/* * 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 || nir_op_is_vec(src_alu->op); } 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_ssa_undef: /* The caller of this function is going to OR the results and we don't * want undefs to count so we return false. */ return false; case nir_instr_type_intrinsic: { nir_intrinsic_instr *src_intrin = nir_instr_as_intrinsic(src_instr); switch (src_intrin->intrinsic) { case nir_intrinsic_load_deref: { /* Don't scalarize if we see a load of a local variable because it * might turn into one of the things we can't scalarize. */ nir_deref_instr *deref = nir_src_as_deref(src_intrin->src[0]); return !nir_deref_mode_may_be(deref, nir_var_function_temp | nir_var_shader_temp); } case nir_intrinsic_interp_deref_at_centroid: case nir_intrinsic_interp_deref_at_sample: case nir_intrinsic_interp_deref_at_offset: case nir_intrinsic_interp_deref_at_vertex: case nir_intrinsic_load_uniform: case nir_intrinsic_load_ubo: case nir_intrinsic_load_ssbo: case nir_intrinsic_load_global: case nir_intrinsic_load_global_constant: case nir_intrinsic_load_input: return true; default: break; } } FALLTHROUGH; 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 = false; nir_foreach_phi_src(src, phi) { /* This loop ignores srcs that are not scalarizable because its likely * still worth copying to temps if another phi source is scalarizable. * This reduces register spilling by a huge amount in the i965 driver for * Deus Ex: MD. */ scalarizable = is_phi_src_scalarizable(src, state); if (scalarizable) break; } /* The hash table entry for 'phi' may have changed while recursing the * dependence graph, so we need to reset it */ entry = _mesa_hash_table_search(state->phi_table, phi); assert(entry); entry->data = (void *)(intptr_t)scalarizable; return scalarizable; } static bool lower_phis_to_scalar_block(nir_block *block, struct lower_phis_to_scalar_state *state) { bool progress = false; /* Find the last phi node in the block */ nir_phi_instr *last_phi = NULL; nir_foreach_instr(instr, block) { 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(instr, block) { if (instr->type != nir_instr_type_phi) break; nir_phi_instr *phi = nir_instr_as_phi(instr); if (!should_lower_phi(phi, state)) continue; unsigned bit_size = phi->dest.ssa.bit_size; /* 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 = nir_op_vec(phi->dest.ssa.num_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, bit_size, 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, phi->dest.ssa.bit_size, NULL); vec->src[i].src = nir_src_for_ssa(&new_phi->dest.ssa); nir_foreach_phi_src(src, phi) { /* 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_mov); nir_ssa_dest_init(&mov->instr, &mov->dest.dest, 1, bit_size, 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(new_phi, 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, &vec->dest.dest.ssa); ralloc_steal(state->dead_ctx, phi); nir_instr_remove(&phi->instr); progress = true; /* 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 progress; } static bool lower_phis_to_scalar_impl(nir_function_impl *impl) { struct lower_phis_to_scalar_state state; bool progress = false; state.mem_ctx = ralloc_parent(impl); state.dead_ctx = ralloc_context(NULL); state.phi_table = _mesa_pointer_hash_table_create(state.dead_ctx); nir_foreach_block(block, impl) { progress = lower_phis_to_scalar_block(block, &state) || progress; } nir_metadata_preserve(impl, nir_metadata_block_index | nir_metadata_dominance); ralloc_free(state.dead_ctx); return progress; } /** 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. */ bool nir_lower_phis_to_scalar(nir_shader *shader) { bool progress = false; nir_foreach_function(function, shader) { if (function->impl) progress = lower_phis_to_scalar_impl(function->impl) || progress; } return progress; }