--- /dev/null
+/*
+ * Copyright © 2018 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.h"
+#include "nir_builder.h"
+#include "nir_deref.h"
+
+#include "util/u_dynarray.h"
+
+/**
+ * Elimination of dead writes based on derefs.
+ *
+ * Dead writes are stores and copies that write to a deref, which then gets
+ * another write before it was used (read or sourced for a copy). Those
+ * writes can be removed since they don't affect anything.
+ *
+ * For derefs that refer to a memory area that can be read after the program,
+ * the last write is considered used. The presence of certain instructions
+ * may also cause writes to be considered used, e.g. memory barrier (in this case
+ * the value must be written as other thread might use it).
+ *
+ * The write mask for store instructions is considered, so it is possible that
+ * a store is removed because of the combination of other stores overwritten
+ * its value.
+ */
+
+/* Entry for unused_writes arrays. */
+struct write_entry {
+ /* If NULL indicates the entry is free to be reused. */
+ nir_intrinsic_instr *intrin;
+ nir_component_mask_t mask;
+ nir_deref_instr *dst;
+};
+
+static void
+clear_unused_for_modes(struct util_dynarray *unused_writes, nir_variable_mode modes)
+{
+ util_dynarray_foreach_reverse(unused_writes, struct write_entry, entry) {
+ nir_variable *var = nir_deref_instr_get_variable(entry->dst);
+ if (var->data.mode & modes)
+ *entry = util_dynarray_pop(unused_writes, struct write_entry);
+ }
+}
+
+static void
+clear_unused_for_read(struct util_dynarray *unused_writes, nir_deref_instr *src)
+{
+ util_dynarray_foreach_reverse(unused_writes, struct write_entry, entry) {
+ if (nir_compare_derefs(src, entry->dst) & nir_derefs_may_alias_bit)
+ *entry = util_dynarray_pop(unused_writes, struct write_entry);
+ }
+}
+
+static bool
+update_unused_writes(struct util_dynarray *unused_writes,
+ nir_intrinsic_instr *intrin,
+ nir_deref_instr *dst, nir_component_mask_t mask)
+{
+ bool progress = false;
+
+ /* This pass assumes that destination of copies and stores are derefs that
+ * end in a vector or scalar (it is OK to have wildcards or indirects for
+ * arrays).
+ */
+ assert(glsl_type_is_vector_or_scalar(dst->type));
+
+ /* Find writes that are unused and can be removed. */
+ util_dynarray_foreach_reverse(unused_writes, struct write_entry, entry) {
+ nir_deref_compare_result comp = nir_compare_derefs(dst, entry->dst);
+ if (comp & nir_derefs_a_contains_b_bit) {
+ entry->mask &= ~mask;
+ if (entry->mask == 0) {
+ nir_instr_remove(&entry->intrin->instr);
+ *entry = util_dynarray_pop(unused_writes, struct write_entry);
+ progress = true;
+ }
+ }
+ }
+
+ /* Add the new write to the unused array. */
+ struct write_entry new_entry = {
+ .intrin = intrin,
+ .mask = mask,
+ .dst = dst,
+ };
+
+ util_dynarray_append(unused_writes, struct write_entry, new_entry);
+
+ return progress;
+}
+
+static bool
+remove_dead_write_vars_local(void *mem_ctx, nir_block *block)
+{
+ bool progress = false;
+
+ struct util_dynarray unused_writes;
+ util_dynarray_init(&unused_writes, mem_ctx);
+
+ nir_foreach_instr_safe(instr, block) {
+ if (instr->type == nir_instr_type_call) {
+ clear_unused_for_modes(&unused_writes, nir_var_shader_out |
+ nir_var_global |
+ nir_var_local |
+ nir_var_shader_storage |
+ nir_var_shared);
+ continue;
+ }
+
+ if (instr->type != nir_instr_type_intrinsic)
+ continue;
+
+ nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
+ switch (intrin->intrinsic) {
+ case nir_intrinsic_barrier:
+ case nir_intrinsic_memory_barrier: {
+ clear_unused_for_modes(&unused_writes, nir_var_shader_out |
+ nir_var_shader_storage |
+ nir_var_shared);
+ break;
+ }
+
+ case nir_intrinsic_emit_vertex:
+ case nir_intrinsic_emit_vertex_with_counter: {
+ clear_unused_for_modes(&unused_writes, nir_var_shader_out);
+ break;
+ }
+
+ case nir_intrinsic_load_deref: {
+ nir_deref_instr *src = nir_src_as_deref(intrin->src[0]);
+ clear_unused_for_read(&unused_writes, src);
+ break;
+ }
+
+ case nir_intrinsic_store_deref: {
+ nir_deref_instr *dst = nir_src_as_deref(intrin->src[0]);
+ nir_component_mask_t mask = nir_intrinsic_write_mask(intrin);
+ progress |= update_unused_writes(&unused_writes, intrin, dst, mask);
+ break;
+ }
+
+ case nir_intrinsic_copy_deref: {
+ nir_deref_instr *src = nir_src_as_deref(intrin->src[1]);
+ nir_deref_instr *dst = nir_src_as_deref(intrin->src[0]);
+
+ /* Self-copy is removed. */
+ if (nir_compare_derefs(src, dst) & nir_derefs_equal_bit) {
+ nir_instr_remove(instr);
+ progress = true;
+ break;
+ }
+
+ clear_unused_for_read(&unused_writes, src);
+ nir_component_mask_t mask = (1 << glsl_get_vector_elements(dst->type)) - 1;
+ progress |= update_unused_writes(&unused_writes, intrin, dst, mask);
+ break;
+ }
+
+ default:
+ break;
+ }
+ }
+
+ /* All unused writes at the end of the block are kept, since we can't be
+ * sure they'll be overwritten or not with local analysis only.
+ */
+
+ return progress;
+}
+
+static bool
+remove_dead_write_vars_impl(void *mem_ctx, nir_function_impl *impl)
+{
+ bool progress = false;
+
+ nir_metadata_require(impl, nir_metadata_block_index);
+
+ nir_foreach_block(block, impl)
+ progress |= remove_dead_write_vars_local(mem_ctx, block);
+
+ if (progress) {
+ nir_metadata_preserve(impl, nir_metadata_block_index |
+ nir_metadata_dominance);
+ }
+
+ return progress;
+}
+
+bool
+nir_opt_dead_write_vars(nir_shader *shader)
+{
+ void *mem_ctx = ralloc_context(NULL);
+ bool progress = false;
+
+ nir_foreach_function(function, shader) {
+ if (!function->impl)
+ continue;
+ progress |= remove_dead_write_vars_impl(mem_ctx, function->impl);
+ }
+
+ ralloc_free(mem_ctx);
+ return progress;
+}
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