src/compiler/nir/nir_opt_move.c

   1 /*
   2  * Copyright © 2016 Intel Corporation
   3  * Copyright © 2019 Valve Corporation
   4  *
   5  * Permission is hereby granted, free of charge, to any person obtaining a
   6  * copy of this software and associated documentation files (the "Software"),
   7  * to deal in the Software without restriction, including without limitation
   8  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   9  * and/or sell copies of the Software, and to permit persons to whom the
  10  * Software is furnished to do so, subject to the following conditions:
  11  *
  12  * The above copyright notice and this permission notice (including the next
  13  * paragraph) shall be included in all copies or substantial portions of the
  14  * Software.
  15  *
  16  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  19  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  21  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  22  * IN THE SOFTWARE.
  23  */
  24
  25 #include "nir.h"
  26
  27 /**
  28  * \file nir_opt_move.c
  29  *
  30  * This pass can move various operations just before their first use inside the
  31  * same basic block. Usually this is to reduce register usage. It's probably
  32  * not a good idea to use this in an optimization loop.
  33  *
  34  * Moving comparisons is useful because many GPUs generate condition codes
  35  * for comparisons, and use predication for conditional selects and control
  36  * flow.  In a sequence such as:
  37  *
  38  *     vec1 32 ssa_1 = flt a b
  39  *     <some other operations>
  40  *     vec1 32 ssa_2 = bcsel ssa_1 c d
  41  *
  42  * the backend would likely do the comparison, producing condition codes,
  43  * then save those to a boolean value.  The intervening operations might
  44  * trash the condition codes.  Then, in order to do the bcsel, it would
  45  * need to re-populate the condition code register based on the boolean.
  46  *
  47  * By moving the comparison just before the bcsel, the condition codes could
  48  * be used directly.  This eliminates the need to reload them from the boolean
  49  * (generally eliminating an instruction).  It may also eliminate the need to
  50  * create a boolean value altogether (unless it's used elsewhere), which could
  51  * lower register pressure.
  52  */
  53
  54 static bool
  55 move_source(nir_src *src, nir_block *block, nir_instr *before, nir_move_options options)
  56 {
  57    if (!src->is_ssa)
  58       return false;
  59
  60    nir_instr *src_instr = src->ssa->parent_instr;
  61
  62    if (src_instr->block == block && nir_can_move_instr(src_instr, options)) {
  63       exec_node_remove(&src_instr->node);
  64
  65       if (before)
  66          exec_node_insert_node_before(&before->node, &src_instr->node);
  67       else
  68          exec_list_push_tail(&block->instr_list, &src_instr->node);
  69
  70       return true;
  71    }
  72    return false;
  73 }
  74
  75 struct source_cb_data {
  76    bool *progress;
  77    nir_move_options options;
  78 };
  79
  80 static bool
  81 move_source_cb(nir_src *src, void *data_ptr)
  82 {
  83    struct source_cb_data data = *(struct source_cb_data*)data_ptr;
  84
  85    nir_instr *instr = src->parent_instr;
  86    if (move_source(src, instr->block, instr, data.options))
  87       *data.progress = true;
  88
  89    return true; /* nir_foreach_src should keep going */
  90 }
  91
  92 static bool
  93 move(nir_block *block, nir_move_options options)
  94 {
  95    bool progress = false;
  96
  97    /* We use a simple approach: walk instructions backwards.
  98     *
  99     * If the instruction's source is a comparison from the same block,
 100     * simply move it here.  This may break SSA if it's used earlier in
 101     * the block as well.  However, as we walk backwards, we'll find the
 102     * earlier use and move it again, further up.  It eventually ends up
 103     * dominating all uses again, restoring SSA form.
 104     *
 105     * Before walking instructions, we consider the if-condition at the
 106     * end of the block, if one exists.  It's effectively a use at the
 107     * bottom of the block.
 108     */
 109    nir_if *iff = nir_block_get_following_if(block);
 110    if (iff) {
 111       progress |= move_source(&iff->condition, block, NULL, options);
 112    }
 113
 114    nir_foreach_instr_reverse(instr, block) {
 115       /* The sources of phi instructions happen after the predecessor block
 116        * but before this block.  (Yes, that's between blocks).  This means
 117        * that we don't need to move them in order for them to be correct.
 118        * We could move them to encourage comparisons that are used in a phi to
 119        * the end of the block, doing so correctly would make the pass
 120        * substantially more complicated and wouldn't gain us anything since
 121        * the phi can't use a flag value anyway.
 122        */
 123
 124       if (instr->type == nir_instr_type_phi) {
 125          /* We're going backwards so everything else is a phi too */
 126          break;
 127       } else if (instr->type == nir_instr_type_alu) {
 128          /* Walk ALU instruction sources backwards so that bcsel's boolean
 129           * condition is processed last for when comparisons are being moved.
 130           */
 131          nir_alu_instr *alu = nir_instr_as_alu(instr);
 132          for (int i = nir_op_infos[alu->op].num_inputs - 1; i >= 0; i--) {
 133             progress |= move_source(&alu->src[i].src, block, instr, options);
 134          }
 135       } else {
 136          struct source_cb_data data;
 137          data.progress = &progress;
 138          data.options = options;
 139          nir_foreach_src(instr, move_source_cb, &data);
 140       }
 141    }
 142
 143    return progress;
 144 }
 145
 146 bool
 147 nir_opt_move(nir_shader *shader, nir_move_options options)
 148 {
 149    bool progress = false;
 150
 151    nir_foreach_function(func, shader) {
 152       if (!func->impl)
 153          continue;
 154
 155       bool impl_progress = false;
 156       nir_foreach_block(block, func->impl) {
 157          if (move(block, options))
 158             impl_progress = true;
 159       }
 160
 161       if (impl_progress) {
 162          nir_metadata_preserve(func->impl, nir_metadata_block_index |
 163                                            nir_metadata_dominance |
 164                                            nir_metadata_live_ssa_defs);
 165          progress = true;
 166       } else {
 167          nir_metadata_preserve(func->impl, nir_metadata_all);
 168       }
 169    }
 170
 171    return progress;
 172 }