i965/fs: Patch the instruction generating discards; don't use CMP.Z.

CMP.Z doesn't work on Gen4-5 because the boolean isn't guaranteed to be
0 or 0xFFFFFFFF - only the low bit is defined.

We can call emit_bool_to_cond_code to generate the condition in f0.0;
the last instruction will generate the flag value.  We can patch it to
use f0.1, and negate the condition.

Fixes discard tests on Gen4-5.

Haswell shader-db stats:
total instructions in shared programs: 5770279 -> 5769112 (-0.02%)
instructions in affected programs:     64342 -> 63175 (-1.81%)
helped:                                1069

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
index 13a3bf2..7a697e8 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
@@ -2422,8 +2422,9 @@ fs_visitor::visit(ir_discard *ir)
     */
    fs_inst *cmp;
    if (ir->condition) {
-      ir->condition->accept(this);
-      cmp = emit(CMP(reg_null_f, this->result, fs_reg(0), BRW_CONDITIONAL_Z));
+      emit_bool_to_cond_code(ir->condition);
+      cmp = (fs_inst *) this->instructions.get_tail();
+      cmp->conditional_mod = brw_negate_cmod(cmp->conditional_mod);
    } else {
       fs_reg some_reg = fs_reg(retype(brw_vec8_grf(0, 0),
                                       BRW_REGISTER_TYPE_UW));