src/intel/compiler/brw_nir_opt_peephole_ffma.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 "brw_nir.h"
  29 #include "compiler/nir/nir_builder.h"
  30
  31 /*
  32  * Implements a small peephole optimization that looks for a multiply that
  33  * is only ever used in an add and replaces both with an fma.
  34  */
  35
  36 static inline bool
  37 are_all_uses_fadd(nir_ssa_def *def)
  38 {
  39    if (!list_is_empty(&def->if_uses))
  40       return false;
  41
  42    nir_foreach_use(use_src, def) {
  43       nir_instr *use_instr = use_src->parent_instr;
  44
  45       if (use_instr->type != nir_instr_type_alu)
  46          return false;
  47
  48       nir_alu_instr *use_alu = nir_instr_as_alu(use_instr);
  49       switch (use_alu->op) {
  50       case nir_op_fadd:
  51          break; /* This one's ok */
  52
  53       case nir_op_mov:
  54       case nir_op_fneg:
  55       case nir_op_fabs:
  56          assert(use_alu->dest.dest.is_ssa);
  57          if (!are_all_uses_fadd(&use_alu->dest.dest.ssa))
  58             return false;
  59          break;
  60
  61       default:
  62          return false;
  63       }
  64    }
  65
  66    return true;
  67 }
  68
  69 static nir_alu_instr *
  70 get_mul_for_src(nir_alu_src *src, unsigned num_components,
  71                 uint8_t swizzle[4], bool *negate, bool *abs)
  72 {
  73    uint8_t swizzle_tmp[4];
  74    assert(src->src.is_ssa && !src->abs && !src->negate);
  75
  76    nir_instr *instr = src->src.ssa->parent_instr;
  77    if (instr->type != nir_instr_type_alu)
  78       return NULL;
  79
  80    nir_alu_instr *alu = nir_instr_as_alu(instr);
  81
  82    /* We want to bail if any of the other ALU operations involved is labled
  83     * exact.  One reason for this is that, while the value that is changing is
  84     * actually the result of the add and not the multiply, the intention of
  85     * the user when they specify an exact multiply is that they want *that*
  86     * value and what they don't care about is the add.  Another reason is that
  87     * SPIR-V explicitly requires this behaviour.
  88     */
  89    if (alu->exact)
  90       return NULL;
  91
  92    switch (alu->op) {
  93    case nir_op_mov:
  94       alu = get_mul_for_src(&alu->src[0], alu->dest.dest.ssa.num_components,
  95                             swizzle, negate, abs);
  96       break;
  97
  98    case nir_op_fneg:
  99       alu = get_mul_for_src(&alu->src[0], alu->dest.dest.ssa.num_components,
 100                             swizzle, negate, abs);
 101       *negate = !*negate;
 102       break;
 103
 104    case nir_op_fabs:
 105       alu = get_mul_for_src(&alu->src[0], alu->dest.dest.ssa.num_components,
 106                             swizzle, negate, abs);
 107       *negate = false;
 108       *abs = true;
 109       break;
 110
 111    case nir_op_fmul:
 112       /* Only absorb a fmul into a ffma if the fmul is only used in fadd
 113        * operations.  This prevents us from being too aggressive with our
 114        * fusing which can actually lead to more instructions.
 115        */
 116       if (!are_all_uses_fadd(&alu->dest.dest.ssa))
 117          return NULL;
 118       break;
 119
 120    default:
 121       return NULL;
 122    }
 123
 124    if (!alu)
 125       return NULL;
 126
 127    /* Copy swizzle data before overwriting it to avoid setting a wrong swizzle.
 128     *
 129     * Example:
 130     *   Former swizzle[] = xyzw
 131     *   src->swizzle[] = zyxx
 132     *
 133     *   Expected output swizzle = zyxx
 134     *   If we reuse swizzle in the loop, then output swizzle would be zyzz.
 135     */
 136    memcpy(swizzle_tmp, swizzle, 4*sizeof(uint8_t));
 137    for (int i = 0; i < num_components; i++)
 138       swizzle[i] = swizzle_tmp[src->swizzle[i]];
 139
 140    return alu;
 141 }
 142
 143 /**
 144  * Given a list of (at least two) nir_alu_src's, tells if any of them is a
 145  * constant value and is used only once.
