src/compiler/nir/nir_opt_move_comparisons.c

   1 /*
   2  * Copyright © 2016 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 "nir.h"
  25
  26 /**
  27  * \file nir_opt_move_comparisons.c
  28  *
  29  * This pass moves ALU comparison operations just before their first use.
  30  *
  31  * It only moves instructions within a single basic block; cross-block
  32  * movement is left to global code motion.
  33  *
  34  * Many GPUs generate condition codes for comparisons, and use predication
  35  * for conditional selects and control flow.  In a sequence such as:
  36  *
  37  *     vec1 32 ssa_1 = flt a b
  38  *     <some other operations>
  39  *     vec1 32 ssa_2 = bcsel ssa_1 c d
  40  *
  41  * the backend would likely do the comparison, producing condition codes,
  42  * then save those to a boolean value.  The intervening operations might
  43  * trash the condition codes.  Then, in order to do the bcsel, it would
  44  * need to re-populate the condition code register based on the boolean.
  45  *
  46  * By moving the comparison just before the bcsel, the condition codes could
  47  * be used directly.  This eliminates the need to reload them from the boolean
  48  * (generally eliminating an instruction).  It may also eliminate the need to
  49  * create a boolean value altogether (unless it's used elsewhere), which could
  50  * lower register pressure.
  51  */
  52
  53 static bool
  54 move_comparison_source(nir_src *src, nir_block *block, nir_instr *before)
  55 {
  56    if (!src->is_ssa)
  57       return false;
  58
  59    nir_instr *src_instr = src->ssa->parent_instr;
  60
  61    if (src_instr->block == block &&
  62        src_instr->type == nir_instr_type_alu &&
  63        nir_alu_instr_is_comparison(nir_instr_as_alu(src_instr))) {
  64
  65       exec_node_remove(&src_instr->node);
  66
  67       if (before)
  68          exec_node_insert_node_before(&before->node, &src_instr->node);
  69       else
  70          exec_list_push_tail(&block->instr_list, &src_instr->node);
  71
  72       return true;
  73    }
  74
  75    return false;
  76 }
  77
  78 static bool
  79 move_comparison_source_cb(nir_src *src, void *data)
  80 {
  81    bool *progress = data;
  82
  83    nir_instr *instr = src->parent_instr;
  84    if (move_comparison_source(src, instr->block, instr))
  85       *progress = true;
  86
  87    return true; /* nir_foreach_src should keep going */
  88 }
  89
  90 static bool
  91 move_comparisons(nir_block *block)
  92 {
  93    bool progress = false;
  94
  95    /* We use a simple approach: walk instructions backwards.
  96     *
  97     * If the instruction's source is a comparison from the same block,
  98     * simply move it here.  This may break SSA if it's used earlier in
  99     * the block as well.  However, as we walk backwards, we'll find the
 100     * earlier use and move it again, further up.  It eventually ends up
 101     * dominating all uses again, restoring SSA form.
 102     *
 103     * Before walking instructions, we consider the if-condition at the
 104     * end of the block, if one exists.  It's effectively a use at the
 105     * bottom of the block.
 106     */
 107    nir_if *iff = nir_block_get_following_if(block);
 108    if (iff) {
 109       progress |= move_comparison_source(&iff->condition, block, NULL);
 110    }
 111
 112    nir_foreach_instr_reverse(instr, block) {
 113       /* The sources of phi instructions happen after the predecessor block
 114        * but before this block.  (Yes, that's between blocks).  This means
 115        * that we don't need to move them in order for them to be correct.
 116        * We could move them to encourage comparisons that are used in a phi to
 117        * the end of the block, doing so correctly would make the pass
 118        * substantially more complicated and wouldn't gain us anything since
 119        * the phi can't use a flag value anyway.
 120        */
 121       if (instr->type == nir_instr_type_phi) {
 122          /* We're going backwards so everything else is a phi too */
 123          break;
 124       } else if (instr->type == nir_instr_type_alu) {
 125          /* Walk ALU instruction sources backwards so that bcsel's boolean
 126           * condition is processed last.
 127           */
 128          nir_alu_instr *alu = nir_instr_as_alu(instr);
 129          for (int i = nir_op_infos[alu->op].num_inputs - 1; i >= 0; i--) {
 130             progress |= move_comparison_source(&alu->src[i].src,
 131                                                block, instr);
 132          }
 133       } else {
 134          nir_foreach_src(instr, move_comparison_source_cb, &progress);
 135       }
 136    }
 137
 138    return progress;
 139 }
 140
 141 bool
 142 nir_opt_move_comparisons(nir_shader *shader)
 143 {
 144    bool progress = false;
 145
 146    nir_foreach_function(func, shader) {
 147       if (!func->impl)
 148          continue;
 149
 150       nir_foreach_block(block, func->impl) {
 151          if (move_comparisons(block)) {
 152             nir_metadata_preserve(func->impl, nir_metadata_block_index |
 153                                               nir_metadata_dominance |
 154                                               nir_metadata_live_ssa_defs);
 155             progress = true;
 156          }
 157       }
 158    }
 159
 160    return progress;
 161 }