gallium: add PIPE_CAP_PREFER_POT_ALIGNED_VARYINGS

Driver should enable this cap if it prefers varyings to be aligned
to power of two in a slot, i.e. vec4 in .xyzw, vec3 in .xyz, vec2 in .xy
or .zw

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13151>

docs/gallium/screen.rst

@@ -629,6 +629,7 @@ The integer capabilities:
 * ``PIPE_CAP_SUPPORTED_PRIM_MODES_WITH_RESTART``: A bitmask of the ``pipe_prim_type`` enum values that the driver can natively support for primitive restart. Only useful if ``PIPE_CAP_PRIMITIVE_RESTART`` is also exported.
 * ``PIPE_CAP_PREFER_BACK_BUFFER_REUSE``: Only applies to DRI_PRIME. If 1, the driver prefers that DRI3 tries to use the same back buffer each frame. If 0, this means DRI3 will at least use 2 back buffers and ping-pong between them to allow the tiled->linear copy to run in parallel.
 * ``PIPE_CAP_DRAW_VERTEX_STATE``: Driver supports `pipe_screen::create_vertex_state/vertex_state_destroy` and `pipe_context::draw_vertex_state`. Only used by display lists and designed to serve vbo_save.
+* ``PIPE_CAP_PREFER_POT_ALIGNED_VARYINGS``: Driver prefers varyings to be aligned to power of two in a slot. If this cap is enabled, vec4 varying will be placed in .xyzw components of the varying slot, vec3 in .xyz and vec2 in .xy or .zw
 
 .. _pipe_capf:
 

src/compiler/glsl/link_varyings.cpp

@@ -1719,6 +1719,7 @@ public:
                    bool disable_xfb_packing,
                    bool xfb_enabled,
                    bool enhanced_layouts_enabled,
+                   bool prefer_pot_aligned_varyings,
                    gl_shader_stage producer_stage,
                    gl_shader_stage consumer_stage);
    ~varying_matches();
@@ -1760,6 +1761,12 @@ private:
 
    const bool enhanced_layouts_enabled;
 
+   /**
+    * If true, this driver prefers varyings to be aligned to power of two
+    * in a slot.
+    */
+   const bool prefer_pot_aligned_varyings;
+
    /**
     * Enum representing the order in which varyings are packed within a
     * packing class.
@@ -1838,12 +1845,14 @@ varying_matches::varying_matches(bool disable_varying_packing,
                                  bool disable_xfb_packing,
                                  bool xfb_enabled,
                                  bool enhanced_layouts_enabled,
+                                 bool prefer_pot_aligned_varyings,
                                  gl_shader_stage producer_stage,
                                  gl_shader_stage consumer_stage)
    : disable_varying_packing(disable_varying_packing),
      disable_xfb_packing(disable_xfb_packing),
      xfb_enabled(xfb_enabled),
      enhanced_layouts_enabled(enhanced_layouts_enabled),
+     prefer_pot_aligned_varyings(prefer_pot_aligned_varyings),
      producer_stage(producer_stage),
      consumer_stage(consumer_stage)
 {
@@ -2046,10 +2055,15 @@ varying_matches::assign_locations(struct gl_shader_program *prog,
     * packing vec3 attributes can cause trouble because splitting a vec3
     * effectively creates an additional transform feedback output.  The
     * extra TFB output may exceed device driver limits.
+    *
+    * Also don't pack vec3 if the driver prefers power of two aligned
+    * varyings. Packing order guarantees that vec4, vec2 and vec1 will be
+    * pot-aligned, we only need to take care of vec3s
     */
    const bool dont_pack_vec3 =
       (prog->TransformFeedback.BufferMode == GL_SEPARATE_ATTRIBS &&
-       prog->TransformFeedback.NumVarying > 0);
+       prog->TransformFeedback.NumVarying > 0) ||
+      this->prefer_pot_aligned_varyings;
 
    for (unsigned i = 0; i < this->num_matches; i++) {
       unsigned *location = &generic_location;
@@ -2794,10 +2808,14 @@ assign_varying_locations(struct gl_context *ctx,
    if (prog->SeparateShader && (producer == NULL || consumer == NULL))
       disable_varying_packing = true;
 
+   bool prefer_pot_aligned_varyings =
+      ctx->Const.PreferPOTAlignedVaryings;
+
    varying_matches matches(disable_varying_packing,
                            disable_xfb_packing,
                            xfb_enabled,
                            ctx->Extensions.ARB_enhanced_layouts,
+                           prefer_pot_aligned_varyings,
                            producer ? producer->Stage : MESA_SHADER_NONE,
                            consumer ? consumer->Stage : MESA_SHADER_NONE);
    void *hash_table_ctx = ralloc_context(NULL);

src/gallium/auxiliary/util/u_screen.c

@@ -476,6 +476,9 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
    case PIPE_CAP_DRAW_VERTEX_STATE:
       return 0;
 
+   case PIPE_CAP_PREFER_POT_ALIGNED_VARYINGS:
+      return 0;
+
    default:
       unreachable("bad PIPE_CAP_*");
    }

src/gallium/include/pipe/p_defines.h

@@ -998,6 +998,7 @@ enum pipe_cap
    PIPE_CAP_SUPPORTED_PRIM_MODES_WITH_RESTART,
    PIPE_CAP_PREFER_BACK_BUFFER_REUSE,
    PIPE_CAP_DRAW_VERTEX_STATE,
+   PIPE_CAP_PREFER_POT_ALIGNED_VARYINGS,
 
    PIPE_CAP_LAST,
    /* XXX do not add caps after PIPE_CAP_LAST! */

src/mesa/main/mtypes.h

@@ -4155,6 +4155,14 @@ struct gl_constants
     */
    GLboolean DisableTransformFeedbackPacking;
 
+   /**
+    * Align varyings to POT in a slot
+    *
+    * Drivers that prefer varyings to be aligned to POT must set this value to GL_TRUE
+    */
+   GLboolean PreferPOTAlignedVaryings;
+
+
    /**
     * UBOs and SSBOs can be packed tightly by the OpenGL implementation when
     * layout is set as shared (the default) or packed. However most Mesa drivers

src/mesa/state_tracker/st_extensions.c

@@ -1550,6 +1550,9 @@ void st_init_extensions(struct pipe_screen *screen,
    if (!screen->get_param(screen, PIPE_CAP_PACKED_STREAM_OUTPUT))
       consts->DisableTransformFeedbackPacking = GL_TRUE;
 
+   if (screen->get_param(screen, PIPE_CAP_PREFER_POT_ALIGNED_VARYINGS))
+      consts->PreferPOTAlignedVaryings = GL_TRUE;
+
    unsigned max_fb_fetch_rts = screen->get_param(screen, PIPE_CAP_FBFETCH);
    bool coherent_fb_fetch =
       screen->get_param(screen, PIPE_CAP_FBFETCH_COHERENT);