src/compiler/nir/nir_lower_phis_to_scalar.c

   1 /*
   2  * Copyright © 2015 Intel Corporation
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining a
   5  * copy of this software and associated documentation files (the "Software"),
   6  * to deal in the Software without restriction, including without limitation
   7  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   8  * and/or sell copies of the Software, and to permit persons to whom the
   9  * Software is furnished to do so, subject to the following conditions:
  10  *
  11  * The above copyright notice and this permission notice (including the next
  12  * paragraph) shall be included in all copies or substantial portions of the
  13  * Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  16  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  18  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  19  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  20  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  21  * IN THE SOFTWARE.
  22  *
  23  * Authors:
  24  *    Jason Ekstrand (jason@jlekstrand.net)
  25  *
  26  */
  27
  28 #include "nir.h"
  29
  30 /*
  31  * Implements a pass that lowers vector phi nodes to scalar phi nodes when
  32  * we don't think it will hurt anything.
  33  */
  34
  35 struct lower_phis_to_scalar_state {
  36    void *mem_ctx;
  37    void *dead_ctx;
  38
  39    /* Hash table marking which phi nodes are scalarizable.  The key is
  40     * pointers to phi instructions and the entry is either NULL for not
  41     * scalarizable or non-null for scalarizable.
  42     */
  43    struct hash_table *phi_table;
  44 };
  45
  46 static bool
  47 should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state);
  48
  49 static bool
  50 is_phi_src_scalarizable(nir_phi_src *src,
  51                         struct lower_phis_to_scalar_state *state)
  52 {
  53    /* Don't know what to do with non-ssa sources */
  54    if (!src->src.is_ssa)
  55       return false;
  56
  57    nir_instr *src_instr = src->src.ssa->parent_instr;
  58    switch (src_instr->type) {
  59    case nir_instr_type_alu: {
  60       nir_alu_instr *src_alu = nir_instr_as_alu(src_instr);
  61
  62       /* ALU operations with output_size == 0 should be scalarized.  We
  63        * will also see a bunch of vecN operations from scalarizing ALU
  64        * operations and, since they can easily be copy-propagated, they
  65        * are ok too.
  66        */
  67       return nir_op_infos[src_alu->op].output_size == 0 ||
  68              src_alu->op == nir_op_vec2 ||
  69              src_alu->op == nir_op_vec3 ||
  70              src_alu->op == nir_op_vec4;
  71    }
  72
  73    case nir_instr_type_phi:
  74       /* A phi is scalarizable if we're going to lower it */
  75       return should_lower_phi(nir_instr_as_phi(src_instr), state);
  76
  77    case nir_instr_type_load_const:
  78    case nir_instr_type_ssa_undef:
  79       /* These are trivially scalarizable */
  80       return true;
  81
  82    case nir_instr_type_intrinsic: {
  83       nir_intrinsic_instr *src_intrin = nir_instr_as_intrinsic(src_instr);
  84
  85       switch (src_intrin->intrinsic) {
  86       case nir_intrinsic_load_var:
  87          return src_intrin->variables[0]->var->data.mode == nir_var_shader_in ||
  88                 src_intrin->variables[0]->var->data.mode == nir_var_uniform;
  89
  90       case nir_intrinsic_interp_var_at_centroid:
  91       case nir_intrinsic_interp_var_at_sample:
  92       case nir_intrinsic_interp_var_at_offset:
  93       case nir_intrinsic_load_uniform:
  94       case nir_intrinsic_load_ubo:
  95       case nir_intrinsic_load_ssbo:
  96       case nir_intrinsic_load_input:
  97          return true;
  98       default:
  99          break;
 100       }
 101    }
 102
 103    default:
 104       /* We can't scalarize this type of instruction */
 105       return false;
 106    }
 107 }
 108
 109 /**
 110  * Determines if the given phi node should be lowered.  The only phi nodes
 111  * we will scalarize at the moment are those where all of the sources are
 112  * scalarizable.
