i965/fs: Port texture projection avoidance optimization from the old backend.

This is part of fixing a ~1% performance regression in OpenArena when
changing the fixed function fragment shader to using the new backend.
Right now this just avoids the LINTERP of the projector, not the math
using it.

1 files changed, 15 insertions, 3 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index 15475fb..9c3180f 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -463,9 +463,21 @@ fs_visitor::emit_general_interpolation(ir_variable *ir)
 	 } else {
 	    /* Perspective interpolation case. */
 	    for (unsigned int k = 0; k < type->vector_elements; k++) {
-	       struct brw_reg interp = interp_reg(location, k);
-	       emit(FS_OPCODE_LINTERP, attr,
-		    this->delta_x, this->delta_y, fs_reg(interp));
+	       /* FINISHME: At some point we probably want to push
+		* this farther by giving similar treatment to the
+		* other potentially constant components of the
+		* attribute, as well as making brw_vs_constval.c
+		* handle varyings other than gl_TexCoord.
+		*/
+	       if (location >= FRAG_ATTRIB_TEX0 &&
+		   location <= FRAG_ATTRIB_TEX7 &&
+		   k == 3 && !(c->key.proj_attrib_mask & (1 << location))) {
+		  emit(BRW_OPCODE_MOV, attr, fs_reg(1.0f));
+	       } else {
+		  struct brw_reg interp = interp_reg(location, k);
+		  emit(FS_OPCODE_LINTERP, attr,
+		       this->delta_x, this->delta_y, fs_reg(interp));
+	       }
 	       attr.reg_offset++;
 	    }