src/compiler/nir/nir_lower_discard_to_demote.c

/*
 * Copyright © 2020 Valve Corporation
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice (including the next
 * paragraph) shall be included in all copies or substantial portions of the
 * Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 *
 */

#include "nir.h"

/**
 * This pass is intended as workaround for game bugs to force correct
 * derivatives after kill. This lowering is not valid in the general case
 * as it might change the result of subgroup operations and loop behavior.
 *
 * discard() will be lowered as demote() and gl_HelperInvocation
 * will be lowered as helperInvocationEXT().
 */
bool
nir_lower_discard_to_demote(nir_shader *shader)
{
   if (shader->info.stage != MESA_SHADER_FRAGMENT)
      return false;

   bool progress = false;

   nir_foreach_function(function, shader) {
      nir_foreach_block(block, function->impl) {
         nir_foreach_instr(instr, block) {
            if (instr->type != nir_instr_type_intrinsic)
               continue;

            nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
            switch (intrin->intrinsic) {
            case nir_intrinsic_discard:
               intrin->intrinsic = nir_intrinsic_demote;
               shader->info.fs.uses_demote = true;
               break;
            case nir_intrinsic_discard_if:
               intrin->intrinsic = nir_intrinsic_demote_if;
               shader->info.fs.uses_demote = true;
               break;
            case nir_intrinsic_load_helper_invocation:
               intrin->intrinsic = nir_intrinsic_is_helper_invocation;
               break;
            default:
               continue;
            }
            progress = true;
         }
      }
   }

   return progress;
}