric Anholt <eric@anholt.net>

src/glsl/Makefile.sources

@@ -87,6 +87,7 @@ LIBGLSL_FILES = \
 	$(GLSL_SRCDIR)/opt_copy_propagation.cpp \
 	$(GLSL_SRCDIR)/opt_copy_propagation_elements.cpp \
 	$(GLSL_SRCDIR)/opt_cse.cpp \
+	$(GLSL_SRCDIR)/opt_dead_builtin_variables.cpp \
 	$(GLSL_SRCDIR)/opt_dead_builtin_varyings.cpp \
 	$(GLSL_SRCDIR)/opt_dead_code.cpp \
 	$(GLSL_SRCDIR)/opt_dead_code_local.cpp \

src/glsl/glsl_parser_extras.cpp

@@ -1483,6 +1483,26 @@ _mesa_glsl_compile_shader(struct gl_context *ctx, struct gl_shader *shader,
          ;
 
       validate_ir_tree(shader->ir);
+
+      enum ir_variable_mode other;
+      switch (shader->Stage) {
+      case MESA_SHADER_VERTEX:
+         other = ir_var_shader_in;
+         break;
+      case MESA_SHADER_FRAGMENT:
+         other = ir_var_shader_out;
+         break;
+      default:
+         /* Something invalid to ensure optimize_dead_builtin_uniforms
+          * doesn't remove anything other than uniforms or constants.
+          */
+         other = ir_var_mode_count;
+         break;
+      }
+
+      optimize_dead_builtin_variables(shader->ir, other);
+
+      validate_ir_tree(shader->ir);
    }
 
    if (shader->InfoLog)

src/glsl/ir_optimization.h

@@ -125,6 +125,8 @@ void lower_named_interface_blocks(void *mem_ctx, gl_shader *shader);
 bool optimize_redundant_jumps(exec_list *instructions);
 bool optimize_split_arrays(exec_list *instructions, bool linked);
 bool lower_offset_arrays(exec_list *instructions);
+void optimize_dead_builtin_variables(exec_list *instructions,
+                                     enum ir_variable_mode other);
 
 bool lower_vertex_id(gl_shader *shader);
 

src/glsl/opt_dead_builtin_variables.cpp

@@ -0,0 +1,81 @@
+/*
+ * Copyright © 2014 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#include "ir.h"
+#include "ir_visitor.h"
+#include "ir_optimization.h"
+
+/**
+ * Pre-linking, optimize unused built-in variables
+ *
+ * Uniforms, constants, system values, inputs (vertex shader only), and
+ * outputs (fragment shader only) that are not used can be removed.
+ */
+void
+optimize_dead_builtin_variables(exec_list *instructions,
+                                enum ir_variable_mode other)
+{
+   foreach_in_list_safe(ir_variable, var, instructions) {
+      if (var->ir_type != ir_type_variable || var->data.used)
+         continue;
+
+      if (var->data.mode != ir_var_uniform
+          && var->data.mode != ir_var_auto
+          && var->data.mode != ir_var_system_value
+          && var->data.mode != other)
+         continue;
+
+      /* So that linker rules can later be enforced, we cannot elimate
+       * variables that were redeclared in the shader code.
+       */
+      if ((var->data.mode == other || var->data.mode == ir_var_system_value)
+          && var->data.how_declared != ir_var_declared_implicitly)
+         continue;
+
+      if (strncmp(var->name, "gl_", 3) != 0)
+         continue;
+
+      /* gl_ModelViewProjectionMatrix and gl_Vertex are special because they
+       * are used by ftransform.  No other built-in variable is used by a
+       * built-in function.  The forward declarations of these variables in
+       * the built-in function shader does not have the "state slot"
+       * information, so removing these variables from the user shader will
+       * cause problems later.
+       *
+       * Matrix uniforms with "Transpose" are not eliminated because there's
+       * an optimization pass that can turn references to the regular matrix
+       * into references to the transpose matrix.  Eliminating the transpose
+       * matrix would cause that pass to generate references to undeclareds
+       * variables (thank you, ir_validate).
+       *
+       * It doesn't seem worth the effort to track when the transpose could be
+       * eliminated (i.e., when the non-transpose was eliminated).
+       */
+      if (strcmp(var->name, "gl_ModelViewProjectionMatrix") == 0
+          || strcmp(var->name, "gl_Vertex") == 0
+          || strstr(var->name, "Transpose") != NULL)
+         continue;
+
+      var->remove();
+   }
+}