src/compiler/nir/nir_lower_multiview.c - platform/external/mesa3d - Git at Google

 /*
  * Copyright © 2016 Intel Corporation
  * 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_control_flow.h"
 #include "nir_builder.h"

 /**
  * This file implements an optimization for multiview. Some GPU's have a
  * special mode which allows the vertex shader (or last stage in the geometry
  * pipeline) to create multiple primitives in different layers of the
  * framebuffer at once by writing multiple copies of gl_Position. The
  * assumption is that in most uses of multiview, the only use of gl_ViewIndex
  * is to change the position to implement the parallax effect, and other
  * varyings will be the same between the different views. We put the body of
  * the original vertex shader in a loop, writing to a different copy of
  * gl_Position each loop iteration, and then let other optimizations clean up
  * the mess.
  */

 static bool
 shader_writes_to_memory(nir_shader *shader)
 {
    /* With multiview, we would need to ensure that memory writes happen either
     * once or once per view. Since combination of multiview and memory writes
     * is not expected, we'll just skip this optimization in this case.
     */

    nir_function_impl *entrypoint = nir_shader_get_entrypoint(shader);

    nir_foreach_block(block, entrypoint) {
       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_deref_atomic_add:
          case nir_intrinsic_deref_atomic_imin:
          case nir_intrinsic_deref_atomic_umin:
          case nir_intrinsic_deref_atomic_imax:
          case nir_intrinsic_deref_atomic_umax:
          case nir_intrinsic_deref_atomic_and:
          case nir_intrinsic_deref_atomic_or:
          case nir_intrinsic_deref_atomic_xor:
          case nir_intrinsic_deref_atomic_exchange:
          case nir_intrinsic_deref_atomic_comp_swap:
          case nir_intrinsic_store_ssbo:
          case nir_intrinsic_ssbo_atomic_add:
          case nir_intrinsic_ssbo_atomic_imin:
          case nir_intrinsic_ssbo_atomic_umin:
          case nir_intrinsic_ssbo_atomic_imax:
          case nir_intrinsic_ssbo_atomic_umax:
          case nir_intrinsic_ssbo_atomic_and:
          case nir_intrinsic_ssbo_atomic_or:
          case nir_intrinsic_ssbo_atomic_xor:
          case nir_intrinsic_ssbo_atomic_exchange:
          case nir_intrinsic_ssbo_atomic_comp_swap:
          case nir_intrinsic_store_shared:
          case nir_intrinsic_shared_atomic_add:
          case nir_intrinsic_shared_atomic_imin:
          case nir_intrinsic_shared_atomic_umin:
          case nir_intrinsic_shared_atomic_imax:
          case nir_intrinsic_shared_atomic_umax:
          case nir_intrinsic_shared_atomic_and:
          case nir_intrinsic_shared_atomic_or:
          case nir_intrinsic_shared_atomic_xor:
          case nir_intrinsic_shared_atomic_exchange:
          case nir_intrinsic_shared_atomic_comp_swap:
          case nir_intrinsic_image_deref_store:
          case nir_intrinsic_image_deref_atomic_add:
          case nir_intrinsic_image_deref_atomic_umin:
          case nir_intrinsic_image_deref_atomic_umax:
          case nir_intrinsic_image_deref_atomic_imin:
          case nir_intrinsic_image_deref_atomic_imax:
          case nir_intrinsic_image_deref_atomic_and:
          case nir_intrinsic_image_deref_atomic_or:
          case nir_intrinsic_image_deref_atomic_xor:
          case nir_intrinsic_image_deref_atomic_exchange:
          case nir_intrinsic_image_deref_atomic_comp_swap:
             return true;

          default:
             /* Keep walking. */
             break;
          }
       }
    }

    return false;
 }

 bool
 nir_shader_uses_view_index(nir_shader *shader)
 {
    nir_function_impl *entrypoint = nir_shader_get_entrypoint(shader);

    nir_foreach_block(block, entrypoint) {
       nir_foreach_instr(instr, block) {
          if (instr->type != nir_instr_type_intrinsic)
             continue;

          nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
          if (intrin->intrinsic == nir_intrinsic_load_view_index)
             return true;
       }
    }

    return false;
 }

 static bool
 shader_only_position_uses_view_index(nir_shader *shader)
 {
    nir_shader *shader_no_position = nir_shader_clone(NULL, shader);
    nir_function_impl *entrypoint = nir_shader_get_entrypoint(shader_no_position);

    /* Remove the store position from a cloned shader. */
    nir_foreach_block(block, entrypoint) {
       nir_foreach_instr_safe(instr, block) {
          if (instr->type != nir_instr_type_intrinsic)
             continue;

          nir_intrinsic_instr *store = nir_instr_as_intrinsic(instr);
          if (store->intrinsic != nir_intrinsic_store_deref)
             continue;

          nir_variable *var = nir_intrinsic_get_var(store, 0);
          if (var->data.location != VARYING_SLOT_POS)
             continue;

          nir_instr_remove(&store->instr);
       }
    }

    /* Clean up shader so unused load_view_index intrinsics are removed. */
    bool progress;
    do {
       progress = false;
       progress |= nir_opt_dead_cf(shader_no_position);

       /* Peephole select will drop if-blocks that have then and else empty,
        * which will remove the usage of an SSA in the condition.
        */
       progress |= nir_opt_peephole_select(shader_no_position, 0, false, false);

       progress |= nir_opt_dce(shader_no_position);
    } while (progress);

    bool uses_view_index = nir_shader_uses_view_index(shader_no_position);

    ralloc_free(shader_no_position);
    return !uses_view_index;
 }

 /* Return true if it's safe to call nir_lower_multiview() on this vertex
  * shader. Note that this only handles driver-agnostic checks, i.e. things
  * which would make nir_lower_multiview() incorrect. Any driver-specific
  * checks, e.g. for sufficient varying space or performance considerations,
  * should be handled in the driver.
  *
  * Note that we don't handle the more complex checks needed for lowering
  * pipelines with geometry or tessellation shaders.
  */

 bool
 nir_can_lower_multiview(nir_shader *shader)
 {
    bool writes_position = false;
    nir_foreach_shader_out_variable(var, shader) {
       if (var->data.location == VARYING_SLOT_POS) {
          writes_position = true;
          break;
       }
    }

    /* Don't bother handling this edge case. */
    if (!writes_position)
       return false;

    return !shader_writes_to_memory(shader) &&
           shader_only_position_uses_view_index(shader);
 }

 /**
  * The lowering. Call with the last active geometry stage.
  */

 bool
 nir_lower_multiview(nir_shader *shader, uint32_t view_mask)
 {
    assert(shader->info.stage != MESA_SHADER_FRAGMENT);
    int view_count = util_bitcount(view_mask);

    nir_function_impl *entrypoint = nir_shader_get_entrypoint(shader);

    /* Update position to refer to an array. */
    nir_variable *pos_var = NULL;
    nir_foreach_shader_out_variable(var, shader) {
       if (var->data.location == VARYING_SLOT_POS) {
          assert(var->type == glsl_vec4_type());
          var->type = glsl_array_type(glsl_vec4_type(), view_count, 0);
          var->data.per_view = true;
          pos_var = var;
          break;
       }
    }

    assert(pos_var);

    nir_cf_list body;
    nir_cf_list_extract(&body, &entrypoint->body);

    nir_builder b;
    nir_builder_init(&b, entrypoint);
    b.cursor = nir_after_cf_list(&entrypoint->body);

    /* Loop Index will go from 0 to view_count. */
    nir_variable *loop_index_var =
       nir_local_variable_create(entrypoint, glsl_uint_type(), "loop_index");
    nir_deref_instr *loop_index_deref = nir_build_deref_var(&b, loop_index_var);
    nir_store_deref(&b, loop_index_deref, nir_imm_int(&b, 0), 1);

    /* Array of view index values that are active in the loop.  Note that the
     * loop index only matches the view index if there are no gaps in the
     * view_mask.
     */
    nir_variable *view_index_var = nir_local_variable_create(
       entrypoint, glsl_array_type(glsl_uint_type(), view_count, 0), "view_index");
    nir_deref_instr *view_index_deref = nir_build_deref_var(&b, view_index_var);
    {
       int array_position = 0;
       uint32_t view_mask_temp = view_mask;
       while (view_mask_temp) {
          uint32_t view_index = u_bit_scan(&view_mask_temp);
          nir_store_deref(&b, nir_build_deref_array_imm(&b, view_index_deref, array_position),
                          nir_imm_int(&b, view_index), 1);
          array_position++;
       }
    }

