i965: Fix glFrontFacing in twoside GLSL demo.

This also cuts instructions by just using the existing bit in the payload
rather than computing it from the determinant in the SF unit and passing it
as a varying down to the WM.  Something still goes wrong with getting the
backface color right, but a simpler shader appears to get the right result.

1 files changed, 32 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_wm_emit.c b/src/mesa/drivers/dri/i965/brw_wm_emit.c
index f2dca9caa6..4372ed3d9a 100644
--- a/src/mesa/drivers/dri/i965/brw_wm_emit.c
+++ b/src/mesa/drivers/dri/i965/brw_wm_emit.c
@@ -254,6 +254,34 @@ static void emit_cinterp( struct brw_compile *p,
    }
 }
 
+/* Sets the destination channels to 1.0 or 0.0 according to glFrontFacing. */
+static void emit_frontfacing( struct brw_compile *p,
+			      const struct brw_reg *dst,
+			      GLuint mask )
+{
+   struct brw_reg r1_6ud = retype(brw_vec1_grf(1, 6), BRW_REGISTER_TYPE_UD);
+   GLuint i;
+
+   if (!(mask & WRITEMASK_XYZW))
+      return;
+
+   for (i = 0; i < 4; i++) {
+      if (mask & (1<<i)) {
+	 brw_MOV(p, dst[i], brw_imm_f(0.0));
+      }
+   }
+
+   /* bit 31 is "primitive is back face", so checking < (1 << 31) gives
+    * us front face
+    */
+   brw_CMP(p, brw_null_reg(), BRW_CONDITIONAL_L, r1_6ud, brw_imm_ud(1 << 31));
+   for (i = 0; i < 4; i++) {
+      if (mask & (1<<i)) {
+	 brw_MOV(p, dst[i], brw_imm_f(1.0));
+      }
+   }
+   brw_set_predicate_control_flag_value(p, 0xff);
+}
 
 static void emit_alu1( struct brw_compile *p, 
 		       struct brw_instruction *(*func)(struct brw_compile *, 
@@ -1160,6 +1188,10 @@ void brw_wm_emit( struct brw_wm_compile *c )
 	 emit_fb_write(c, args[0], args[1], args[2], inst->target, inst->eot);
 	 break;
 
+      case WM_FRONTFACING:
+	 emit_frontfacing(p, dst, dst_flags);
+	 break;
+
 	 /* Straightforward arithmetic:
 	  */
       case OPCODE_ADD: