--- /dev/null
+/*
+ * Copyright © 2015 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.
+ *
+ * Authors:
+ * Jason Ekstrand (jason@jlekstrand.net)
+ *
+ */
+
+#include "nir.h"
+
+/*
+ * Implements a pass that lowers vector phi nodes to scalar phi nodes when
+ * we don't think it will hurt anything.
+ */
+
+struct lower_phis_to_scalar_state {
+ void *mem_ctx;
+ void *dead_ctx;
+
+ /* Hash table marking which phi nodes are scalarizable. The key is
+ * pointers to phi instructions and the entry is either NULL for not
+ * scalarizable or non-null for scalarizable.
+ */
+ struct hash_table *phi_table;
+};
+
+static bool
+should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state);
+
+static bool
+is_phi_src_scalarizable(nir_phi_src *src,
+ struct lower_phis_to_scalar_state *state)
+{
+ /* Don't know what to do with non-ssa sources */
+ if (!src->src.is_ssa)
+ return false;
+
+ nir_instr *src_instr = src->src.ssa->parent_instr;
+ switch (src_instr->type) {
+ case nir_instr_type_alu: {
+ nir_alu_instr *src_alu = nir_instr_as_alu(src_instr);
+
+ /* ALU operations with output_size == 0 should be scalarized. We
+ * will also see a bunch of vecN operations from scalarizing ALU
+ * operations and, since they can easily be copy-propagated, they
+ * are ok too.
+ */
+ return nir_op_infos[src_alu->op].output_size == 0 ||
+ src_alu->op != nir_op_vec2 ||
+ src_alu->op != nir_op_vec3 ||
+ src_alu->op != nir_op_vec4;
+ }
+
+ case nir_instr_type_phi:
+ /* A phi is scalarizable if we're going to lower it */
+ return should_lower_phi(nir_instr_as_phi(src_instr), state);
+
+ case nir_instr_type_load_const:
+ /* These are trivially scalarizable */
+ return true;
+
+ case nir_instr_type_intrinsic: {
+ nir_intrinsic_instr *src_intrin = nir_instr_as_intrinsic(src_instr);
+
+ switch (src_intrin->intrinsic) {
+ case nir_intrinsic_load_var:
+ return src_intrin->variables[0]->var->data.mode == nir_var_shader_in ||
+ src_intrin->variables[0]->var->data.mode == nir_var_uniform;
+
+ case nir_intrinsic_interp_var_at_centroid:
+ case nir_intrinsic_interp_var_at_sample:
+ case nir_intrinsic_interp_var_at_offset:
+ case nir_intrinsic_load_uniform:
+ case nir_intrinsic_load_uniform_indirect:
+ case nir_intrinsic_load_ubo:
+ case nir_intrinsic_load_ubo_indirect:
+ case nir_intrinsic_load_input:
+ case nir_intrinsic_load_input_indirect:
+ return true;
+ default:
+ break;
+ }
+ }
+
+ default:
+ /* We can't scalarize this type of instruction */
+ return false;
+ }
+}
+
+/**
+ * Determines if the given phi node should be lowered. The only phi nodes
+ * we will scalarize at the moment are those where all of the sources are
+ * scalarizable.
+ *
+ * The reason for this comes down to coalescing. Since phi sources can't
+ * swizzle, swizzles on phis have to be resolved by inserting a mov right
+ * before the phi. The choice then becomes between movs to pick off
+ * components for a scalar phi or potentially movs to recombine components
+ * for a vector phi. The problem is that the movs generated to pick off
+ * the components are almost uncoalescable. We can't coalesce them in NIR
+ * because we need them to pick off components and we can't coalesce them
+ * in the backend because the source register is a vector and the
+ * destination is a scalar that may be used at other places in the program.
+ * On the other hand, if we have a bunch of scalars going into a vector
+ * phi, the situation is much better. In this case, if the SSA def is
+ * generated in the predecessor block to the corresponding phi source, the
+ * backend code will be an ALU op into a temporary and then a mov into the
+ * given vector component; this move can almost certainly be coalesced
+ * away.
+ */
+static bool
+should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state)
+{
+ /* Already scalar */
+ if (phi->dest.ssa.num_components == 1)
+ return false;
+
+ struct hash_entry *entry = _mesa_hash_table_search(state->phi_table, phi);
+ if (entry)
+ return entry->data != NULL;
+
+ /* Insert an entry and mark it as scalarizable for now. That way
+ * we don't recurse forever and a cycle in the dependence graph
+ * won't automatically make us fail to scalarize.
+ */
+ entry = _mesa_hash_table_insert(state->phi_table, phi, (void *)(intptr_t)1);
+
+ bool scalarizable = true;
+
+ nir_foreach_phi_src(phi, src) {
+ scalarizable = is_phi_src_scalarizable(src, state);
+ if (!scalarizable)
+ break;
+ }
+
+ entry->data = (void *)(intptr_t)scalarizable;
+
+ return scalarizable;
+}
+
+static bool
+lower_phis_to_scalar_block(nir_block *block, void *void_state)
+{
+ struct lower_phis_to_scalar_state *state = void_state;
+
+ /* Find the last phi node in the block */
+ nir_phi_instr *last_phi = NULL;
+ nir_foreach_instr(block, instr) {
+ if (instr->type != nir_instr_type_phi)
+ break;
+
+ last_phi = nir_instr_as_phi(instr);
+ }
+
+ /* We have to handle the phi nodes in their own pass due to the way
+ * we're modifying the linked list of instructions.