 146  */
 147 static bool
 148 any_alu_src_is_a_constant(nir_alu_src srcs[])
 149 {
 150    for (unsigned i = 0; i < 2; i++) {
 151       if (srcs[i].src.ssa->parent_instr->type == nir_instr_type_load_const) {
 152          nir_load_const_instr *load_const =
 153             nir_instr_as_load_const (srcs[i].src.ssa->parent_instr);
 154
 155          if (list_is_singular(&load_const->def.uses) &&
 156              list_is_empty(&load_const->def.if_uses)) {
 157             return true;
 158          }
 159       }
 160    }
 161
 162    return false;
 163 }
 164
 165 static bool
 166 brw_nir_opt_peephole_ffma_block(nir_builder *b, nir_block *block)
 167 {
 168    bool progress = false;
 169
 170    nir_foreach_instr_safe(instr, block) {
 171       if (instr->type != nir_instr_type_alu)
 172          continue;
 173
 174       nir_alu_instr *add = nir_instr_as_alu(instr);
 175       if (add->op != nir_op_fadd)
 176          continue;
 177
 178       assert(add->dest.dest.is_ssa);
 179       if (add->exact)
 180          continue;
 181
 182       assert(add->src[0].src.is_ssa && add->src[1].src.is_ssa);
 183
 184       /* This, is the case a + a.  We would rather handle this with an
 185        * algebraic reduction than fuse it.  Also, we want to only fuse
 186        * things where the multiply is used only once and, in this case,
 187        * it would be used twice by the same instruction.
 188        */
 189       if (add->src[0].src.ssa == add->src[1].src.ssa)
 190          continue;
 191
 192       nir_alu_instr *mul;
 193       uint8_t add_mul_src, swizzle[4];
 194       bool negate, abs;
 195       for (add_mul_src = 0; add_mul_src < 2; add_mul_src++) {
 196          for (unsigned i = 0; i < 4; i++)
 197             swizzle[i] = i;
 198
 199          negate = false;
 200          abs = false;
 201
 202          mul = get_mul_for_src(&add->src[add_mul_src],
 203                                add->dest.dest.ssa.num_components,
 204                                swizzle, &negate, &abs);
 205
 206          if (mul != NULL)
 207             break;
 208       }
 209
 210       if (mul == NULL)
 211          continue;
 212
 213       unsigned bit_size = add->dest.dest.ssa.bit_size;
 214
 215       nir_ssa_def *mul_src[2];
 216       mul_src[0] = mul->src[0].src.ssa;
 217       mul_src[1] = mul->src[1].src.ssa;
 218
 219       /* If any of the operands of the fmul and any of the fadd is a constant,
 220        * we bypass because it will be more efficient as the constants will be
 221        * propagated as operands, potentially saving two load_const instructions.
 222        */
 223       if (any_alu_src_is_a_constant(mul->src) &&
 224           any_alu_src_is_a_constant(add->src)) {
 225          continue;
 226       }
 227
 228       b->cursor = nir_before_instr(&add->instr);
 229
 230       if (abs) {
 231          for (unsigned i = 0; i < 2; i++)
 232             mul_src[i] = nir_fabs(b, mul_src[i]);
 233       }
 234
 235       if (negate)
 236          mul_src[0] = nir_fneg(b, mul_src[0]);
 237
 238       nir_alu_instr *ffma = nir_alu_instr_create(b->shader, nir_op_ffma);
 239       ffma->dest.saturate = add->dest.saturate;
 240       ffma->dest.write_mask = add->dest.write_mask;
 241
 242       for (unsigned i = 0; i < 2; i++) {
 243          ffma->src[i].src = nir_src_for_ssa(mul_src[i]);
 244          for (unsigned j = 0; j < add->dest.dest.ssa.num_components; j++)
 245             ffma->src[i].swizzle[j] = mul->src[i].swizzle[swizzle[j]];
 246       }
 247       nir_alu_src_copy(&ffma->src[2], &add->src[1 - add_mul_src], ffma);
 248
 249       assert(add->dest.dest.is_ssa);
 250
 251       nir_ssa_dest_init(&ffma->instr, &ffma->dest.dest,
 252                         add->dest.dest.ssa.num_components,
 253                         bit_size,
 254                         add->dest.dest.ssa.name);
 255       nir_ssa_def_rewrite_uses(&add->dest.dest.ssa,
 256                                nir_src_for_ssa(&ffma->dest.dest.ssa));
 257
 258       nir_builder_instr_insert(b, &ffma->instr);
 259       assert(list_is_empty(&add->dest.dest.ssa.uses));
 260       nir_instr_remove(&add->instr);
 261
 262       progress = true;
 263    }
 264
 265    return progress;
 266 }
 267
 268 static bool
 269 brw_nir_opt_peephole_ffma_impl(nir_function_impl *impl)
 270 {
 271    bool progress = false;
 272
 273    nir_builder builder;
 274    nir_builder_init(&builder, impl);
 275
 276    nir_foreach_block(block, impl) {
 277       progress |= brw_nir_opt_peephole_ffma_block(&builder, block);
 278    }
 279
 280    if (progress) {
 281       nir_metadata_preserve(impl, nir_metadata_block_index |
 282                                   nir_metadata_dominance);
 283    } else {
 284       nir_metadata_preserve(impl, nir_metadata_all);
 285    }
 286
 287    return progress;
 288 }
 289
 290 bool
 291 brw_nir_opt_peephole_ffma(nir_shader *shader)
 292 {
 293    bool progress = false;
 294
 295    nir_foreach_function(function, shader) {
 296       if (function->impl)
 297          progress |= brw_nir_opt_peephole_ffma_impl(function->impl);
 298    }
 299
 300    return progress;
 301 }