src/compiler/nir/nir_lower_gs_intrinsics.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 "nir.h"
  25 #include "nir_builder.h"
  26
  27 /**
  28  * \file nir_lower_gs_intrinsics.c
  29  *
  30  * Geometry Shaders can call EmitVertex()/EmitStreamVertex() to output an
  31  * arbitrary number of vertices.  However, the shader must declare the maximum
  32  * number of vertices that it will ever output - further attempts to emit
  33  * vertices result in undefined behavior according to the GLSL specification.
  34  *
  35  * Drivers might use this maximum number of vertices to allocate enough space
  36  * to hold the geometry shader's output.  Some drivers (such as i965) need to
  37  * implement "safety checks" which ensure that the shader hasn't emitted too
  38  * many vertices, to avoid overflowing that space and trashing other memory.
  39  *
  40  * The count of emitted vertices can also be useful in buffer offset
  41  * calculations, so drivers know where to write the GS output.
  42  *
  43  * However, for simple geometry shaders that emit a statically determinable
  44  * number of vertices, this extra bookkeeping is unnecessary and inefficient.
  45  * By tracking the vertex count in NIR, we allow constant folding/propagation
  46  * and dead control flow optimizations to eliminate most of it where possible.
  47  *
  48  * This pass introduces a new global variable which stores the current vertex
  49  * count (initialized to 0), and converts emit_vertex/end_primitive intrinsics
  50  * to their *_with_counter variants.  emit_vertex is also wrapped in a safety
  51  * check to avoid buffer overflows.  Finally, it adds a set_vertex_count
  52  * intrinsic at the end of the program, informing the driver of the final
  53  * vertex count.
  54  */
  55
  56 struct state {
  57    nir_builder *builder;
  58    nir_variable *vertex_count_var;
  59    bool progress;
  60 };
  61
  62 /**
  63  * Replace emit_vertex intrinsics with:
  64  *
  65  * if (vertex_count < max_vertices) {
  66  *    emit_vertex_with_counter vertex_count ...
  67  *    vertex_count += 1
  68  * }
  69  */
  70 static void
  71 rewrite_emit_vertex(nir_intrinsic_instr *intrin, struct state *state)
  72 {
  73    nir_builder *b = state->builder;
  74
  75    /* Load the vertex count */
  76    b->cursor = nir_before_instr(&intrin->instr);
  77    nir_ssa_def *count = nir_load_var(b, state->vertex_count_var);
  78
  79    nir_ssa_def *max_vertices =
  80       nir_imm_int(b, b->shader->info.gs.vertices_out);
  81
  82    /* Create: if (vertex_count < max_vertices) and insert it.
  83     *
  84     * The new if statement needs to be hooked up to the control flow graph
  85     * before we start inserting instructions into it.
  86     */
  87    nir_push_if(b, nir_ilt(b, count, max_vertices));
  88
  89    nir_intrinsic_instr *lowered =
  90       nir_intrinsic_instr_create(b->shader,
  91                                  nir_intrinsic_emit_vertex_with_counter);
  92    nir_intrinsic_set_stream_id(lowered, nir_intrinsic_stream_id(intrin));
  93    lowered->src[0] = nir_src_for_ssa(count);
  94    nir_builder_instr_insert(b, &lowered->instr);
  95
  96    /* Increment the vertex count by 1 */
  97    nir_store_var(b, state->vertex_count_var,
  98                  nir_iadd(b, count, nir_imm_int(b, 1)),
  99                  0x1); /* .x */
 100
 101    nir_pop_if(b, NULL);
 102
 103    nir_instr_remove(&intrin->instr);
 104
 105    state->progress = true;
 106 }
 107
 108 /**
 109  * Replace end_primitive with end_primitive_with_counter.
 110  */
 111 static void
 112 rewrite_end_primitive(nir_intrinsic_instr *intrin, struct state *state)
 113 {
 114    nir_builder *b = state->builder;
 115
 116    b->cursor = nir_before_instr(&intrin->instr);
 117    nir_ssa_def *count = nir_load_var(b, state->vertex_count_var);
 118
 119    nir_intrinsic_instr *lowered =
 120       nir_intrinsic_instr_create(b->shader,
 121                                  nir_intrinsic_end_primitive_with_counter);
 122    nir_intrinsic_set_stream_id(lowered, nir_intrinsic_stream_id(intrin));
 123    lowered->src[0] = nir_src_for_ssa(count);
 124    nir_builder_instr_insert(b, &lowered->instr);
 125
 126    nir_instr_remove(&intrin->instr);
 127
 128    state->progress = true;
 129 }
 130
 131 static bool
 132 rewrite_intrinsics(nir_block *block, struct state *state)
 133 {
 134    nir_foreach_instr_safe(instr, block) {
 135       if (instr->type != nir_instr_type_intrinsic)
 136          continue;
 137
 138       nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
 139       switch (intrin->intrinsic) {
 140       case nir_intrinsic_emit_vertex:
 141          rewrite_emit_vertex(intrin, state);
 142          break;
 143       case nir_intrinsic_end_primitive:
 144          rewrite_end_primitive(intrin, state);
 145          break;
 146       default:
 147          /* not interesting; skip this */
 148          break;
 149       }
 150    }
 151
 152    return true;
 153 }
 154
 155 /**
 156  * Add a set_vertex_count intrinsic at the end of the program
 157  * (representing the final vertex count).
 158  */
 159 static void
 160 append_set_vertex_count(nir_block *end_block, struct state *state)
 161 {
 162    nir_builder *b = state->builder;
 163    nir_shader *shader = state->builder->shader;
 164
 165    /* Insert the new intrinsic in all of the predecessors of the end block,
 166     * but before any jump instructions (return).
 167     */
 168    set_foreach(end_block->predecessors, entry) {
 169       nir_block *pred = (nir_block *) entry->key;
 170       b->cursor = nir_after_block_before_jump(pred);
 171
 172       nir_ssa_def *count = nir_load_var(b, state->vertex_count_var);
 173
 174       nir_intrinsic_instr *set_vertex_count =
 175          nir_intrinsic_instr_create(shader, nir_intrinsic_set_vertex_count);
 176       set_vertex_count->src[0] = nir_src_for_ssa(count);
 177
 178       nir_builder_instr_insert(b, &set_vertex_count->instr);
 179    }
 180 }
 181
 182 bool
 183 nir_lower_gs_intrinsics(nir_shader *shader)
 184 {
 185    struct state state;
 186    state.progress = false;
 187
 188    nir_function_impl *impl = nir_shader_get_entrypoint(shader);
 189    assert(impl);
 190
 191    nir_builder b;
 192    nir_builder_init(&b, impl);
 193    state.builder = &b;
 194
 195    /* Create the counter variable */
 196    state.vertex_count_var =
 197       nir_local_variable_create(impl, glsl_uint_type(), "vertex_count");
 198    /* initialize to 0 */
 199    b.cursor = nir_before_cf_list(&impl->body);
 200    nir_store_var(&b, state.vertex_count_var, nir_imm_int(&b, 0), 0x1);
 201
 202    nir_foreach_block_safe(block, impl)
 203       rewrite_intrinsics(block, &state);
 204
 205    /* This only works because we have a single main() function. */
 206    append_set_vertex_count(impl->end_block, &state);
 207
 208    nir_metadata_preserve(impl, 0);
 209
 210    return state.progress;
 211 }