i965/fs: Handle conditional discards.

The discard condition tells us which channels we want killed.  We want
to invert that condition to get the channels that should survive (remain
live) in f0.1.  Emit a CMP to negate it.

Nothing generates these today, but that will change shortly.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/dri/i965/brw_fs_nir.cpp

    bool has_indirect = false;
 
    switch (instr->intrinsic) {
-   case nir_intrinsic_discard: {
+   case nir_intrinsic_discard:
+   case nir_intrinsic_discard_if: {
       /* We track our discarded pixels in f0.1.  By predicating on it, we can
-       * update just the flag bits that aren't yet discarded.  By emitting a
-       * CMP of g0 != g0, all our currently executing channels will get turned
-       * off.
+       * update just the flag bits that aren't yet discarded.  If there's no
+       * condition, we emit a CMP of g0 != g0, so all currently executing
+       * channels will get turned off.
        */
-      fs_reg some_reg = fs_reg(retype(brw_vec8_grf(0, 0),
-                                    BRW_REGISTER_TYPE_UW));
-      fs_inst *cmp = emit(CMP(reg_null_f, some_reg, some_reg,
-                              BRW_CONDITIONAL_NZ));
+      fs_inst *cmp;
+      if (instr->intrinsic == nir_intrinsic_discard_if) {
+         cmp = emit(CMP(reg_null_f, get_nir_src(instr->src[0]),
+                        fs_reg(0), BRW_CONDITIONAL_Z));
+      } else {
+         fs_reg some_reg = fs_reg(retype(brw_vec8_grf(0, 0),
+                                       BRW_REGISTER_TYPE_UW));
+         cmp = emit(CMP(reg_null_f, some_reg, some_reg, BRW_CONDITIONAL_NZ));
+      }
       cmp->predicate = BRW_PREDICATE_NORMAL;
       cmp->flag_subreg = 1;
 

src/mesa/drivers/dri/i965/brw_fs_visitor.cpp

 void
 fs_visitor::visit(ir_discard *ir)
 {
-   assert(ir->condition == NULL); /* FINISHME */
-
    /* We track our discarded pixels in f0.1.  By predicating on it, we can
-    * update just the flag bits that aren't yet discarded.  By emitting a
-    * CMP of g0 != g0, all our currently executing channels will get turned
-    * off.
+    * update just the flag bits that aren't yet discarded.  If there's no
+    * condition, we emit a CMP of g0 != g0, so all currently executing
+    * channels will get turned off.
     */
-   fs_reg some_reg = fs_reg(retype(brw_vec8_grf(0, 0),
-                                   BRW_REGISTER_TYPE_UW));
-   fs_inst *cmp = emit(CMP(reg_null_f, some_reg, some_reg,
-                           BRW_CONDITIONAL_NZ));
+   fs_inst *cmp;
+   if (ir->condition) {
+      ir->condition->accept(this);
+      cmp = emit(CMP(reg_null_f, this->result, fs_reg(0), BRW_CONDITIONAL_Z));
+   } else {
+      fs_reg some_reg = fs_reg(retype(brw_vec8_grf(0, 0),
+                                      BRW_REGISTER_TYPE_UW));
+      cmp = emit(CMP(reg_null_f, some_reg, some_reg, BRW_CONDITIONAL_NZ));
+   }
    cmp->predicate = BRW_PREDICATE_NORMAL;
    cmp->flag_subreg = 1;