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_deref: {
  87          nir_deref_instr *deref = nir_src_as_deref(src_intrin->src[0]);
  88          return deref->mode == nir_var_shader_in ||
  89                 deref->mode == nir_var_uniform;
  90       }
  91
  92       case nir_intrinsic_interp_deref_at_centroid:
  93       case nir_intrinsic_interp_deref_at_sample:
  94       case nir_intrinsic_interp_deref_at_offset:
  95       case nir_intrinsic_load_uniform:
  96       case nir_intrinsic_load_ubo:
  97       case nir_intrinsic_load_ssbo:
  98       case nir_intrinsic_load_input:
  99          return true;
 100       default:
 101          break;
 102       }
 103    }
 104
 105    default:
 106       /* We can't scalarize this type of instruction */
 107       return false;
 108    }
 109 }
 110
 111 /**
 112  * Determines if the given phi node should be lowered.  The only phi nodes
 113  * we will scalarize at the moment are those where all of the sources are
 114  * scalarizable.
 115  *
 116  * The reason for this comes down to coalescing.  Since phi sources can't
 117  * swizzle, swizzles on phis have to be resolved by inserting a mov right
 118  * before the phi.  The choice then becomes between movs to pick off
 119  * components for a scalar phi or potentially movs to recombine components
 120  * for a vector phi.  The problem is that the movs generated to pick off
 121  * the components are almost uncoalescable.  We can't coalesce them in NIR
 122  * because we need them to pick off components and we can't coalesce them
 123  * in the backend because the source register is a vector and the
 124  * destination is a scalar that may be used at other places in the program.
 125  * On the other hand, if we have a bunch of scalars going into a vector
 126  * phi, the situation is much better.  In this case, if the SSA def is
 127  * generated in the predecessor block to the corresponding phi source, the
 128  * backend code will be an ALU op into a temporary and then a mov into the
 129  * given vector component;  this move can almost certainly be coalesced
 130  * away.
 131  */
 132 static bool
 133 should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state)
 134 {
 135    /* Already scalar */
 136    if (phi->dest.ssa.num_components == 1)
 137       return false;
 138
 139    struct hash_entry *entry = _mesa_hash_table_search(state->phi_table, phi);
 140    if (entry)
 141       return entry->data != NULL;
 142
 143    /* Insert an entry and mark it as scalarizable for now. That way
 144     * we don't recurse forever and a cycle in the dependence graph
 145     * won't automatically make us fail to scalarize.
 146     */
 147    entry = _mesa_hash_table_insert(state->phi_table, phi, (void *)(intptr_t)1);
 148
 149    bool scalarizable = true;
 150
 151    nir_foreach_phi_src(src, phi) {
 152       scalarizable = is_phi_src_scalarizable(src, state);
 153       if (!scalarizable)
 154          break;
 155    }
 156
 157    /* The hash table entry for 'phi' may have changed while recursing the
 158     * dependence graph, so we need to reset it */
 159    entry = _mesa_hash_table_search(state->phi_table, phi);
 160    assert(entry);
 161
 162    entry->data = (void *)(intptr_t)scalarizable;
 163
 164    return scalarizable;
 165 }
 166
 167 static bool
 168 lower_phis_to_scalar_block(nir_block *block,
 169                            struct lower_phis_to_scalar_state *state)
 170 {
 171    bool progress = false;
 172
 173    /* Find the last phi node in the block */
 174    nir_phi_instr *last_phi = NULL;
 175    nir_foreach_instr(instr, block) {
 176       if (instr->type != nir_instr_type_phi)
 177          break;
 178
 179       last_phi = nir_instr_as_phi(instr);
 180    }
 181
 182    /* We have to handle the phi nodes in their own pass due to the way
 183     * we're modifying the linked list of instructions.
 184     */
 185    nir_foreach_instr_safe(instr, block) {
 186       if (instr->type != nir_instr_type_phi)
 187          break;
 188
 189       nir_phi_instr *phi = nir_instr_as_phi(instr);
 190
 191       if (!should_lower_phi(phi, state))
 192          continue;
 193
 194       unsigned bit_size = phi->dest.ssa.bit_size;
 195
 196       /* Create a vecN operation to combine the results.  Most of these
 197        * will be redundant, but copy propagation should clean them up for
 198        * us.  No need to add the complexity here.
