src/intel/compiler/brw_nir_analyze_ubo_ranges.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
  24 #include "brw_nir.h"
  25 #include "compiler/nir/nir.h"
  26 #include "util/u_dynarray.h"
  27
  28 /**
  29  * \file brw_nir_analyze_ubo_ranges.c
  30  *
  31  * This pass decides which portions of UBOs to upload as push constants,
  32  * so shaders can access them as part of the thread payload, rather than
  33  * having to issue expensive memory reads to pull the data.
  34  *
  35  * The 3DSTATE_CONSTANT_* mechanism can push data from up to 4 different
  36  * buffers, in GRF (256-bit/32-byte) units.
  37  *
  38  * To do this, we examine NIR load_ubo intrinsics, recording the number of
  39  * loads at each offset.  We track offsets at a 32-byte granularity, so even
  40  * fields with a bit of padding between them tend to fall into contiguous
  41  * ranges.  We build a list of these ranges, tracking their "cost" (number
  42  * of registers required) and "benefit" (number of pull loads eliminated
  43  * by pushing the range).  We then sort the list to obtain the four best
  44  * ranges (most benefit for the least cost).
  45  */
  46
  47 struct ubo_range_entry
  48 {
  49    struct brw_ubo_range range;
  50    int benefit;
  51 };
  52
  53 static int
  54 score(const struct ubo_range_entry *entry)
  55 {
  56    return 2 * entry->benefit - entry->range.length;
  57 }
  58
  59 /**
  60  * Compares score for two UBO range entries.
  61  *
  62  * For a descending qsort().
  63  */
  64 static int
  65 cmp_ubo_range_entry(const void *va, const void *vb)
  66 {
  67    const struct ubo_range_entry *a = va;
  68    const struct ubo_range_entry *b = vb;
  69
  70    /* Rank based on scores */
  71    int delta = score(b) - score(a);
  72
  73    /* Then use the UBO block index as a tie-breaker */
  74    if (delta == 0)
  75       delta = b->range.block - a->range.block;
  76
  77    /* Finally use the UBO offset as a second tie-breaker */
  78    if (delta == 0)
  79       delta = b->range.block - a->range.block;
  80
  81    return delta;
  82 }
  83
  84 struct ubo_block_info
  85 {
  86    /* Each bit in the offsets bitfield represents a 32-byte section of data.
  87     * If it's set to one, there is interesting UBO data at that offset.  If
  88     * not, there's a "hole" - padding between data - or just nothing at all.
  89     */
  90    uint64_t offsets;
  91    uint8_t uses[64];
  92 };
  93
  94 struct ubo_analysis_state
  95 {
  96    struct hash_table *blocks;
  97    bool uses_regular_uniforms;
  98 };
  99
 100 static struct ubo_block_info *
 101 get_block_info(struct ubo_analysis_state *state, int block)
 102 {
 103    uint32_t hash = block + 1;
 104    void *key = (void *) (uintptr_t) hash;
 105
 106    struct hash_entry *entry =
 107       _mesa_hash_table_search_pre_hashed(state->blocks, hash, key);
 108
 109    if (entry)
 110       return (struct ubo_block_info *) entry->data;
 111
 112    struct ubo_block_info *info =
 113       rzalloc(state->blocks, struct ubo_block_info);
 114    _mesa_hash_table_insert_pre_hashed(state->blocks, hash, key, info);
 115
 116    return info;
 117 }
 118
 119 static void
 120 analyze_ubos_block(struct ubo_analysis_state *state, nir_block *block)
 121 {
 122    nir_foreach_instr(instr, block) {
 123       if (instr->type != nir_instr_type_intrinsic)
 124          continue;
 125
 126       nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
 127       if (intrin->intrinsic == nir_intrinsic_load_uniform)
 128          state->uses_regular_uniforms = true;
 129
 130       if (intrin->intrinsic != nir_intrinsic_load_ubo)
 131          continue;
 132
 133       nir_const_value *block_const = nir_src_as_const_value(intrin->src[0]);
 134       nir_const_value *offset_const = nir_src_as_const_value(intrin->src[1]);
 135
 136       if (block_const && offset_const) {
 137          const int block = block_const->u32[0];
 138          const int offset = offset_const->u32[0] / 32;
 139
 140          /* Won't fit in our bitfield */
 141          if (offset >= 64)
 142             continue;
 143
 144          /* TODO: should we count uses in loops as higher benefit? */
 145
 146          struct ubo_block_info *info = get_block_info(state, block);
 147          info->offsets |= 1ull << offset;
 148          info->uses[offset]++;
 149       }
 150    }
 151 }
 152
 153 static void
 154 print_ubo_entry(FILE *file,
 155                 const struct ubo_range_entry *entry,
 156                 struct ubo_analysis_state *state)
 157 {
 158    struct ubo_block_info *info = get_block_info(state, entry->range.block);
 159
 160    fprintf(file,
 161            "block %2d, start %2d, length %2d, bits = %"PRIx64", "
 162            "benefit %2d, cost %2d, score = %2d\n",
 163            entry->range.block, entry->range.start, entry->range.length,
 164            info->offsets, entry->benefit, entry->range.length, score(entry));
 165 }
 166
 167 void
 168 brw_nir_analyze_ubo_ranges(const struct brw_compiler *compiler,
 169                            nir_shader *nir,
 170                            struct brw_ubo_range out_ranges[4])
 171 {
