i965/msaa: Fix centroid interpolation of unlit pixels.

From the Ivy Bridge PRM, Vol 2 Part 1 p280-281 (3DSTATE_WM:
Barycentric Interpolation Mode):

    "Errata: When Centroid Barycentric mode is required, HW may
    produce incorrect interpolation results when a 2X2 pixels have
    unlit pixels."

To work around this problem, after doing centroid interpolation, we
replace the centroid-interpolated values for unlit pixels with
non-centroid-interpolated values (which are interpolated at pixel
centers).  This produces correct rendering at the expense of a slight
increase in shader execution time.

I've conditioned the workaround with a runtime flag
(brw->needs_unlit_centroid_workaround) in the hopes that we won't need
it in future chip generations.

Fixes piglit tests "EXT_framebuffer_multisample/interpolation {2,4}
{centroid-deriv,centroid-deriv-disabled}".  All MSAA interpolation
tests pass now.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/dri/i965/brw_context.c

@@ -296,6 +296,10 @@ brwCreateContext(int api,
       brw->has_negative_rhw_bug = true;
    }
 
+   if (intel->gen <= 7) {
+      brw->needs_unlit_centroid_workaround = true;
+   }
+
    brw->prim_restart.in_progress = false;
    brw->prim_restart.enable_cut_index = false;
 

src/mesa/drivers/dri/i965/brw_context.h

@@ -725,6 +725,15 @@ struct brw_context
    bool has_pln;
    bool precompile;
 
+   /**
+    * Some versions of Gen hardware don't do centroid interpolation correctly
+    * on unlit pixels, causing incorrect values for derivatives near triangle
+    * edges.  Enabling this flag causes the fragment shader to use
+    * non-centroid interpolation for unlit pixels, at the expense of two extra
+    * fragment shader instructions.
+    */
+   bool needs_unlit_centroid_workaround;
+
    struct {
       struct brw_state_flags dirty;
    } state;

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -506,6 +506,18 @@ fs_visitor::emit_general_interpolation(ir_variable *ir)
 		  struct brw_reg interp = interp_reg(location, k);
                   emit_linterp(attr, fs_reg(interp), interpolation_mode,
                                ir->centroid);
+                  if (brw->needs_unlit_centroid_workaround && ir->centroid) {
+                     /* Get the pixel/sample mask into f0 so that we know
+                      * which pixels are lit.  Then, for each channel that is
+                      * unlit, replace the centroid data with non-centroid
+                      * data.
+                      */
+                     emit(FS_OPCODE_MOV_DISPATCH_TO_FLAGS, attr);
+                     fs_inst *inst = emit_linterp(attr, fs_reg(interp),
+                                                  interpolation_mode, false);
+                     inst->predicated = true;
+                     inst->predicate_inverse = true;
+                  }
 		  if (intel->gen < 6) {
 		     emit(BRW_OPCODE_MUL, attr, attr, this->pixel_w);
 		  }

src/mesa/drivers/dri/i965/brw_wm.c

@@ -130,7 +130,8 @@ brw_wm_non_glsl_emit(struct brw_context *brw, struct brw_wm_compile *c)
  * (see enum brw_wm_barycentric_interp_mode) is needed by the fragment shader.
  */
 static unsigned
-brw_compute_barycentric_interp_modes(bool shade_model_flat,
+brw_compute_barycentric_interp_modes(struct brw_context *brw,
+                                     bool shade_model_flat,
                                      const struct gl_fragment_program *fprog)
 {
    unsigned barycentric_interp_modes = 0;
@@ -154,11 +155,18 @@ brw_compute_barycentric_interp_modes(bool shade_model_flat,
       if (attr == FRAG_ATTRIB_WPOS || attr == FRAG_ATTRIB_FACE)
          continue;
 
+      /* Determine the set (or sets) of barycentric coordinates needed to
+       * interpolate this variable.  Note that when
+       * brw->needs_unlit_centroid_workaround is set, centroid interpolation
+       * uses PIXEL interpolation for unlit pixels and CENTROID interpolation
+       * for lit pixels, so we need both sets of barycentric coordinates.
+       */
       if (interp_qualifier == INTERP_QUALIFIER_NOPERSPECTIVE) {
          if (is_centroid) {
             barycentric_interp_modes |=
                1 << BRW_WM_NONPERSPECTIVE_CENTROID_BARYCENTRIC;
-         } else {
+         }
+         if (!is_centroid || brw->needs_unlit_centroid_workaround) {
             barycentric_interp_modes |=
                1 << BRW_WM_NONPERSPECTIVE_PIXEL_BARYCENTRIC;
          }
@@ -168,7 +176,8 @@ brw_compute_barycentric_interp_modes(bool shade_model_flat,
          if (is_centroid) {
             barycentric_interp_modes |=
                1 << BRW_WM_PERSPECTIVE_CENTROID_BARYCENTRIC;
-         } else {
+         }
+         if (!is_centroid || brw->needs_unlit_centroid_workaround) {
             barycentric_interp_modes |=
                1 << BRW_WM_PERSPECTIVE_PIXEL_BARYCENTRIC;
          }
@@ -289,7 +298,8 @@ bool do_wm_prog(struct brw_context *brw,
    brw_init_compile(brw, &c->func, c);
 
    c->prog_data.barycentric_interp_modes =
-      brw_compute_barycentric_interp_modes(c->key.flat_shade, &fp->program);
+      brw_compute_barycentric_interp_modes(brw, c->key.flat_shade,
+                                           &fp->program);
 
    if (prog && prog->_LinkedShaders[MESA_SHADER_FRAGMENT]) {
       if (!brw_wm_fs_emit(brw, c, prog))