ir_to_mesa: use RSQ+MUL instead of RSQ+RCP for SQRT

sqrt(x) = 1/rsq(x) = x*rsq(x)

This optimization already was in the old GLSL compiler.

Acked on irc by Eric Anholt.

diff --git a/src/mesa/program/ir_to_mesa.cpp b/src/mesa/program/ir_to_mesa.cpp
index af6d7345a54a4d142b35053d2ebdac68f9a952de..516c991855d8078c06d95b5e721b596823fd43d9 100644 (file)
--- a/src/mesa/program/ir_to_mesa.cpp
+++ b/src/mesa/program/ir_to_mesa.cpp
@@ -880,8 +880,9 @@ ir_to_mesa_visitor::visit(ir_expression *ir)
       break;
 
    case ir_unop_sqrt:
+      /* sqrt(x) = x * rsq(x). */
       ir_to_mesa_emit_scalar_op1(ir, OPCODE_RSQ, result_dst, op[0]);
-      ir_to_mesa_emit_scalar_op1(ir, OPCODE_RCP, result_dst, result_src);
+      ir_to_mesa_emit_op2(ir, OPCODE_MUL, result_dst, result_src, op[0]);
       /* For incoming channels < 0, set the result to 0. */
       ir_to_mesa_emit_op3(ir, OPCODE_CMP, result_dst,
                          op[0], src_reg_for_float(0.0), result_src);