 172    const struct gen_device_info *devinfo = compiler->devinfo;
 173
 174    if ((devinfo->gen <= 7 && !devinfo->is_haswell) ||
 175        !compiler->scalar_stage[nir->info.stage]) {
 176       memset(out_ranges, 0, 4 * sizeof(struct brw_ubo_range));
 177       return;
 178    }
 179
 180    void *mem_ctx = ralloc_context(NULL);
 181
 182    struct ubo_analysis_state state = {
 183       .uses_regular_uniforms = false,
 184       .blocks =
 185          _mesa_hash_table_create(mem_ctx, NULL, _mesa_key_pointer_equal),
 186    };
 187
 188    /* Walk the IR, recording how many times each UBO block/offset is used. */
 189    nir_foreach_function(function, nir) {
 190       if (function->impl) {
 191          nir_foreach_block(block, function->impl) {
 192             analyze_ubos_block(&state, block);
 193          }
 194       }
 195    }
 196
 197    /* Find ranges: a block, starting 32-byte offset, and length. */
 198    struct util_dynarray ranges;
 199    util_dynarray_init(&ranges, mem_ctx);
 200
 201    struct hash_entry *entry;
 202    hash_table_foreach(state.blocks, entry) {
 203       const int b = entry->hash - 1;
 204       const struct ubo_block_info *info = entry->data;
 205       uint64_t offsets = info->offsets;
 206
 207       /* Walk through the offsets bitfield, finding contiguous regions of
 208        * set bits:
 209        *
 210        *   0000000001111111111111000000000000111111111111110000000011111100
 211        *            ^^^^^^^^^^^^^            ^^^^^^^^^^^^^^        ^^^^^^
 212        *
 213        * Each of these will become a UBO range.
 214        */
 215       while (offsets != 0) {
 216          /* Find the first 1 in the offsets bitfield.  This represents the
 217           * start of a range of interesting UBO data.  Make it zero-indexed.
 218           */
 219          int first_bit = ffsll(offsets) - 1;
 220
 221          /* Find the first 0 bit in offsets beyond first_bit.  To find the
 222           * first zero bit, we find the first 1 bit in the complement.  In
 223           * order to ignore bits before first_bit, we mask off those bits.
 224           */
 225          int first_hole = ffsll(~offsets & ~((1ull << first_bit) - 1)) - 1;
 226
 227          if (first_hole == -1) {
 228             /* If we didn't find a hole, then set it to the end of the
 229              * bitfield.  There are no more ranges to process.
 230              */
 231             first_hole = 64;
 232             offsets = 0;
 233          } else {
 234             /* We've processed all bits before first_hole.  Mask them off. */
 235             offsets &= ~((1ull << first_hole) - 1);
 236          }
 237
 238          struct ubo_range_entry *entry =
 239             util_dynarray_grow(&ranges, sizeof(struct ubo_range_entry));
 240
 241          entry->range.block = b;
 242          entry->range.start = first_bit;
 243          /* first_hole is one beyond the end, so we don't need to add 1 */
 244          entry->range.length = first_hole - first_bit;
 245          entry->benefit = 0;
 246
 247          for (int i = 0; i < entry->range.length; i++)
 248             entry->benefit += info->uses[first_bit + i];
 249       }
 250    }
 251
 252    int nr_entries = ranges.size / sizeof(struct ubo_range_entry);
 253
 254    if (0) {
 255       util_dynarray_foreach(&ranges, struct ubo_range_entry, entry) {
 256          print_ubo_entry(stderr, entry, &state);
 257       }
 258    }
 259
 260    /* TODO: Consider combining ranges.
 261     *
 262     * We can only push 3-4 ranges via 3DSTATE_CONSTANT_XS.  If there are
 263     * more ranges, and two are close by with only a small hole, it may be
 264     * worth combining them.  The holes will waste register space, but the
 265     * benefit of removing pulls may outweigh that cost.
 266     */
 267
 268    /* Sort the list so the most beneficial ranges are at the front. */
 269    qsort(ranges.data, nr_entries, sizeof(struct ubo_range_entry),
 270          cmp_ubo_range_entry);
 271
 272    struct ubo_range_entry *entries = ranges.data;
 273
 274    /* Return the top 4 or so.  We drop by one if regular uniforms are in
 275     * use, assuming one push buffer will be dedicated to those.  We may
 276     * also only get 3 on Haswell if we can't write INSTPM.
 277     *
 278     * The backend may need to shrink these ranges to ensure that they
 279     * don't exceed the maximum push constant limits.  It can simply drop
 280     * the tail of the list, as that's the least valuable portion.  We
 281     * unfortunately can't truncate it here, because we don't know what
 282     * the backend is planning to do with regular uniforms.
 283     */
 284    const int max_ubos = (compiler->constant_buffer_0_is_relative ? 3 : 4) -
 285                         state.uses_regular_uniforms;
 286    nr_entries = MIN2(nr_entries, max_ubos);
 287
 288    for (int i = 0; i < nr_entries; i++) {
 289       out_ranges[i] = entries[i].range;
 290    }
 291    for (int i = nr_entries; i < 4; i++) {
 292       out_ranges[i].block = 0;
 293       out_ranges[i].start = 0;
 294       out_ranges[i].length = 0;
 295    }
 296
 297    ralloc_free(ranges.mem_ctx);
 298 }