 113  *
 114  * The reason for this comes down to coalescing.  Since phi sources can't
 115  * swizzle, swizzles on phis have to be resolved by inserting a mov right
 116  * before the phi.  The choice then becomes between movs to pick off
 117  * components for a scalar phi or potentially movs to recombine components
 118  * for a vector phi.  The problem is that the movs generated to pick off
 119  * the components are almost uncoalescable.  We can't coalesce them in NIR
 120  * because we need them to pick off components and we can't coalesce them
 121  * in the backend because the source register is a vector and the
 122  * destination is a scalar that may be used at other places in the program.
 123  * On the other hand, if we have a bunch of scalars going into a vector
 124  * phi, the situation is much better.  In this case, if the SSA def is
 125  * generated in the predecessor block to the corresponding phi source, the
 126  * backend code will be an ALU op into a temporary and then a mov into the
 127  * given vector component;  this move can almost certainly be coalesced
 128  * away.
 129  */
 130 static bool
 131 should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state)
 132 {
 133    /* Already scalar */
 134    if (phi->dest.ssa.num_components == 1)
 135       return false;
 136
 137    struct hash_entry *entry = _mesa_hash_table_search(state->phi_table, phi);
 138    if (entry)
 139       return entry->data != NULL;
 140
 141    /* Insert an entry and mark it as scalarizable for now. That way
 142     * we don't recurse forever and a cycle in the dependence graph
 143     * won't automatically make us fail to scalarize.
 144     */
 145    entry = _mesa_hash_table_insert(state->phi_table, phi, (void *)(intptr_t)1);
 146
 147    bool scalarizable = true;
 148
 149    nir_foreach_phi_src(phi, src) {
 150       scalarizable = is_phi_src_scalarizable(src, state);
 151       if (!scalarizable)
 152          break;
 153    }
 154
 155    /* The hash table entry for 'phi' may have changed while recursing the
 156     * dependence graph, so we need to reset it */
 157    entry = _mesa_hash_table_search(state->phi_table, phi);
 158    assert(entry);
 159
 160    entry->data = (void *)(intptr_t)scalarizable;
 161
 162    return scalarizable;
 163 }
 164
 165 static bool
 166 lower_phis_to_scalar_block(nir_block *block, void *void_state)
 167 {
 168    struct lower_phis_to_scalar_state *state = void_state;
 169
 170    /* Find the last phi node in the block */
 171    nir_phi_instr *last_phi = NULL;
 172    nir_foreach_instr(block, instr) {
 173       if (instr->type != nir_instr_type_phi)
 174          break;
 175
 176       last_phi = nir_instr_as_phi(instr);
 177    }
 178
 179    /* We have to handle the phi nodes in their own pass due to the way
 180     * we're modifying the linked list of instructions.
 181     */
 182    nir_foreach_instr_safe(block, instr) {
 183       if (instr->type != nir_instr_type_phi)
 184          break;
 185
 186       nir_phi_instr *phi = nir_instr_as_phi(instr);
 187
 188       if (!should_lower_phi(phi, state))
 189          continue;
 190
 191       /* Create a vecN operation to combine the results.  Most of these
 192        * will be redundant, but copy propagation should clean them up for
 193        * us.  No need to add the complexity here.
