intel/compiler: add a new pass to lower shading rate into HW format

Rework:
 * Jason: Modernize brw_nir_lower_shading_rate_output:
   1. Use nir_shader_instructions_pass()
   2. Use *_imm builder helpers.
   3. Use nir_intrinsic_base() instead of ->const_index[0]

v2: Also lower loads (Caio)

v3: Update stage check to trigger lowering (Caio)

v4: Assert on != MESH (Caio)

v5: Fixup instruction insertion (Caio)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13739>

src/compiler/shader_enums.h

@@ -98,6 +98,19 @@ gl_shader_stage_is_callable(gl_shader_stage stage)
           stage == MESA_SHADER_CALLABLE;
 }
 
+static inline bool
+gl_shader_stage_can_set_fragment_shading_rate(gl_shader_stage stage)
+{
+   /* According to EXT_fragment_shading_rate :
+    *
+    *    "This extension adds support for setting the fragment shading rate
+    *     for a primitive in vertex, geometry, and mesh shading stages"
+    */
+   return stage == MESA_SHADER_VERTEX ||
+          stage == MESA_SHADER_GEOMETRY ||
+          stage == MESA_SHADER_MESH;
+}
+
 /**
  * Number of STATE_* values we need to address any GL state.
  * Used to dimension arrays.

src/intel/compiler/brw_nir.c

@@ -1110,6 +1110,9 @@ brw_postprocess_nir(nir_shader *nir, const struct brw_compiler *compiler,
       OPT(nir_lower_idiv, &options);
    }
 
+   if (gl_shader_stage_can_set_fragment_shading_rate(nir->info.stage))
+      brw_nir_lower_shading_rate_output(nir);
+
    brw_nir_optimize(nir, compiler, is_scalar, false);
 
    if (is_scalar && nir_shader_has_local_variables(nir)) {

src/intel/compiler/brw_nir.h

@@ -120,6 +120,8 @@ bool brw_nir_lower_conversions(nir_shader *nir);
 
 bool brw_nir_lower_scoped_barriers(nir_shader *nir);
 
+bool brw_nir_lower_shading_rate_output(nir_shader *nir);
+
 bool brw_nir_lower_storage_image(nir_shader *nir,
                                  const struct intel_device_info *devinfo);
 

src/intel/compiler/brw_nir_lower_shading_rate_output.c

@@ -0,0 +1,111 @@
+/*
+ * Copyright (c) 2021 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+/*
+ * Lower the shading rate output from the bit field format described in the
+ * SPIRV spec :
+ *
+ * bit | name              | description
+ *   0 | Vertical2Pixels   | Fragment invocation covers 2 pixels vertically
+ *   1 | Vertical4Pixels   | Fragment invocation covers 4 pixels vertically
+ *   2 | Horizontal2Pixels | Fragment invocation covers 2 pixels horizontally
+ *   3 | Horizontal4Pixels | Fragment invocation covers 4 pixels horizontally
+ *
+ * into a single dword composed of 2 fp16 to be stored in the dword 0 of the
+ * VUE header.
+ *
+ * When no horizontal/vertical bits are set, the size in pixel size in that
+ * dimension is assumed to be 1.
+ *
+ * According to the specification, the shading rate output can be read &
+ * written. A read after a write should report a different value if the
+ * implemention decides on different primitive shading rate for some reason.
+ * This is never the case in our implementation.
+ */
+
+#include "brw_nir.h"
+#include "compiler/nir/nir_builder.h"
+
+static bool
+lower_shading_rate_output_instr(nir_builder *b, nir_instr *instr,
+                                UNUSED void *_state)
+{
+   if (instr->type != nir_instr_type_intrinsic)
+      return false;
+
+   nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+   if (intrin->intrinsic != nir_intrinsic_load_output &&
+       intrin->intrinsic != nir_intrinsic_store_output)
+      return false;
+
+   if (nir_intrinsic_base(intrin) != VARYING_SLOT_PRIMITIVE_SHADING_RATE)
+      return false;
+
+   b->cursor = intrin->intrinsic == nir_intrinsic_load_output ?
+      nir_after_instr(instr) : nir_before_instr(instr);
+
+   if (intrin->intrinsic == nir_intrinsic_store_output) {
+      assert(intrin->src[0].is_ssa);
+      nir_ssa_def *bit_field = intrin->src[0].ssa;
+      nir_ssa_def *fp16_x =
+         nir_i2f16(b,
+                   nir_ishl(b, nir_imm_int(b, 1),
+                            nir_ishr_imm(b, bit_field, 2)));
+      nir_ssa_def *fp16_y =
+         nir_i2f16(b,
+                   nir_ishl(b, nir_imm_int(b, 1),
+                            nir_iand_imm(b, bit_field, 0x3)));
+      nir_ssa_def *packed_fp16_xy = nir_pack_32_2x16_split(b, fp16_x, fp16_y);
+
+      nir_instr_rewrite_src(instr, &intrin->src[0],
+                            nir_src_for_ssa(packed_fp16_xy));
+   } else {
+      assert(intrin->intrinsic == nir_intrinsic_load_output);
+      nir_ssa_def *packed_fp16_xy = &intrin->dest.ssa;
+
+      nir_ssa_def *u32_x =
+         nir_i2i32(b, nir_unpack_32_2x16_split_x(b, packed_fp16_xy));
+      nir_ssa_def *u32_y =
+         nir_i2i32(b, nir_unpack_32_2x16_split_y(b, packed_fp16_xy));
+
+      nir_ssa_def *bit_field =
+         nir_ior(b, nir_ishl_imm(b, nir_ushr_imm(b, u32_x, 1), 2),
+                    nir_ushr_imm(b, u32_y, 1));
+
+      nir_ssa_def_rewrite_uses_after(packed_fp16_xy, bit_field,
+                                     bit_field->parent_instr);
+   }
+
+   return true;
+}
+
+bool
+brw_nir_lower_shading_rate_output(nir_shader *nir)
+{
+   /* TODO(mesh): Add Shading Rate support. */
+   assert(nir->info.stage != MESA_SHADER_MESH);
+
+   return nir_shader_instructions_pass(nir, lower_shading_rate_output_instr,
+                                       nir_metadata_block_index |
+                                       nir_metadata_dominance, NULL);
+}

src/intel/compiler/meson.build

@@ -90,6 +90,7 @@ libintel_compiler_files = files(
   'brw_nir_lower_rt_intrinsics.c',
   'brw_nir_lower_scoped_barriers.c',
   'brw_nir_lower_shader_calls.c',
+  'brw_nir_lower_shading_rate_output.c',
   'brw_nir_lower_storage_image.c',
   'brw_nir_opt_peephole_ffma.c',
   'brw_nir_rt.h',