st/mesa: fix quad strip trimming bug

The translate_prim() function tries to convert quad strips into
tri strips.  This is normally OK but we have to check for an odd
number of vertices so that we don't accidentally draw an extra
triangle.  The mesa-demos/src/samples/prim.c demo exercises that.
With this fix the stray yellow triangle is no longer drawn.

Use the u_trim_pipe_prim() function to make sure that prims have
the right number of vertices and avoid calling gallium drawing
functions when the prim has a degenerate number of vertices.

Plus add comments, clean-up formatting, etc.

NOTE: This is a candidate for the 7.8 branch.

1 files changed, 57 insertions, 27 deletions

diff --git a/src/mesa/state_tracker/st_draw.c b/src/mesa/state_tracker/st_draw.c
index eed8e2aa5d..7fdaff45f7 100644
--- a/src/mesa/state_tracker/st_draw.c
+++ b/src/mesa/state_tracker/st_draw.c
@@ -57,6 +57,7 @@
 #include "pipe/p_defines.h"
 #include "util/u_inlines.h"
 #include "util/u_format.h"
+#include "util/u_prim.h"
 #include "draw/draw_context.h"
 #include "cso_cache/cso_context.h"
 
@@ -517,10 +518,21 @@ check_uniforms(GLcontext *ctx)
 }
 
 
-static unsigned translate_prim( GLcontext *ctx,
-                                unsigned prim )
+/**
+ * Translate OpenGL primtive type (GL_POINTS, GL_TRIANGLE_STRIP, etc) to
+ * the corresponding Gallium type.
+ */
+static unsigned
+translate_prim(const GLcontext *ctx, unsigned prim)
 {
+   /* GL prims should match Gallium prims, spot-check a few */
+   assert(GL_POINTS == PIPE_PRIM_POINTS);
+   assert(GL_QUADS == PIPE_PRIM_QUADS);
+   assert(GL_TRIANGLE_STRIP_ADJACENCY == PIPE_PRIM_TRIANGLE_STRIP_ADJACENCY);
+
    /* Avoid quadstrips if it's easy to do so:
+    * Note: it's imporant to do the correct trimming if we change the prim type!
+    * We do that wherever this function is called.
     */
    if (prim == GL_QUAD_STRIP &&
        ctx->Light.ShadeModel != GL_FLAT &&
@@ -531,6 +543,8 @@ static unsigned translate_prim( GLcontext *ctx,
    return prim;
 }
 
+
+
 /**
  * This function gets plugged into the VBO module and is called when
  * we have something to render.
@@ -639,7 +653,6 @@ st_draw_vbo(GLcontext *ctx,
       struct gl_buffer_object *bufobj = ib->obj;
       struct pipe_resource *indexBuf = NULL;
       unsigned indexSize, indexOffset, i;
-      unsigned prim;
 
       switch (ib->type) {
       case GL_UNSIGNED_INT:
@@ -678,27 +691,40 @@ st_draw_vbo(GLcontext *ctx,
           * need a bit of work...
           */
          for (i = 0; i < nr_prims; i++) {
-            prim = translate_prim( ctx, prims[i].mode );
+            unsigned vcount = prims[i].count;
+            unsigned prim = translate_prim(ctx, prims[i].mode);
 
-            pipe->draw_range_elements(pipe, indexBuf, indexSize, 0,
-                                      min_index, max_index, prim,
-                                      prims[i].start + indexOffset, prims[i].count);
+            if (u_trim_pipe_prim(prims[i].mode, &vcount)) {
+               pipe->draw_range_elements(pipe, indexBuf, indexSize, 0,
+                                         min_index, max_index, prim,
+                                         prims[i].start + indexOffset, vcount);
+            }
          }
       }
       else {
          for (i = 0; i < nr_prims; i++) {
-            prim = translate_prim( ctx, prims[i].mode );
+            unsigned vcount = prims[i].count;
+            unsigned prim = translate_prim(ctx, prims[i].mode);
             
-            if (prims[i].num_instances == 1) {
-               pipe->draw_elements(pipe, indexBuf, indexSize, 0, prim,
-                                   prims[i].start + indexOffset,
-                                   prims[i].count);
-            }
-            else {
-               pipe->draw_elements_instanced(pipe, indexBuf, indexSize, 0, prim,
-                                             prims[i].start + indexOffset,
-                                             prims[i].count,
-                                             0, prims[i].num_instances);
+            if (u_trim_pipe_prim(prims[i].mode, &vcount)) {
+               if (prims[i].num_instances == 1) {
+                  pipe->draw_elements(pipe, indexBuf,
+                                      indexSize,
+                                      0, /* indexBias */
+                                      prim,
+                                      prims[i].start + indexOffset,
+                                      vcount);
+               }
+               else {
+                  pipe->draw_elements_instanced(pipe, indexBuf,
+                                                indexSize,
+                                                0, /* indexBias */
+                                                prim,
+                                                prims[i].start + indexOffset,
+                                                vcount,
+                                                0, /* startInstance */
+                                                prims[i].num_instances);
+               }
             }
          }
       }
@@ -708,18 +734,22 @@ st_draw_vbo(GLcontext *ctx,
    else {
       /* non-indexed */
       GLuint i;
-      GLuint prim;
 
       for (i = 0; i < nr_prims; i++) {
-         prim = translate_prim( ctx, prims[i].mode );
+         unsigned vcount = prims[i].count;
+         unsigned prim = translate_prim(ctx, prims[i].mode);
 
-         if (prims[i].num_instances == 1) {
-            pipe->draw_arrays(pipe, prim, prims[i].start, prims[i].count);
-         }
-         else {
-            pipe->draw_arrays_instanced(pipe, prim, prims[i].start,
-                                        prims[i].count,
-                                        0, prims[i].num_instances);
+         if (u_trim_pipe_prim(prims[i].mode, &vcount)) {
+            if (prims[i].num_instances == 1) {
+               pipe->draw_arrays(pipe, prim, prims[i].start, vcount);
+            }
+            else {
+               pipe->draw_arrays_instanced(pipe, prim,
+                                           prims[i].start,
+                                           vcount,
+                                           0, /* startInstance */
+                                           prims[i].num_instances);
+            }
          }
       }
    }