    /* Create the equivalent of
     *
     *    while (true):
     *       if (loop_index >= view_count):
     *          break
     *
     *       view_index = active_indices[loop_index]
     *       pos_deref = &pos[loop_index]
     *
     *       # Placeholder for the body to be reinserted.
     *
     *       loop_index += 1
     *
     * Later both `view_index` and `pos_deref` will be used to rewrite the
     * original shader body.
     */

    nir_loop* loop = nir_push_loop(&b);

    nir_ssa_def *loop_index = nir_load_deref(&b, loop_index_deref);
    nir_ssa_def *cmp = nir_ige(&b, loop_index, nir_imm_int(&b, view_count));
    nir_if *loop_check = nir_push_if(&b, cmp);
    nir_jump(&b, nir_jump_break);
    nir_pop_if(&b, loop_check);

    nir_ssa_def *view_index =
       nir_load_deref(&b, nir_build_deref_array(&b, view_index_deref, loop_index));
    nir_deref_instr *pos_deref =
       nir_build_deref_array(&b, nir_build_deref_var(&b, pos_var), loop_index);

    nir_store_deref(&b, loop_index_deref, nir_iadd_imm(&b, loop_index, 1), 1);
    nir_pop_loop(&b, loop);

    /* Reinsert the body. */
    b.cursor = nir_after_instr(&pos_deref->instr);
    nir_cf_reinsert(&body, b.cursor);

    nir_foreach_block(block, entrypoint) {
       nir_foreach_instr_safe(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_load_view_index: {
             assert(intrin->dest.is_ssa);
             nir_ssa_def_rewrite_uses(&intrin->dest.ssa, nir_src_for_ssa(view_index));
             break;
          }

          case nir_intrinsic_store_deref: {
             nir_variable *var = nir_intrinsic_get_var(intrin, 0);
             if (var == pos_var) {
                nir_deref_instr *old_deref = nir_src_as_deref(intrin->src[0]);

                nir_instr_rewrite_src(instr, &intrin->src[0],
                                      nir_src_for_ssa(&pos_deref->dest.ssa));

                /* Remove old deref since it has the wrong type. */
                nir_deref_instr_remove_if_unused(old_deref);
             }
             break;
          }

          case nir_intrinsic_load_deref:
             if (nir_intrinsic_get_var(intrin, 0) == pos_var) {
                unreachable("Should have lowered I/O to temporaries "
                            "so no load_deref on position output is expected.");
             }
             break;

          case nir_intrinsic_copy_deref:
             unreachable("Should have lowered copy_derefs at this point");
             break;

          default:
             /* Do nothing. */
             break;
          }
       }
    }

    nir_metadata_preserve(entrypoint, nir_metadata_none);
    return true;
 }
	/*
	* Copyright © 2016 Intel Corporation
	* 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_control_flow.h"
	#include "nir_builder.h"

	/**
	* This file implements an optimization for multiview. Some GPU's have a
	* special mode which allows the vertex shader (or last stage in the geometry
	* pipeline) to create multiple primitives in different layers of the
	* framebuffer at once by writing multiple copies of gl_Position. The
	* assumption is that in most uses of multiview, the only use of gl_ViewIndex
	* is to change the position to implement the parallax effect, and other
	* varyings will be the same between the different views. We put the body of
	* the original vertex shader in a loop, writing to a different copy of
	* gl_Position each loop iteration, and then let other optimizations clean up
	* the mess.
	*/

	static bool
	shader_writes_to_memory(nir_shader *shader)
	{
	/* With multiview, we would need to ensure that memory writes happen either
	* once or once per view. Since combination of multiview and memory writes
	* is not expected, we'll just skip this optimization in this case.
	*/

	nir_function_impl *entrypoint = nir_shader_get_entrypoint(shader);

