nir: Add a memcpy optimization pass

This pass attempts to optimize three broad categories of memcpy:

 1. Self-copies: These we can discard out-of-hand.

 2. Vector copies: It doesn't matter what the vector size is or if the
    source and destination have different vector types, it's still easy
    enough to emit a load/store pair.

 3. Tightly packed copies:  In the case where a type is tightly packed
    (no padding bits), we can replace the memcpy with a copy_deref
    instruction which the optimizer is far better at handling.

This has proven capable of getting rid of many of the memcpy instances
in some rather gnarly OpenCL C kernels I've been looking at, even after
coming out of LLVM's optimizer.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6871>

src/compiler/Makefile.sources

@@ -329,6 +329,7 @@ NIR_FILES = \
 	nir/nir_opt_loop_unroll.c \
 	nir/nir_opt_large_constants.c \
 	nir/nir_opt_load_store_vectorize.c \
+	nir/nir_opt_memcpy.c \
 	nir/nir_opt_move.c \
 	nir/nir_opt_peephole_select.c \
 	nir/nir_opt_rematerialize_compares.c \

src/compiler/nir/meson.build

@@ -210,6 +210,7 @@ files_libnir = files(
   'nir_opt_large_constants.c',
   'nir_opt_load_store_vectorize.c',
   'nir_opt_loop_unroll.c',
+  'nir_opt_memcpy.c',
   'nir_opt_move.c',
   'nir_opt_peephole_select.c',
   'nir_opt_rematerialize_compares.c',

src/compiler/nir/nir.h

@@ -4126,6 +4126,7 @@ void nir_lower_deref_copy_instr(struct nir_builder *b,
                                 nir_intrinsic_instr *copy);
 bool nir_lower_var_copies(nir_shader *shader);
 
+bool nir_opt_memcpy(nir_shader *shader);
 bool nir_lower_memcpy(nir_shader *shader);
 
 void nir_fixup_deref_modes(nir_shader *shader);

