llvmpipe: abstract the code to set number of subpixel bits

As we're moving towards expanding the number of subpixel
bits and the width of the variables used in the computations
we need to make this code a bit more centralized.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/drivers/llvmpipe/lp_rast.h

@@ -46,9 +46,18 @@ struct lp_scene;
 struct lp_fence;
 struct cmd_bin;
 
+#define FIXED_TYPE_WIDTH 32
 /** For sub-pixel positioning */
 #define FIXED_ORDER 4
 #define FIXED_ONE (1<<FIXED_ORDER)
+/** Maximum length of an edge in a primitive in pixels.
+ *  If the framebuffer is large we have to think about fixed-point
+ *  integer overflow. Coordinates need ((FIXED_TYPE_WIDTH/2) - 1) bits
+ *  to be able to fit product of two such coordinates inside
+ *  FIXED_TYPE_WIDTH, any larger and we could overflow a
+ *  FIXED_TYPE_WIDTH_-bit int.
+ */
+#define MAX_FIXED_LENGTH (1 << (((FIXED_TYPE_WIDTH/2) - 1) - FIXED_ORDER))
 
 /* Rasterizer output size going to jit fs, width/height */
 #define LP_RASTER_BLOCK_SIZE 4

src/gallium/drivers/llvmpipe/lp_setup.c

@@ -1007,16 +1007,12 @@ try_update_scene_state( struct lp_setup_context *setup )
                                          &setup->draw_regions[i]);
          }
       }
-      /* If the framebuffer is large we have to think about fixed-point
-       * integer overflow.  For 2K by 2K images, coordinates need 15 bits
-       * (2^11 + 4 subpixel bits).  The product of two such numbers would
-       * use 30 bits.  Any larger and we could overflow a 32-bit int.
-       *
-       * To cope with this problem we check if triangles are large and
-       * subdivide them if needed.
+      /*
+       * Subdivide triangles if the framebuffer is larger than the
+       * MAX_FIXED_LENGTH.
        */
-      setup->subdivide_large_triangles = (setup->fb.width > 2048 ||
-                                          setup->fb.height > 2048);
+      setup->subdivide_large_triangles = (setup->fb.width > MAX_FIXED_LENGTH ||
+                                          setup->fb.height > MAX_FIXED_LENGTH);
    }
                                       
    setup->dirty = 0;

src/gallium/drivers/llvmpipe/lp_setup_tri.c

@@ -988,7 +988,7 @@ check_subdivide_triangle(struct lp_setup_context *setup,
                          const float (*v2)[4],
                          triangle_func_t tri)
 {
-   const float maxLen = 2048.0f;  /* longest permissible edge, in pixels */
+   const float maxLen = (float) MAX_FIXED_LENGTH;  /* longest permissible edge, in pixels */
    float dx10, dy10, len10;
    float dx21, dy21, len21;
    float dx02, dy02, len02;