	nir_foreach_block(block, entrypoint) {
	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_deref_atomic_add:
	case nir_intrinsic_deref_atomic_imin:
	case nir_intrinsic_deref_atomic_umin:
	case nir_intrinsic_deref_atomic_imax:
	case nir_intrinsic_deref_atomic_umax:
	case nir_intrinsic_deref_atomic_and:
	case nir_intrinsic_deref_atomic_or:
	case nir_intrinsic_deref_atomic_xor:
	case nir_intrinsic_deref_atomic_exchange:
	case nir_intrinsic_deref_atomic_comp_swap:
	case nir_intrinsic_store_ssbo:
	case nir_intrinsic_ssbo_atomic_add:
	case nir_intrinsic_ssbo_atomic_imin:
	case nir_intrinsic_ssbo_atomic_umin:
	case nir_intrinsic_ssbo_atomic_imax:
	case nir_intrinsic_ssbo_atomic_umax:
	case nir_intrinsic_ssbo_atomic_and:
	case nir_intrinsic_ssbo_atomic_or:
	case nir_intrinsic_ssbo_atomic_xor:
	case nir_intrinsic_ssbo_atomic_exchange:
	case nir_intrinsic_ssbo_atomic_comp_swap:
	case nir_intrinsic_store_shared:
	case nir_intrinsic_shared_atomic_add:
	case nir_intrinsic_shared_atomic_imin:
	case nir_intrinsic_shared_atomic_umin:
	case nir_intrinsic_shared_atomic_imax:
	case nir_intrinsic_shared_atomic_umax:
	case nir_intrinsic_shared_atomic_and:
	case nir_intrinsic_shared_atomic_or:
	case nir_intrinsic_shared_atomic_xor:
	case nir_intrinsic_shared_atomic_exchange:
	case nir_intrinsic_shared_atomic_comp_swap:
	case nir_intrinsic_image_deref_store:
	case nir_intrinsic_image_deref_atomic_add:
	case nir_intrinsic_image_deref_atomic_umin:
	case nir_intrinsic_image_deref_atomic_umax:
	case nir_intrinsic_image_deref_atomic_imin:
	case nir_intrinsic_image_deref_atomic_imax:
	case nir_intrinsic_image_deref_atomic_and:
	case nir_intrinsic_image_deref_atomic_or:
	case nir_intrinsic_image_deref_atomic_xor:
	case nir_intrinsic_image_deref_atomic_exchange:
	case nir_intrinsic_image_deref_atomic_comp_swap:
	return true;

	default:
	/* Keep walking. */
	break;
	}
	}
	}

	return false;
	}

	bool
	nir_shader_uses_view_index(nir_shader *shader)
	{
	nir_function_impl *entrypoint = nir_shader_get_entrypoint(shader);

	nir_foreach_block(block, entrypoint) {
	nir_foreach_instr(instr, block) {
	if (instr->type != nir_instr_type_intrinsic)
	continue;

	nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
	if (intrin->intrinsic == nir_intrinsic_load_view_index)
	return true;
	}
	}

	return false;
	}

	static bool
	shader_only_position_uses_view_index(nir_shader *shader)
	{
	nir_shader *shader_no_position = nir_shader_clone(NULL, shader);
	nir_function_impl *entrypoint = nir_shader_get_entrypoint(shader_no_position);

	/* Remove the store position from a cloned shader. */
	nir_foreach_block(block, entrypoint) {
	nir_foreach_instr_safe(instr, block) {
	if (instr->type != nir_instr_type_intrinsic)
	continue;

	nir_intrinsic_instr *store = nir_instr_as_intrinsic(instr);
	if (store->intrinsic != nir_intrinsic_store_deref)
	continue;

	nir_variable *var = nir_intrinsic_get_var(store, 0);
	if (var->data.location != VARYING_SLOT_POS)
	continue;

	nir_instr_remove(&store->instr);
	}
	}

	/* Clean up shader so unused load_view_index intrinsics are removed. */
	bool progress;
	do {
	progress = false;
	progress \|= nir_opt_dead_cf(shader_no_position);

	/* Peephole select will drop if-blocks that have then and else empty,
	* which will remove the usage of an SSA in the condition.
	*/
	progress \|= nir_opt_peephole_select(shader_no_position, 0, false, false);

	progress \|= nir_opt_dce(shader_no_position);
	} while (progress);

	bool uses_view_index = nir_shader_uses_view_index(shader_no_position);

	ralloc_free(shader_no_position);
	return !uses_view_index;
	}

	/* Return true if it's safe to call nir_lower_multiview() on this vertex
	* shader. Note that this only handles driver-agnostic checks, i.e. things
	* which would make nir_lower_multiview() incorrect. Any driver-specific
	* checks, e.g. for sufficient varying space or performance considerations,
	* should be handled in the driver.
	*
	* Note that we don't handle the more complex checks needed for lowering
	* pipelines with geometry or tessellation shaders.
	*/

	bool
	nir_can_lower_multiview(nir_shader *shader)
	{
	bool writes_position = false;
	nir_foreach_shader_out_variable(var, shader) {
	if (var->data.location == VARYING_SLOT_POS) {
	writes_position = true;
	break;
	}
	}

	/* Don't bother handling this edge case. */
	if (!writes_position)
	return false;

	return !shader_writes_to_memory(shader) &&
	shader_only_position_uses_view_index(shader);
	}

	/**
	* The lowering. Call with the last active geometry stage.
	*/

	bool
	nir_lower_multiview(nir_shader *shader, uint32_t view_mask)
	{
	assert(shader->info.stage != MESA_SHADER_FRAGMENT);
	int view_count = util_bitcount(view_mask);

	nir_function_impl *entrypoint = nir_shader_get_entrypoint(shader);

