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Diffstat (limited to 'src/compiler/nir/nir_lower_phis_to_scalar.c')
| -rw-r--r-- | src/compiler/nir/nir_lower_phis_to_scalar.c | 293 |
1 files changed, 293 insertions, 0 deletions
diff --git a/src/compiler/nir/nir_lower_phis_to_scalar.c b/src/compiler/nir/nir_lower_phis_to_scalar.c new file mode 100644 index 0000000..dd2abcf --- /dev/null +++ b/src/compiler/nir/nir_lower_phis_to_scalar.c @@ -0,0 +1,293 @@ +/* + * 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: + case nir_instr_type_ssa_undef: + /* 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_ubo: + case nir_intrinsic_load_ssbo: + case nir_intrinsic_load_input: + 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; + } + + /* 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, 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(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, + nir_src_for_ssa(&vec->dest.dest.ssa)); + + 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_function(shader, function) { + if (function->impl) + lower_phis_to_scalar_impl(function->impl); + } +} |