src/compiler/nir/nir_opt_peephole_select.c

   1 /*
   2  * Copyright © 2014 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 #include "nir_control_flow.h"
  30
  31 /*
  32  * Implements a small peephole optimization that looks for
  33  *
  34  * if (cond) {
  35  *    <then SSA defs>
  36  * } else {
  37  *    <else SSA defs>
  38  * }
  39  * phi
  40  * ...
  41  * phi
  42  *
  43  * and replaces it with:
  44  *
  45  * <then SSA defs>
  46  * <else SSA defs>
  47  * bcsel
  48  * ...
  49  * bcsel
  50  *
  51  * where the SSA defs are ALU operations or other cheap instructions (not
  52  * texturing, for example).
  53  *
  54  * If the number of ALU operations in the branches is greater than the limit
  55  * parameter, then the optimization is skipped.  In limit=0 mode, the SSA defs
  56  * must only be MOVs which we expect to get copy-propagated away once they're
  57  * out of the inner blocks.
  58  */
  59
  60 static bool
  61 block_check_for_allowed_instrs(nir_block *block, unsigned *count, bool alu_ok)
  62 {
  63    nir_foreach_instr(instr, block) {
  64       switch (instr->type) {
  65       case nir_instr_type_intrinsic: {
  66          nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
  67
  68          switch (intrin->intrinsic) {
  69          case nir_intrinsic_load_deref:
  70             switch (nir_src_as_deref(intrin->src[0])->mode) {
  71             case nir_var_shader_in:
  72             case nir_var_uniform:
  73                break;
  74
  75             default:
  76                return false;
  77             }
  78             break;
  79
  80          case nir_intrinsic_load_uniform:
  81             if (!alu_ok)
  82                return false;
  83             break;
  84
  85          default:
  86             return false;
  87          }
  88
  89          break;
  90       }
  91
  92       case nir_instr_type_deref:
  93       case nir_instr_type_load_const:
  94          break;
  95
  96       case nir_instr_type_alu: {
  97          nir_alu_instr *mov = nir_instr_as_alu(instr);
  98          switch (mov->op) {
  99          case nir_op_fmov:
 100          case nir_op_imov:
 101          case nir_op_fneg:
 102          case nir_op_ineg:
 103          case nir_op_fabs:
 104          case nir_op_iabs:
 105          case nir_op_vec2:
 106          case nir_op_vec3:
 107          case nir_op_vec4:
 108             break;
 109          default:
 110             if (!alu_ok) {
 111                /* It must be a move-like operation. */
 112                return false;
 113             }
 114             break;
 115          }
 116
 117          /* It must be SSA */
 118          if (!mov->dest.dest.is_ssa)
 119             return false;
 120
 121          if (alu_ok) {
 122             (*count)++;
 123          } else {
 124             /* Can't handle saturate */
 125             if (mov->dest.saturate)
 126                return false;
 127
 128             /* It cannot have any if-uses */
 129             if (!list_empty(&mov->dest.dest.ssa.if_uses))
 130                return false;
 131
 132             /* The only uses of this definition must be phis in the successor */
 133             nir_foreach_use(use, &mov->dest.dest.ssa) {
 134                if (use->parent_instr->type != nir_instr_type_phi ||
 135                    use->parent_instr->block != block->successors[0])
 136                   return false;
 137             }
 138          }
 139          break;
 140       }
 141
 142       default:
 143          return false;
 144       }
 145    }
 146
 147    return true;
 148 }
 149
 150 static bool
 151 nir_opt_peephole_select_block(nir_block *block, nir_shader *shader,
 152                               unsigned limit)
 153 {
 154    if (nir_cf_node_is_first(&block->cf_node))
 155       return false;
 156
 157    nir_cf_node *prev_node = nir_cf_node_prev(&block->cf_node);
 158    if (prev_node->type != nir_cf_node_if)
 159       return false;
 160
 161    nir_if *if_stmt = nir_cf_node_as_if(prev_node);
 162    nir_block *then_block = nir_if_first_then_block(if_stmt);
 163    nir_block *else_block = nir_if_first_else_block(if_stmt);
 164
 165    /* We can only have one block in each side ... */
 166    if (nir_if_last_then_block(if_stmt) != then_block ||
 167        nir_if_last_else_block(if_stmt) != else_block)
 168       return false;
 169
 170    /* ... and those blocks must only contain "allowed" instructions. */
 171    unsigned count = 0;
 172    if (!block_check_for_allowed_instrs(then_block, &count, limit != 0) ||
 173        !block_check_for_allowed_instrs(else_block, &count, limit != 0))
 174       return false;
 175
 176    if (count > limit)
 177       return false;
 178
 179    /* At this point, we know that the previous CFG node is an if-then
 180     * statement containing only moves to phi nodes in this block.  We can
 181     * just remove that entire CF node and replace all of the phi nodes with
 182     * selects.