+ */
+ nir_foreach_instr_safe(block, instr) {
+ if (instr->type != nir_instr_type_phi)
+ break;
+
+ nir_phi_instr *phi = nir_instr_as_phi(instr);
+
+ if (!should_lower_phi(phi, state))
+ continue;
+
+ /* Create a vecN operation to combine the results. Most of these
+ * will be redundant, but copy propagation should clean them up for
+ * us. No need to add the complexity here.
+ */
+ nir_op vec_op;
+ switch (phi->dest.ssa.num_components) {
+ case 2: vec_op = nir_op_vec2; break;
+ case 3: vec_op = nir_op_vec3; break;
+ case 4: vec_op = nir_op_vec4; break;
+ default: unreachable("Invalid number of components");
+ }
+
+ nir_alu_instr *vec = nir_alu_instr_create(state->mem_ctx, vec_op);
+ nir_ssa_dest_init(&vec->instr, &vec->dest.dest,
+ phi->dest.ssa.num_components, NULL);
+ vec->dest.write_mask = (1 << phi->dest.ssa.num_components) - 1;
+
+ for (unsigned i = 0; i < phi->dest.ssa.num_components; i++) {
+ nir_phi_instr *new_phi = nir_phi_instr_create(state->mem_ctx);
+ nir_ssa_dest_init(&new_phi->instr, &new_phi->dest, 1, NULL);
+
+ vec->src[i].src = nir_src_for_ssa(&new_phi->dest.ssa);
+
+ nir_foreach_phi_src(phi, src) {
+ /* We need to insert a mov to grab the i'th component of src */
+ nir_alu_instr *mov = nir_alu_instr_create(state->mem_ctx,
+ nir_op_imov);
+ nir_ssa_dest_init(&mov->instr, &mov->dest.dest, 1, NULL);
+ mov->dest.write_mask = 1;
+ nir_src_copy(&mov->src[0].src, &src->src, state->mem_ctx);
+ mov->src[0].swizzle[0] = i;
+
+ /* Insert at the end of the predecessor but before the jump */
+ nir_instr *pred_last_instr = nir_block_last_instr(src->pred);
+ if (pred_last_instr && pred_last_instr->type == nir_instr_type_jump)
+ nir_instr_insert_before(pred_last_instr, &mov->instr);
+ else
+ nir_instr_insert_after_block(src->pred, &mov->instr);
+
+ nir_phi_src *new_src = ralloc(state->mem_ctx, nir_phi_src);
+ new_src->pred = src->pred;
+ new_src->src = nir_src_for_ssa(&mov->dest.dest.ssa);
+
+ exec_list_push_tail(&new_phi->srcs, &new_src->node);
+ }
+
+ nir_instr_insert_before(&phi->instr, &new_phi->instr);
+ }
+
+ nir_instr_insert_after(&last_phi->instr, &vec->instr);
+
+ nir_ssa_def_rewrite_uses(&phi->dest.ssa,
+ nir_src_for_ssa(&vec->dest.dest.ssa),
+ state->mem_ctx);
+
+ ralloc_steal(state->dead_ctx, phi);
+ nir_instr_remove(&phi->instr);
+
+ /* We're using the safe iterator and inserting all the newly
+ * scalarized phi nodes before their non-scalarized version so that's
+ * ok. However, we are also inserting vec operations after all of
+ * the last phi node so once we get here, we can't trust even the
+ * safe iterator to stop properly. We have to break manually.
+ */
+ if (instr == &last_phi->instr)
+ break;
+ }
+
+ return true;
+}
+
+static void
+lower_phis_to_scalar_impl(nir_function_impl *impl)
+{
+ struct lower_phis_to_scalar_state state;
+
+ state.mem_ctx = ralloc_parent(impl);
+ state.dead_ctx = ralloc_context(NULL);
+ state.phi_table = _mesa_hash_table_create(state.dead_ctx, _mesa_hash_pointer,
+ _mesa_key_pointer_equal);
+
+ nir_foreach_block(impl, lower_phis_to_scalar_block, &state);
+
+ nir_metadata_preserve(impl, nir_metadata_block_index |
+ nir_metadata_dominance);
+
+ ralloc_free(state.dead_ctx);
+}
+
+/** A pass that lowers vector phi nodes to scalar
+ *
+ * This pass loops through the blocks and lowers looks for vector phi nodes
+ * it can lower to scalar phi nodes. Not all phi nodes are lowered. For
+ * instance, if one of the sources is a non-scalarizable vector, then we
+ * don't bother lowering because that would generate hard-to-coalesce movs.
+ */
+void
+nir_lower_phis_to_scalar(nir_shader *shader)
+{
+ nir_foreach_overload(shader, overload) {
+ if (overload->impl)
+ lower_phis_to_scalar_impl(overload->impl);
+ }
+}
projects / mesa.git / commitdiff