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, 33 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_wm_glsl.c b/src/mesa/drivers/dri/i965/brw_wm_glsl.c
index 4cf092226c..b3c15fe87f 100644
--- a/src/mesa/drivers/dri/i965/brw_wm_glsl.c
+++ b/src/mesa/drivers/dri/i965/brw_wm_glsl.c
@@ -653,6 +653,36 @@ static void emit_pinterp(struct brw_wm_compile *c,
     }
 }
 
+/* Sets the destination channels to 1.0 or 0.0 according to glFrontFacing. */
+static void emit_frontfacing(struct brw_wm_compile *c,
+			     struct prog_instruction *inst)
+{
+    struct brw_compile *p = &c->func;
+    struct brw_reg r1_6ud = retype(brw_vec1_grf(1, 6), BRW_REGISTER_TYPE_UD);
+    struct brw_reg dst;
+    GLuint mask = inst->DstReg.WriteMask;
+    int i;
+
+    for (i = 0; i < 4; i++) {
+	if (mask & (1<<i)) {
+	    dst = get_dst_reg(c, inst, i, 1);
+	    brw_MOV(p, dst, 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)) {
+	    dst = get_dst_reg(c, inst, i, 1);
+	    brw_MOV(p, dst, brw_imm_f(1.0));
+	}
+    }
+    brw_set_predicate_control_flag_value(p, 0xff);
+}
+
 static void emit_xpd(struct brw_wm_compile *c,
 		struct prog_instruction *inst)
 {
@@ -2435,6 +2465,9 @@ static void brw_wm_emit_glsl(struct brw_context *brw, struct brw_wm_compile *c)
 	    case WM_FB_WRITE:
 		emit_fb_write(c, inst);
 		break;
+	    case WM_FRONTFACING:
+		emit_frontfacing(c, inst);
+		break;
 	    case OPCODE_ABS:
 		emit_abs(c, inst);
 		break;