i965/fs: Add FS_OPCODE_MOV_DISPATCH_TO_FLAGS to fragment shader backend.

In order to compute centroid varyings correctly, the fragment shader
needs to be able to load the current pixel/sample mask into a flag
register.  This patch adds an opcode to the fragment shader back-end
to do this; the opcode gets translated into the instruction

mov(1)  f0<1>UW  g1.14<0,1,0>UW  { align1 WE_all }

Since this instruction clobbers f0, instruction scheduling has to
treat it the same as instructions that have a conditional modifier.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/dri/i965/brw_defines.h

@@ -675,6 +675,7 @@ enum opcode {
    FS_OPCODE_SPILL,
    FS_OPCODE_UNSPILL,
    FS_OPCODE_PULL_CONSTANT_LOAD,
+   FS_OPCODE_MOV_DISPATCH_TO_FLAGS,
 
    VS_OPCODE_URB_WRITE,
    VS_OPCODE_SCRATCH_READ,

src/mesa/drivers/dri/i965/brw_fs.h

@@ -539,6 +539,7 @@ public:
    void generate_spill(fs_inst *inst, struct brw_reg src);
    void generate_unspill(fs_inst *inst, struct brw_reg dst);
    void generate_pull_constant_load(fs_inst *inst, struct brw_reg dst);
+   void generate_mov_dispatch_to_flags();
 
    void emit_dummy_fs();
    fs_reg *emit_fragcoord_interpolation(ir_variable *ir);

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

@@ -591,6 +591,27 @@ fs_visitor::generate_pull_constant_load(fs_inst *inst, struct brw_reg dst)
    }
 }
 
+
+/**
+ * Cause the current pixel/sample mask (from R1.7 bits 15:0) to be transferred
+ * into the flags register (f0.0).
+ *
+ * Used only on Gen6 and above.
+ */
+void
+fs_visitor::generate_mov_dispatch_to_flags()
+{
+   struct brw_reg f0 = brw_flag_reg();
+   struct brw_reg g1 = retype(brw_vec1_grf(1, 7), BRW_REGISTER_TYPE_UW);
+
+   assert (intel->gen >= 6);
+   brw_push_insn_state(p);
+   brw_set_mask_control(p, BRW_MASK_DISABLE);
+   brw_MOV(p, f0, g1);
+   brw_pop_insn_state(p);
+}
+
+
 static uint32_t brw_file_from_reg(fs_reg *reg)
 {
    switch (reg->file) {
@@ -928,6 +949,11 @@ fs_visitor::generate_code()
       case FS_OPCODE_FB_WRITE:
 	 generate_fb_write(inst);
 	 break;
+
+      case FS_OPCODE_MOV_DISPATCH_TO_FLAGS:
+         generate_mov_dispatch_to_flags();
+         break;
+
       default:
 	 if (inst->opcode < (int)ARRAY_SIZE(brw_opcodes)) {
 	    _mesa_problem(ctx, "Unsupported opcode `%s' in FS",

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

@@ -334,7 +334,11 @@ instruction_scheduler::calculate_deps()
 	 }
       }
 
-      if (inst->conditional_mod) {
+      /* Treat FS_OPCODE_MOV_DISPATCH_TO_FLAGS as though it had a
+       * conditional_mod, because it sets the flag register.
+       */
+      if (inst->conditional_mod ||
+          inst->opcode == FS_OPCODE_MOV_DISPATCH_TO_FLAGS) {
 	 add_dep(last_conditional_mod, n, 0);
 	 last_conditional_mod = n;
       }
@@ -413,8 +417,13 @@ instruction_scheduler::calculate_deps()
 	 }
       }
 
-      if (inst->conditional_mod)
+      /* Treat FS_OPCODE_MOV_DISPATCH_TO_FLAGS as though it had a
+       * conditional_mod, because it sets the flag register.
+       */
+      if (inst->conditional_mod ||
+          inst->opcode == FS_OPCODE_MOV_DISPATCH_TO_FLAGS) {
 	 last_conditional_mod = n;
+      }
    }
 }