gallium: add a new cap PIPE_CAP_GLSL_ZERO_INIT

Allows driver to select a zero init mode between the 3 possible values.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4607>

src/gallium/auxiliary/util/u_screen.c

@@ -345,6 +345,7 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
    case PIPE_CAP_SHADER_SAMPLES_IDENTICAL:
    case PIPE_CAP_TGSI_ATOMINC_WRAP:
    case PIPE_CAP_TGSI_TG4_COMPONENT_IN_SWIZZLE:
+   case PIPE_CAP_GLSL_ZERO_INIT:
       return 0;
 
    case PIPE_CAP_MAX_GS_INVOCATIONS:

src/gallium/docs/source/screen.rst

@@ -579,6 +579,7 @@ The integer capabilities:
 * ``PIPE_CAP_SYSTEM_SVM``: True if all application memory can be shared with the GPU without explicit mapping.
 * ``PIPE_CAP_VIEWPORT_MASK``: Whether ``TGSI_SEMANTIC_VIEWPORT_MASK`` and ``TGSI_PROPERTY_LAYER_VIEWPORT_RELATIVE`` are supported (see GL_NV_viewport_array2).
 * ``PIPE_CAP_MAP_UNSYNCHRONIZED_THREAD_SAFE``: Whether mapping a buffer as unsynchronized from any thread is safe.
+* ``PIPE_CAP_GLSL_ZERO_INIT``: Choose a default zero initialization some glsl variables. If `1`, then all glsl shader variables and gl_FragColor are initialized to zero. If `2`, then shader out variables are not initialized but function out variables are.
 
 .. _pipe_capf:
 

src/gallium/drivers/radeonsi/si_get.c

@@ -164,6 +164,9 @@ static int si_get_param(struct pipe_screen *pscreen, enum pipe_cap param)
    case PIPE_CAP_MAP_UNSYNCHRONIZED_THREAD_SAFE:
       return 1;
 
+   case PIPE_CAP_GLSL_ZERO_INIT:
+      return 2;
+
    case PIPE_CAP_QUERY_SO_OVERFLOW:
       return !sscreen->use_ngg_streamout;
 

src/gallium/include/pipe/p_defines.h

@@ -947,6 +947,7 @@ enum pipe_cap
    PIPE_CAP_VIEWPORT_MASK,
    PIPE_CAP_ALPHA_TO_COVERAGE_DITHER_CONTROL,
    PIPE_CAP_MAP_UNSYNCHRONIZED_THREAD_SAFE,
+   PIPE_CAP_GLSL_ZERO_INIT,
 };
 
 /**

src/mesa/main/mtypes.h

@@ -3868,9 +3868,13 @@ struct gl_constants
    GLboolean ForceGLSLAbsSqrt;
 
    /**
-    * Types of variable to default initialized to zero.
+    * Types of variable to default initialized to zero. Supported values are:
+    *   - 0: no zero initialization
+    *   - 1: all shader variables and gl_FragColor are initialiazed to 0
+    *   - 2: same as 1, but shader out variables are *not* initialized, while
+    *        function out variables are now initialized.
     */
-   GLuint GLSLZeroInit;
+   GLchar GLSLZeroInit;
 
    /**
     * Treat integer textures using GL_LINEAR filters as GL_NEAREST.

src/mesa/state_tracker/st_extensions.c

@@ -1205,6 +1205,8 @@ void st_init_extensions(struct pipe_screen *screen,
 
    if (options->glsl_zero_init) {
       consts->GLSLZeroInit = 1;
+   } else {
+      consts->GLSLZeroInit = screen->get_param(screen, PIPE_CAP_GLSL_ZERO_INIT);
    }
 
    consts->ForceIntegerTexNearest = options->force_integer_tex_nearest;