glsl: Convert mix() to use a new ir_triop_lrp opcode.

Many GPUs have an instruction to do linear interpolation which is more
efficient than simply performing the algebra necessary (two multiplies,
an add, and a subtract).

Pattern matching or peepholing this is more desirable, but can be
tricky.  By using an opcode, we can at least make shaders which use the
mix() built-in get the more efficient behavior.

Currently, all consumers lower ir_triop_lrp.  Subsequent patches will
actually generate different code.

v2 [mattst88]:
   - Add LRP_TO_ARITH flag to ir_to_mesa.cpp. Will be removed in a
     subsequent patch and ir_triop_lrp translated directly.
v3 [mattst88]:
   - Move changes from the next patch to opt_algebraic.cpp to accept
     3-src operations.

Reviewed-by: Matt Turner <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>
Signed-off-by: Kenneth Graunke <[email protected]>

1 files changed, 2 insertions, 1 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_shader.cpp b/src/mesa/drivers/dri/i965/brw_shader.cpp
index c71715ee512..9ab18cc7d63 100644
--- a/src/mesa/drivers/dri/i965/brw_shader.cpp
+++ b/src/mesa/drivers/dri/i965/brw_shader.cpp
@@ -155,7 +155,8 @@ brw_link_shader(struct gl_context *ctx, struct gl_shader_program *shProg)
 			 DIV_TO_MUL_RCP |
 			 SUB_TO_ADD_NEG |
 			 EXP_TO_EXP2 |
-			 LOG_TO_LOG2);
+			 LOG_TO_LOG2 |
+			 LRP_TO_ARITH);
 
       /* Pre-gen6 HW can only nest if-statements 16 deep.  Beyond this,
        * if-statements need to be flattened.