 183     */
 184
 185    nir_block *prev_block = nir_cf_node_as_block(nir_cf_node_prev(prev_node));
 186
 187    /* First, we move the remaining instructions from the blocks to the
 188     * block before.  We have already guaranteed that this is safe by
 189     * calling block_check_for_allowed_instrs()
 190     */
 191    nir_foreach_instr_safe(instr, then_block) {
 192       exec_node_remove(&instr->node);
 193       instr->block = prev_block;
 194       exec_list_push_tail(&prev_block->instr_list, &instr->node);
 195    }
 196
 197    nir_foreach_instr_safe(instr, else_block) {
 198       exec_node_remove(&instr->node);
 199       instr->block = prev_block;
 200       exec_list_push_tail(&prev_block->instr_list, &instr->node);
 201    }
 202
 203    nir_foreach_instr_safe(instr, block) {
 204       if (instr->type != nir_instr_type_phi)
 205          break;
 206
 207       nir_phi_instr *phi = nir_instr_as_phi(instr);
 208       nir_alu_instr *sel = nir_alu_instr_create(shader, nir_op_b32csel);
 209       nir_src_copy(&sel->src[0].src, &if_stmt->condition, sel);
 210       /* Splat the condition to all channels */
 211       memset(sel->src[0].swizzle, 0, sizeof sel->src[0].swizzle);
 212
 213       assert(exec_list_length(&phi->srcs) == 2);
 214       nir_foreach_phi_src(src, phi) {
 215          assert(src->pred == then_block || src->pred == else_block);
 216          assert(src->src.is_ssa);
 217
 218          unsigned idx = src->pred == then_block ? 1 : 2;
 219          nir_src_copy(&sel->src[idx].src, &src->src, sel);
 220       }
 221
 222       nir_ssa_dest_init(&sel->instr, &sel->dest.dest,
 223                         phi->dest.ssa.num_components,
 224                         phi->dest.ssa.bit_size, phi->dest.ssa.name);
 225       sel->dest.write_mask = (1 << phi->dest.ssa.num_components) - 1;
 226
 227       nir_ssa_def_rewrite_uses(&phi->dest.ssa,
 228                                nir_src_for_ssa(&sel->dest.dest.ssa));
 229
 230       nir_instr_insert_before(&phi->instr, &sel->instr);
 231       nir_instr_remove(&phi->instr);
 232    }
 233
 234    nir_cf_node_remove(&if_stmt->cf_node);
 235    return true;
 236 }
 237
 238 static bool
 239 nir_opt_peephole_select_impl(nir_function_impl *impl, unsigned limit)
 240 {
 241    nir_shader *shader = impl->function->shader;
 242    bool progress = false;
 243
 244    nir_foreach_block_safe(block, impl) {
 245       progress |= nir_opt_peephole_select_block(block, shader, limit);
 246    }
 247
 248    if (progress)
 249       nir_metadata_preserve(impl, nir_metadata_none);
 250
 251    return progress;
 252 }
 253
 254 bool
 255 nir_opt_peephole_select(nir_shader *shader, unsigned limit)
 256 {
 257    bool progress = false;
 258
 259    nir_foreach_function(function, shader) {
 260       if (function->impl)
 261          progress |= nir_opt_peephole_select_impl(function->impl, limit);
 262    }
 263
 264    return progress;
 265 }