glsl/drirc: add an option for gl_ClipVertex / gl_CullDistance checks

The GLSL spec says it's an error if a shader statically writes to these
2 variables.

Until this commit, Mesa refused to link a shader if it had an unused
function writing to one of these variables while another (used) function
wrote to the other.

This commit adds an option to perform dead function elimination after
the intra-stage linking step but before performing these checks.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12897>

src/compiler/glsl/linker.cpp

@@ -103,6 +103,8 @@ struct find_variable {
 
 /**
  * Visitor that determines whether or not a variable is ever written.
+ * Note: this is only considering if the variable is statically written
+ * (= regardless of the runtime flow of control)
  *
  * Use \ref find_assignments for convenience.
  */
@@ -622,6 +624,13 @@ analyze_clip_cull_usage(struct gl_shader_program *prog,
                         struct gl_context *ctx,
                         struct shader_info *info)
 {
+   if (ctx->Const.DoDCEBeforeClipCullAnalysis) {
+      /* Remove dead functions to avoid raising an error (eg: dead function
+       * writes to gl_ClipVertex, and main() writes to gl_ClipDistance).
+       */
+      do_dead_functions(shader->ir);
+   }
+
    info->clip_distance_array_size = 0;
    info->cull_distance_array_size = 0;
 

src/gallium/auxiliary/pipe-loader/driinfo_gallium.h

@@ -26,6 +26,7 @@ DRI_CONF_SECTION_DEBUG
    DRI_CONF_ALLOW_GLSL_RELAXED_ES(false)
    DRI_CONF_ALLOW_GLSL_BUILTIN_VARIABLE_REDECLARATION(false)
    DRI_CONF_ALLOW_GLSL_CROSS_STAGE_INTERPOLATION_MISMATCH(false)
+   DRI_CONF_DO_DCE_BEFORE_CLIP_CULL_ANALYSIS(false)
    DRI_CONF_ALLOW_HIGHER_COMPAT_VERSION(false)
    DRI_CONF_FORCE_GLSL_ABS_SQRT(false)
    DRI_CONF_GLSL_CORRECT_DERIVATIVES_AFTER_DISCARD(false)

src/gallium/auxiliary/util/u_driconf.c

@@ -57,6 +57,7 @@ u_driconf_fill_st_options(struct st_config_options *options,
    query_bool_option(vs_position_always_precise);
    query_bool_option(force_glsl_abs_sqrt);
    query_bool_option(allow_glsl_cross_stage_interpolation_mismatch);
+   query_bool_option(do_dce_before_clip_cull_analysis);
    query_bool_option(allow_draw_out_of_order);
    query_bool_option(glthread_nop_check_framebuffer_status);
    query_bool_option(ignore_map_unsynchronized);

src/gallium/include/frontend/api.h

@@ -233,6 +233,7 @@ struct st_config_options
    bool vs_position_always_precise;
    bool force_glsl_abs_sqrt;
    bool allow_glsl_cross_stage_interpolation_mismatch;
+   bool do_dce_before_clip_cull_analysis;
    bool allow_draw_out_of_order;
    bool glthread_nop_check_framebuffer_status;
    bool ignore_map_unsynchronized;

src/mesa/main/mtypes.h

@@ -3955,6 +3955,15 @@ struct gl_constants
     */
    GLboolean AllowExtraPPTokens;
 
+   /**
+    * The spec forbids a shader to "statically write both gl_ClipVertex
+    * and gl_ClipDistance".
+    * This option adds a tolerance for shader that statically writes to
+    * both but at least one of the write can be removed by a dead code
+    * elimination pass.
+    */
+   GLboolean DoDCEBeforeClipCullAnalysis;
+
    /**
     * Force computing the absolute value for sqrt() and inversesqrt() to follow
     * D3D9 when apps rely on this behaviour.

src/mesa/state_tracker/st_extensions.c

@@ -1201,6 +1201,8 @@ void st_init_extensions(struct pipe_screen *screen,
 
    consts->AllowGLSLCrossStageInterpolationMismatch = options->allow_glsl_cross_stage_interpolation_mismatch;
 
+   consts->DoDCEBeforeClipCullAnalysis = options->do_dce_before_clip_cull_analysis;
+
    consts->GLSLIgnoreWriteToReadonlyVar = options->glsl_ignore_write_to_readonly_var;
 
    consts->PrimitiveRestartFixedIndex =

src/util/driconf.h

@@ -212,6 +212,10 @@
    DRI_CONF_OPT_B(allow_glsl_cross_stage_interpolation_mismatch, def,   \
                   "Allow interpolation qualifier mismatch across shader stages")
 
+#define DRI_CONF_DO_DCE_BEFORE_CLIP_CULL_ANALYSIS(def) \
+   DRI_CONF_OPT_B(do_dce_before_clip_cull_analysis, def,   \
+                  "Use dead code elimitation before checking for invalid Clip*/CullDistance variables usage.")
+
 #define DRI_CONF_ALLOW_DRAW_OUT_OF_ORDER(def) \
    DRI_CONF_OPT_B(allow_draw_out_of_order, def, \
                   "Allow out-of-order draw optimizations. Set when Z fighting doesn't have to be accurate.")