src/compiler/nir/nir_opt_memcpy.c

@@ -0,0 +1,224 @@
+/*
+ * Copyright © 2020 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.
+ */
+
+#include "nir_builder.h"
+
+static bool
+opt_memcpy_deref_cast(nir_intrinsic_instr *cpy, nir_src *deref_src)
+{
+   assert(cpy->intrinsic == nir_intrinsic_memcpy_deref);
+
+   nir_deref_instr *cast = nir_src_as_deref(*deref_src);
+   if (cast == NULL || cast->deref_type != nir_deref_type_cast)
+      return false;
+
+   /* We always have to replace the source with a deref, not a bare uint
+    * pointer.  If it's the first deref in the chain, bail.
+    */
+   nir_deref_instr *parent = nir_src_as_deref(cast->parent);
+   if (parent == NULL)
+      return false;
+
+   /* If it has useful alignment information, we want to keep that */
+   if (cast->cast.align_mul > 0)
+      return false;
+
+   /* Casts to uint8 or int8 never do us any good; get rid of them */
+   if (cast->type == glsl_int8_t_type() ||
+       cast->type == glsl_uint8_t_type()) {
+      nir_instr_rewrite_src(&cpy->instr, deref_src,
+                            nir_src_for_ssa(&parent->dest.ssa));
+      return true;
+   }
+
+   int64_t parent_type_size = glsl_get_explicit_size(parent->type, false);
+   if (parent_type_size < 0)
+      return false;
+
+   if (!nir_src_is_const(cpy->src[2]))
+      return false;
+
+   /* We don't want to get rid of the cast if the resulting type would be
+    * smaller than the amount of data we're copying.
+    */
+   if (nir_src_as_uint(cpy->src[2]) < (uint64_t)parent_type_size)
+      return false;
+
+   nir_instr_rewrite_src(&cpy->instr, deref_src,
+                         nir_src_for_ssa(&parent->dest.ssa));
+   return true;
+}
+
+static bool
+type_is_tightly_packed(const struct glsl_type *type, unsigned *size_out)
+{
+   unsigned size = 0;
+   if (glsl_type_is_struct_or_ifc(type)) {
+      unsigned num_fields = glsl_get_length(type);
+      for (unsigned i = 0; i < num_fields; i++) {
+         const struct glsl_struct_field *field =
+            glsl_get_struct_field_data(type, i);
+
+         if (field->offset < 0 || field->offset != size)
+            return false;
+
+         unsigned field_size;
+         if (!type_is_tightly_packed(field->type, &field_size))
+            return false;
+
+         size = field->offset + field_size;
+      }
+   } else if (glsl_type_is_array_or_matrix(type)) {
+      if (glsl_type_is_unsized_array(type))
+         return false;
+
+      unsigned stride = glsl_get_explicit_stride(type);
+      if (stride == 0)
+         return false;
+
+      const struct glsl_type *elem_type = glsl_get_array_element(type);
+
+      unsigned elem_size;
+      if (!type_is_tightly_packed(elem_type, &elem_size))
+         return false;
+
+      if (elem_size != stride)
+         return false;
+
+      size = stride * glsl_get_length(type);
+   } else {
+      assert(glsl_type_is_vector_or_scalar(type));
+      if (glsl_get_explicit_stride(type) > 0)
+         return false;
+
+      if (glsl_type_is_boolean(type))
+         return false;
+
+      size = glsl_get_explicit_size(type, false);
+   }
+
+   if (size_out)
+      *size_out = size;
+   return true;
+}
+
+static bool
+try_lower_memcpy(nir_builder *b, nir_intrinsic_instr *cpy)
+{
+   nir_deref_instr *dst = nir_src_as_deref(cpy->src[0]);
+   nir_deref_instr *src = nir_src_as_deref(cpy->src[1]);
+
+   /* A self-copy can always be eliminated */
+   if (dst == src) {
+      nir_instr_remove(&cpy->instr);
+      return true;
+   }
+
+   if (!nir_src_is_const(cpy->src[2]))
+      return false;
+
+   uint64_t size = nir_src_as_uint(cpy->src[2]);
+   if (size == 0) {
+      nir_instr_remove(&cpy->instr);
+      return true;
+   }
+
+   if (glsl_type_is_vector_or_scalar(src->type) &&
+       glsl_type_is_vector_or_scalar(dst->type) &&
+       glsl_get_explicit_size(dst->type, false) == size &&
+       glsl_get_explicit_size(src->type, false) == size) {
+      b->cursor = nir_instr_remove(&cpy->instr);
+      nir_ssa_def *data =
+         nir_load_deref_with_access(b, src, nir_intrinsic_src_access(cpy));
+      data = nir_bitcast_vector(b, data, glsl_get_bit_size(dst->type));
+      assert(data->num_components == glsl_get_vector_elements(dst->type));
+      nir_store_deref_with_access(b, src, data, ~0 /* write mask */,
+                                  nir_intrinsic_dst_access(cpy));
+      return true;
+   }
+
+   unsigned type_size;
+   if (dst->type == src->type &&
+       type_is_tightly_packed(dst->type, &type_size) &&
+       type_size == size) {
+      b->cursor = nir_instr_remove(&cpy->instr);
+      nir_copy_deref_with_access(b, dst, src,
+                                 nir_intrinsic_dst_access(cpy),
+                                 nir_intrinsic_src_access(cpy));
+      return true;
+   }
+
+   return false;
+}
+
+static bool
+opt_memcpy_impl(nir_function_impl *impl)
+{
+   bool progress = false;
+
+   nir_builder b;
+   nir_builder_init(&b, impl);
+
+   nir_foreach_block(block, impl) {
+      nir_foreach_instr_safe(instr, block) {
+         if (instr->type != nir_instr_type_intrinsic)
+            continue;
+
+         nir_intrinsic_instr *cpy = nir_instr_as_intrinsic(instr);
+         if (cpy->intrinsic != nir_intrinsic_memcpy_deref)
+            continue;
+
+         while (opt_memcpy_deref_cast(cpy, &cpy->src[0]))
+            progress = true;
+         while (opt_memcpy_deref_cast(cpy, &cpy->src[1]))
+            progress = true;
+
+         if (try_lower_memcpy(&b, cpy)) {
+            progress = true;
+            continue;
+         }
+      }
+   }
+
+   if (progress) {
+      nir_metadata_preserve(impl, nir_metadata_block_index |
+                                  nir_metadata_dominance);
+   } else {
+      nir_metadata_preserve(impl, nir_metadata_all);
+   }
+
+   return progress;
+}
+
+bool
+nir_opt_memcpy(nir_shader *shader)
+{
+   bool progress = false;
+
+   nir_foreach_function(function, shader) {
+      if (function->impl && opt_memcpy_impl(function->impl))
+         progress = true;
+   }
+
+   return progress;
+}