 199        */
 200       nir_op vec_op;
 201       switch (phi->dest.ssa.num_components) {
 202       case 2: vec_op = nir_op_vec2; break;
 203       case 3: vec_op = nir_op_vec3; break;
 204       case 4: vec_op = nir_op_vec4; break;
 205       default: unreachable("Invalid number of components");
 206       }
 207
 208       nir_alu_instr *vec = nir_alu_instr_create(state->mem_ctx, vec_op);
 209       nir_ssa_dest_init(&vec->instr, &vec->dest.dest,
 210                         phi->dest.ssa.num_components,
 211                         bit_size, NULL);
 212       vec->dest.write_mask = (1 << phi->dest.ssa.num_components) - 1;
 213
 214       for (unsigned i = 0; i < phi->dest.ssa.num_components; i++) {
 215          nir_phi_instr *new_phi = nir_phi_instr_create(state->mem_ctx);
 216          nir_ssa_dest_init(&new_phi->instr, &new_phi->dest, 1,
 217                            phi->dest.ssa.bit_size, NULL);
 218
 219          vec->src[i].src = nir_src_for_ssa(&new_phi->dest.ssa);
 220
 221          nir_foreach_phi_src(src, phi) {
 222             /* We need to insert a mov to grab the i'th component of src */
 223             nir_alu_instr *mov = nir_alu_instr_create(state->mem_ctx,
 224                                                       nir_op_imov);
 225             nir_ssa_dest_init(&mov->instr, &mov->dest.dest, 1, bit_size, NULL);
 226             mov->dest.write_mask = 1;
 227             nir_src_copy(&mov->src[0].src, &src->src, state->mem_ctx);
 228             mov->src[0].swizzle[0] = i;
 229
 230             /* Insert at the end of the predecessor but before the jump */
 231             nir_instr *pred_last_instr = nir_block_last_instr(src->pred);
 232             if (pred_last_instr && pred_last_instr->type == nir_instr_type_jump)
 233                nir_instr_insert_before(pred_last_instr, &mov->instr);
 234             else
 235                nir_instr_insert_after_block(src->pred, &mov->instr);
 236
 237             nir_phi_src *new_src = ralloc(new_phi, nir_phi_src);
 238             new_src->pred = src->pred;
 239             new_src->src = nir_src_for_ssa(&mov->dest.dest.ssa);
 240
 241             exec_list_push_tail(&new_phi->srcs, &new_src->node);
 242          }
 243
 244          nir_instr_insert_before(&phi->instr, &new_phi->instr);
 245       }
 246
 247       nir_instr_insert_after(&last_phi->instr, &vec->instr);
 248
 249       nir_ssa_def_rewrite_uses(&phi->dest.ssa,
 250                                nir_src_for_ssa(&vec->dest.dest.ssa));
 251
 252       ralloc_steal(state->dead_ctx, phi);
 253       nir_instr_remove(&phi->instr);
 254
 255       progress = true;
 256
 257       /* We're using the safe iterator and inserting all the newly
 258        * scalarized phi nodes before their non-scalarized version so that's
 259        * ok.  However, we are also inserting vec operations after all of
 260        * the last phi node so once we get here, we can't trust even the
 261        * safe iterator to stop properly.  We have to break manually.
 262        */
 263       if (instr == &last_phi->instr)
 264          break;
 265    }
 266
 267    return progress;
 268 }
 269
 270 static bool
 271 lower_phis_to_scalar_impl(nir_function_impl *impl)
 272 {
 273    struct lower_phis_to_scalar_state state;
 274    bool progress = false;
 275
 276    state.mem_ctx = ralloc_parent(impl);
 277    state.dead_ctx = ralloc_context(NULL);
 278    state.phi_table = _mesa_hash_table_create(state.dead_ctx, _mesa_hash_pointer,
 279                                              _mesa_key_pointer_equal);
 280
 281    nir_foreach_block(block, impl) {
 282       progress = lower_phis_to_scalar_block(block, &state) || progress;
 283    }
 284
 285    nir_metadata_preserve(impl, nir_metadata_block_index |
 286                                nir_metadata_dominance);
 287
 288    ralloc_free(state.dead_ctx);
 289    return progress;
 290 }
 291
 292 /** A pass that lowers vector phi nodes to scalar
 293  *
 294  * This pass loops through the blocks and lowers looks for vector phi nodes
 295  * it can lower to scalar phi nodes.  Not all phi nodes are lowered.  For
 296  * instance, if one of the sources is a non-scalarizable vector, then we
 297  * don't bother lowering because that would generate hard-to-coalesce movs.
 298  */
 299 bool
 300 nir_lower_phis_to_scalar(nir_shader *shader)
 301 {
 302    bool progress = false;
 303
 304    nir_foreach_function(function, shader) {
 305       if (function->impl)
 306          progress = lower_phis_to_scalar_impl(function->impl) || progress;
 307    }
 308
 309    return progress;
 310 }