 194        */
 195       nir_op vec_op;
 196       switch (phi->dest.ssa.num_components) {
 197       case 2: vec_op = nir_op_vec2; break;
 198       case 3: vec_op = nir_op_vec3; break;
 199       case 4: vec_op = nir_op_vec4; break;
 200       default: unreachable("Invalid number of components");
 201       }
 202
 203       nir_alu_instr *vec = nir_alu_instr_create(state->mem_ctx, vec_op);
 204       nir_ssa_dest_init(&vec->instr, &vec->dest.dest,
 205                         phi->dest.ssa.num_components, NULL);
 206       vec->dest.write_mask = (1 << phi->dest.ssa.num_components) - 1;
 207
 208       for (unsigned i = 0; i < phi->dest.ssa.num_components; i++) {
 209          nir_phi_instr *new_phi = nir_phi_instr_create(state->mem_ctx);
 210          nir_ssa_dest_init(&new_phi->instr, &new_phi->dest, 1, NULL);
 211
 212          vec->src[i].src = nir_src_for_ssa(&new_phi->dest.ssa);
 213
 214          nir_foreach_phi_src(phi, src) {
 215             /* We need to insert a mov to grab the i'th component of src */
 216             nir_alu_instr *mov = nir_alu_instr_create(state->mem_ctx,
 217                                                       nir_op_imov);
 218             nir_ssa_dest_init(&mov->instr, &mov->dest.dest, 1, NULL);
 219             mov->dest.write_mask = 1;
 220             nir_src_copy(&mov->src[0].src, &src->src, state->mem_ctx);
 221             mov->src[0].swizzle[0] = i;
 222
 223             /* Insert at the end of the predecessor but before the jump */
 224             nir_instr *pred_last_instr = nir_block_last_instr(src->pred);
 225             if (pred_last_instr && pred_last_instr->type == nir_instr_type_jump)
 226                nir_instr_insert_before(pred_last_instr, &mov->instr);
 227             else
 228                nir_instr_insert_after_block(src->pred, &mov->instr);
 229
 230             nir_phi_src *new_src = ralloc(new_phi, nir_phi_src);
 231             new_src->pred = src->pred;
 232             new_src->src = nir_src_for_ssa(&mov->dest.dest.ssa);
 233
 234             exec_list_push_tail(&new_phi->srcs, &new_src->node);
 235          }
 236
 237          nir_instr_insert_before(&phi->instr, &new_phi->instr);
 238       }
 239
 240       nir_instr_insert_after(&last_phi->instr, &vec->instr);
 241
 242       nir_ssa_def_rewrite_uses(&phi->dest.ssa,
 243                                nir_src_for_ssa(&vec->dest.dest.ssa));
 244
 245       ralloc_steal(state->dead_ctx, phi);
 246       nir_instr_remove(&phi->instr);
 247
 248       /* We're using the safe iterator and inserting all the newly
 249        * scalarized phi nodes before their non-scalarized version so that's
 250        * ok.  However, we are also inserting vec operations after all of
 251        * the last phi node so once we get here, we can't trust even the
 252        * safe iterator to stop properly.  We have to break manually.
 253        */
 254       if (instr == &last_phi->instr)
 255          break;
 256    }
 257
 258    return true;
 259 }
 260
 261 static void
 262 lower_phis_to_scalar_impl(nir_function_impl *impl)
 263 {
 264    struct lower_phis_to_scalar_state state;
 265
 266    state.mem_ctx = ralloc_parent(impl);
 267    state.dead_ctx = ralloc_context(NULL);
 268    state.phi_table = _mesa_hash_table_create(state.dead_ctx, _mesa_hash_pointer,
 269                                              _mesa_key_pointer_equal);
 270
 271    nir_foreach_block(impl, lower_phis_to_scalar_block, &state);
 272
 273    nir_metadata_preserve(impl, nir_metadata_block_index |
 274                                nir_metadata_dominance);
 275
 276    ralloc_free(state.dead_ctx);
 277 }
 278
 279 /** A pass that lowers vector phi nodes to scalar
 280  *
 281  * This pass loops through the blocks and lowers looks for vector phi nodes
 282  * it can lower to scalar phi nodes.  Not all phi nodes are lowered.  For
 283  * instance, if one of the sources is a non-scalarizable vector, then we
 284  * don't bother lowering because that would generate hard-to-coalesce movs.
 285  */
 286 void
 287 nir_lower_phis_to_scalar(nir_shader *shader)
 288 {
 289    nir_foreach_function(shader, function) {
 290       if (function->impl)
 291          lower_phis_to_scalar_impl(function->impl);
 292    }
 293 }