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              nir_op_is_vec(src_alu->op);
  69    }
  70
  71    case nir_instr_type_phi:
  72       /* A phi is scalarizable if we're going to lower it */
  73       return should_lower_phi(nir_instr_as_phi(src_instr), state);
  74
  75    case nir_instr_type_load_const:
  76       /* These are trivially scalarizable */
  77       return true;
  78
  79    case nir_instr_type_ssa_undef:
  80       /* The caller of this function is going to OR the results and we don't
  81        * want undefs to count so we return false.
  82        */
  83       return false;
  84
  85    case nir_instr_type_intrinsic: {
  86       nir_intrinsic_instr *src_intrin = nir_instr_as_intrinsic(src_instr);
  87
  88       switch (src_intrin->intrinsic) {
  89       case nir_intrinsic_load_deref: {
  90          nir_deref_instr *deref = nir_src_as_deref(src_intrin->src[0]);
  91          return deref->mode == nir_var_shader_in ||
  92                 deref->mode == nir_var_uniform ||
  93                 deref->mode == nir_var_mem_ubo ||
  94                 deref->mode == nir_var_mem_ssbo ||
  95                 deref->mode == nir_var_mem_global;
  96       }
  97
  98       case nir_intrinsic_interp_deref_at_centroid:
  99       case nir_intrinsic_interp_deref_at_sample:
 100       case nir_intrinsic_interp_deref_at_offset:
 101       case nir_intrinsic_interp_deref_at_vertex:
 102       case nir_intrinsic_load_uniform:
 103       case nir_intrinsic_load_ubo:
 104       case nir_intrinsic_load_ssbo:
 105       case nir_intrinsic_load_global:
 106       case nir_intrinsic_load_input:
 107          return true;
 108       default:
 109          break;
 110       }
 111    }
 112    /* fallthrough */
 113
 114    default:
 115       /* We can't scalarize this type of instruction */
 116       return false;
 117    }
 118 }
 119
 120 /**
 121  * Determines if the given phi node should be lowered.  The only phi nodes
 122  * we will scalarize at the moment are those where all of the sources are
 123  * scalarizable.
 124  *
 125  * The reason for this comes down to coalescing.  Since phi sources can't
 126  * swizzle, swizzles on phis have to be resolved by inserting a mov right
 127  * before the phi.  The choice then becomes between movs to pick off
 128  * components for a scalar phi or potentially movs to recombine components
 129  * for a vector phi.  The problem is that the movs generated to pick off
 130  * the components are almost uncoalescable.  We can't coalesce them in NIR
 131  * because we need them to pick off components and we can't coalesce them
 132  * in the backend because the source register is a vector and the
 133  * destination is a scalar that may be used at other places in the program.
 134  * On the other hand, if we have a bunch of scalars going into a vector
 135  * phi, the situation is much better.  In this case, if the SSA def is
 136  * generated in the predecessor block to the corresponding phi source, the
 137  * backend code will be an ALU op into a temporary and then a mov into the
 138  * given vector component;  this move can almost certainly be coalesced
 139  * away.
 140  */
 141 static bool
 142 should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state)
 143 {
 144    /* Already scalar */
 145    if (phi->dest.ssa.num_components == 1)
 146       return false;
 147
 148    struct hash_entry *entry = _mesa_hash_table_search(state->phi_table, phi);
 149    if (entry)
 150       return entry->data != NULL;
 151
 152    /* Insert an entry and mark it as scalarizable for now. That way
 153     * we don't recurse forever and a cycle in the dependence graph
 154     * won't automatically make us fail to scalarize.
 155     */
 156    entry = _mesa_hash_table_insert(state->phi_table, phi, (void *)(intptr_t)1);
 157
 158    bool scalarizable = false;
 159
 160    nir_foreach_phi_src(src, phi) {
 161       /* This loop ignores srcs that are not scalarizable because its likely
 162        * still worth copying to temps if another phi source is scalarizable.
 163        * This reduces register spilling by a huge amount in the i965 driver for
 164        * Deus Ex: MD.
 165        */
 166       scalarizable = is_phi_src_scalarizable(src, state);
 167       if (scalarizable)
 168          break;
 169    }
 170
 171    /* The hash table entry for 'phi' may have changed while recursing the
 172     * dependence graph, so we need to reset it */
 173    entry = _mesa_hash_table_search(state->phi_table, phi);
 174    assert(entry);
 175
 176    entry->data = (void *)(intptr_t)scalarizable;
 177
 178    return scalarizable;
 179 }
 180
 181 static bool
 182 lower_phis_to_scalar_block(nir_block *block,
 183                            struct lower_phis_to_scalar_state *state)
 184 {
 185    bool progress = false;
 186
 187    /* Find the last phi node in the block */
 188    nir_phi_instr *last_phi = NULL;
 189    nir_foreach_instr(instr, block) {
 190       if (instr->type != nir_instr_type_phi)
 191          break;
 192
 193       last_phi = nir_instr_as_phi(instr);
 194    }
 195
 196    /* We have to handle the phi nodes in their own pass due to the way
 197     * we're modifying the linked list of instructions.