	/* Update position to refer to an array. */
	nir_variable *pos_var = NULL;
	nir_foreach_shader_out_variable(var, shader) {
	if (var->data.location == VARYING_SLOT_POS) {
	assert(var->type == glsl_vec4_type());
	var->type = glsl_array_type(glsl_vec4_type(), view_count, 0);
	var->data.per_view = true;
	pos_var = var;
	break;
	}
	}

	assert(pos_var);

	nir_cf_list body;
	nir_cf_list_extract(&body, &entrypoint->body);

	nir_builder b;
	nir_builder_init(&b, entrypoint);
	b.cursor = nir_after_cf_list(&entrypoint->body);

	/* Loop Index will go from 0 to view_count. */
	nir_variable *loop_index_var =
	nir_local_variable_create(entrypoint, glsl_uint_type(), "loop_index");
	nir_deref_instr *loop_index_deref = nir_build_deref_var(&b, loop_index_var);
	nir_store_deref(&b, loop_index_deref, nir_imm_int(&b, 0), 1);

	/* Array of view index values that are active in the loop. Note that the
	* loop index only matches the view index if there are no gaps in the
	* view_mask.
	*/
	nir_variable *view_index_var = nir_local_variable_create(
	entrypoint, glsl_array_type(glsl_uint_type(), view_count, 0), "view_index");
	nir_deref_instr *view_index_deref = nir_build_deref_var(&b, view_index_var);
	{
	int array_position = 0;
	uint32_t view_mask_temp = view_mask;
	while (view_mask_temp) {
	uint32_t view_index = u_bit_scan(&view_mask_temp);
	nir_store_deref(&b, nir_build_deref_array_imm(&b, view_index_deref, array_position),
	nir_imm_int(&b, view_index), 1);
	array_position++;
	}
	}

	/* Create the equivalent of
	*
	* while (true):
	* if (loop_index >= view_count):
	* break
	*
	* view_index = active_indices[loop_index]
	* pos_deref = &pos[loop_index]
	*
	* # Placeholder for the body to be reinserted.
	*
	* loop_index += 1
	*
	* Later both `view_index` and `pos_deref` will be used to rewrite the
	* original shader body.
	*/

	nir_loop* loop = nir_push_loop(&b);

	nir_ssa_def *loop_index = nir_load_deref(&b, loop_index_deref);
	nir_ssa_def *cmp = nir_ige(&b, loop_index, nir_imm_int(&b, view_count));
	nir_if *loop_check = nir_push_if(&b, cmp);
	nir_jump(&b, nir_jump_break);
	nir_pop_if(&b, loop_check);

	nir_ssa_def *view_index =
	nir_load_deref(&b, nir_build_deref_array(&b, view_index_deref, loop_index));
	nir_deref_instr *pos_deref =
	nir_build_deref_array(&b, nir_build_deref_var(&b, pos_var), loop_index);

	nir_store_deref(&b, loop_index_deref, nir_iadd_imm(&b, loop_index, 1), 1);
	nir_pop_loop(&b, loop);

	/* Reinsert the body. */
	b.cursor = nir_after_instr(&pos_deref->instr);
	nir_cf_reinsert(&body, b.cursor);

	nir_foreach_block(block, entrypoint) {
	nir_foreach_instr_safe(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_load_view_index: {
	assert(intrin->dest.is_ssa);
	nir_ssa_def_rewrite_uses(&intrin->dest.ssa, nir_src_for_ssa(view_index));
	break;
	}

	case nir_intrinsic_store_deref: {
	nir_variable *var = nir_intrinsic_get_var(intrin, 0);
	if (var == pos_var) {
	nir_deref_instr *old_deref = nir_src_as_deref(intrin->src[0]);

	nir_instr_rewrite_src(instr, &intrin->src[0],
	nir_src_for_ssa(&pos_deref->dest.ssa));

	/* Remove old deref since it has the wrong type. */
	nir_deref_instr_remove_if_unused(old_deref);
	}
	break;
	}

	case nir_intrinsic_load_deref:
	if (nir_intrinsic_get_var(intrin, 0) == pos_var) {
	unreachable("Should have lowered I/O to temporaries "
	"so no load_deref on position output is expected.");
	}
	break;

	case nir_intrinsic_copy_deref:
	unreachable("Should have lowered copy_derefs at this point");
	break;

	default:
	/* Do nothing. */
	break;
	}
	}
	}

	nir_metadata_preserve(entrypoint, nir_metadata_none);
	return true;
	}