i965/fs: Combine pixel center calculation into one inst.

The X and Y values come interleaved in g1 (.4-.11 inclusive), so we can
calculate them together with a single add(32) instruction on some
platforms like Broadwell and newer or in SIMD8 elsewhere.

Note that I also moved the PIXEL_X/PIXEL_Y virtual opcodes from before
LINTERP to after it. That's because the writes_accumulator_implicitly()
function in backend_instruction tests for <= LINTERP for determining
whether the instruction indeed writes the accumulator implicitly. The
old FS_OPCODE_PIXEL_X/Y emitted ADD instructions, which did, but the new
opcodes just emit MOVs, which don't. It doesn't matter, since we don't
use these opcodes on Gen4/5 anymore, but in the case that we do...

On Broadwell:
total instructions in shared programs: 7192355 -> 7186224 (-0.09%)
instructions in affected programs:     1190700 -> 1184569 (-0.51%)
helped:                                6131

On Haswell:
total instructions in shared programs: 6155979 -> 6152800 (-0.05%)
instructions in affected programs:     652362 -> 649183 (-0.49%)
helped:                                3179

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

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

@@ -925,6 +925,8 @@ enum opcode {
    FS_OPCODE_DDY_FINE,
    FS_OPCODE_CINTERP,
    FS_OPCODE_LINTERP,
+   FS_OPCODE_PIXEL_X,
+   FS_OPCODE_PIXEL_Y,
    FS_OPCODE_UNIFORM_PULL_CONSTANT_LOAD,
    FS_OPCODE_UNIFORM_PULL_CONSTANT_LOAD_GEN7,
    FS_OPCODE_VARYING_PULL_CONSTANT_LOAD,

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

@@ -1940,6 +1940,16 @@ fs_generator::generate_code(const cfg_t *cfg, int dispatch_width)
       case FS_OPCODE_LINTERP:
 	 generate_linterp(inst, dst, src);
 	 break;
+      case FS_OPCODE_PIXEL_X:
+         assert(src[0].type == BRW_REGISTER_TYPE_UW);
+         src[0].subnr = 0 * type_sz(src[0].type);
+         brw_MOV(p, dst, stride(src[0], 8, 4, 1));
+         break;
+      case FS_OPCODE_PIXEL_Y:
+         assert(src[0].type == BRW_REGISTER_TYPE_UW);
+         src[0].subnr = 4 * type_sz(src[0].type);
+         brw_MOV(p, dst, stride(src[0], 8, 4, 1));
+         break;
       case SHADER_OPCODE_TEX:
       case FS_OPCODE_TXB:
       case SHADER_OPCODE_TXD:

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

@@ -3478,27 +3478,58 @@ fs_visitor::emit_interpolation_setup_gen6()
 {
    struct brw_reg g1_uw = retype(brw_vec1_grf(1, 0), BRW_REGISTER_TYPE_UW);
 
-   /* If the pixel centers end up used, the setup is the same as for gen4. */
    this->current_annotation = "compute pixel centers";
-   fs_reg int_pixel_x = vgrf(glsl_type::uint_type);
-   fs_reg int_pixel_y = vgrf(glsl_type::uint_type);
-   int_pixel_x.type = BRW_REGISTER_TYPE_UW;
-   int_pixel_y.type = BRW_REGISTER_TYPE_UW;
-   emit(ADD(int_pixel_x,
-            fs_reg(stride(suboffset(g1_uw, 4), 2, 4, 0)),
-            fs_reg(brw_imm_v(0x10101010))));
-   emit(ADD(int_pixel_y,
-            fs_reg(stride(suboffset(g1_uw, 5), 2, 4, 0)),
-            fs_reg(brw_imm_v(0x11001100))));
-
-   /* As of gen6, we can no longer mix float and int sources.  We have
-    * to turn the integer pixel centers into floats for their actual
-    * use.
-    */
-   this->pixel_x = vgrf(glsl_type::float_type);
-   this->pixel_y = vgrf(glsl_type::float_type);
-   emit(MOV(this->pixel_x, int_pixel_x));
-   emit(MOV(this->pixel_y, int_pixel_y));
+   if (brw->gen >= 8 || dispatch_width == 8) {
+      /* The "Register Region Restrictions" page says for BDW (and newer,
+       * presumably):
+       *
+       *     "When destination spans two registers, the source may be one or
+       *      two registers. The destination elements must be evenly split
+       *      between the two registers."
+       *
+       * Thus we can do a single add(16) in SIMD8 or an add(32) in SIMD16 to
+       * compute our pixel centers.
+       */
+      fs_reg int_pixel_xy(GRF, alloc.allocate(dispatch_width / 8),
+                          BRW_REGISTER_TYPE_UW, dispatch_width * 2);
+      emit(ADD(int_pixel_xy,
+               fs_reg(stride(suboffset(g1_uw, 4), 1, 4, 0)),
+               fs_reg(brw_imm_v(0x11001010))))
+         ->force_writemask_all = true;
+
+      this->pixel_x = vgrf(glsl_type::float_type);
+      this->pixel_y = vgrf(glsl_type::float_type);
+      emit(FS_OPCODE_PIXEL_X, this->pixel_x, int_pixel_xy);
+      emit(FS_OPCODE_PIXEL_Y, this->pixel_y, int_pixel_xy);
+   } else {
+      /* The "Register Region Restrictions" page says for SNB, IVB, HSW:
+       *
+       *     "When destination spans two registers, the source MUST span two
+       *      registers."
+       *
+       * Since the GRF source of the ADD will only read a single register, we
+       * must do two separate ADDs in SIMD16.
+       */
+      fs_reg int_pixel_x = vgrf(glsl_type::uint_type);
+      fs_reg int_pixel_y = vgrf(glsl_type::uint_type);
+      int_pixel_x.type = BRW_REGISTER_TYPE_UW;
+      int_pixel_y.type = BRW_REGISTER_TYPE_UW;
+      emit(ADD(int_pixel_x,
+               fs_reg(stride(suboffset(g1_uw, 4), 2, 4, 0)),
+               fs_reg(brw_imm_v(0x10101010))));
+      emit(ADD(int_pixel_y,
+               fs_reg(stride(suboffset(g1_uw, 5), 2, 4, 0)),
+               fs_reg(brw_imm_v(0x11001100))));
+
+      /* As of gen6, we can no longer mix float and int sources.  We have
+       * to turn the integer pixel centers into floats for their actual
+       * use.
+       */
+      this->pixel_x = vgrf(glsl_type::float_type);
+      this->pixel_y = vgrf(glsl_type::float_type);
+      emit(MOV(this->pixel_x, int_pixel_x));
+      emit(MOV(this->pixel_y, int_pixel_y));
+   }
 
    this->current_annotation = "compute pos.w";
    this->pixel_w = fs_reg(brw_vec8_grf(payload.source_w_reg, 0));