 198     */
 199    nir_foreach_instr_safe(instr, block) {
 200       if (instr->type != nir_instr_type_phi)
 201          break;
 202
 203       nir_phi_instr *phi = nir_instr_as_phi(instr);
 204
 205       if (!should_lower_phi(phi, state))
 206          continue;
 207
 208       unsigned bit_size = phi->dest.ssa.bit_size;
 209
 210       /* Create a vecN operation to combine the results.  Most of these
 211        * will be redundant, but copy propagation should clean them up for
 212        * us.  No need to add the complexity here.
 213        */
 214       nir_op vec_op = nir_op_vec(phi->dest.ssa.num_components);
 215
 216       nir_alu_instr *vec = nir_alu_instr_create(state->mem_ctx, vec_op);
 217       nir_ssa_dest_init(&vec->instr, &vec->dest.dest,
 218                         phi->dest.ssa.num_components,
 219                         bit_size, NULL);
 220       vec->dest.write_mask = (1 << phi->dest.ssa.num_components) - 1;
 221
 222       for (unsigned i = 0; i < phi->dest.ssa.num_components; i++) {
 223          nir_phi_instr *new_phi = nir_phi_instr_create(state->mem_ctx);
 224          nir_ssa_dest_init(&new_phi->instr, &new_phi->dest, 1,
 225                            phi->dest.ssa.bit_size, NULL);
 226
 227          vec->src[i].src = nir_src_for_ssa(&new_phi->dest.ssa);
 228
 229          nir_foreach_phi_src(src, phi) {
 230             /* We need to insert a mov to grab the i'th component of src */
 231             nir_alu_instr *mov = nir_alu_instr_create(state->mem_ctx,
 232                                                       nir_op_mov);
 233             nir_ssa_dest_init(&mov->instr, &mov->dest.dest, 1, bit_size, NULL);
 234             mov->dest.write_mask = 1;
 235             nir_src_copy(&mov->src[0].src, &src->src, state->mem_ctx);
 236             mov->src[0].swizzle[0] = i;
 237
 238             /* Insert at the end of the predecessor but before the jump */
 239             nir_instr *pred_last_instr = nir_block_last_instr(src->pred);
 240             if (pred_last_instr && pred_last_instr->type == nir_instr_type_jump)
 241                nir_instr_insert_before(pred_last_instr, &mov->instr);
 242             else
 243                nir_instr_insert_after_block(src->pred, &mov->instr);
 244
 245             nir_phi_src *new_src = ralloc(new_phi, nir_phi_src);
 246             new_src->pred = src->pred;
 247             new_src->src = nir_src_for_ssa(&mov->dest.dest.ssa);
 248
 249             exec_list_push_tail(&new_phi->srcs, &new_src->node);
 250          }
 251
 252          nir_instr_insert_before(&phi->instr, &new_phi->instr);
 253       }
 254
 255       nir_instr_insert_after(&last_phi->instr, &vec->instr);
 256
 257       nir_ssa_def_rewrite_uses(&phi->dest.ssa,
 258                                nir_src_for_ssa(&vec->dest.dest.ssa));
 259
 260       ralloc_steal(state->dead_ctx, phi);
 261       nir_instr_remove(&phi->instr);
 262
 263       progress = true;
 264
 265       /* We're using the safe iterator and inserting all the newly
 266        * scalarized phi nodes before their non-scalarized version so that's
 267        * ok.  However, we are also inserting vec operations after all of
 268        * the last phi node so once we get here, we can't trust even the
 269        * safe iterator to stop properly.  We have to break manually.
 270        */
 271       if (instr == &last_phi->instr)
 272          break;
 273    }
 274
 275    return progress;
 276 }
 277
 278 static bool
 279 lower_phis_to_scalar_impl(nir_function_impl *impl)
 280 {
 281    struct lower_phis_to_scalar_state state;
 282    bool progress = false;
 283
 284    state.mem_ctx = ralloc_parent(impl);
 285    state.dead_ctx = ralloc_context(NULL);
 286    state.phi_table = _mesa_pointer_hash_table_create(state.dead_ctx);
 287
 288    nir_foreach_block(block, impl) {
 289       progress = lower_phis_to_scalar_block(block, &state) || progress;
 290    }
 291
 292    nir_metadata_preserve(impl, nir_metadata_block_index |
 293                                nir_metadata_dominance);
 294
 295    ralloc_free(state.dead_ctx);
 296    return progress;
 297 }
 298
 299 /** A pass that lowers vector phi nodes to scalar
 300  *
 301  * This pass loops through the blocks and lowers looks for vector phi nodes
 302  * it can lower to scalar phi nodes.  Not all phi nodes are lowered.  For
 303  * instance, if one of the sources is a non-scalarizable vector, then we
 304  * don't bother lowering because that would generate hard-to-coalesce movs.
 305  */
 306 bool
 307 nir_lower_phis_to_scalar(nir_shader *shader)
 308 {
 309    bool progress = false;
 310
 311    nir_foreach_function(function, shader) {
 312       if (function->impl)
 313          progress = lower_phis_to_scalar_impl(function->impl) || progress;
 314    }
 315
 316    return progress;
 317 }