aboutsummaryrefslogtreecommitdiffstats
path: root/src/compiler/nir/nir_lower_gs_intrinsics.c
blob: 78c12e0547503dae490b6aab9b85bae690b01e34 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
/*
 * Copyright © 2015 Intel 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"
#include "nir_builder.h"

/**
 * \file nir_lower_gs_intrinsics.c
 *
 * Geometry Shaders can call EmitVertex()/EmitStreamVertex() to output an
 * arbitrary number of vertices.  However, the shader must declare the maximum
 * number of vertices that it will ever output - further attempts to emit
 * vertices result in undefined behavior according to the GLSL specification.
 *
 * Drivers might use this maximum number of vertices to allocate enough space
 * to hold the geometry shader's output.  Some drivers (such as i965) need to
 * implement "safety checks" which ensure that the shader hasn't emitted too
 * many vertices, to avoid overflowing that space and trashing other memory.
 *
 * The count of emitted vertices can also be useful in buffer offset
 * calculations, so drivers know where to write the GS output.
 *
 * However, for simple geometry shaders that emit a statically determinable
 * number of vertices, this extra bookkeeping is unnecessary and inefficient.
 * By tracking the vertex count in NIR, we allow constant folding/propagation
 * and dead control flow optimizations to eliminate most of it where possible.
 *
 * This pass introduces a new global variable which stores the current vertex
 * count (initialized to 0), and converts emit_vertex/end_primitive intrinsics
 * to their *_with_counter variants.  emit_vertex is also wrapped in a safety
 * check to avoid buffer overflows.  Finally, it adds a set_vertex_count
 * intrinsic at the end of the program, informing the driver of the final
 * vertex count.
 */

struct state {
   nir_builder *builder;
   nir_variable *vertex_count_var;
   bool progress;
};

/**
 * Replace emit_vertex intrinsics with:
 *
 * if (vertex_count < max_vertices) {
 *    emit_vertex_with_counter vertex_count ...
 *    vertex_count += 1
 * }
 */
static void
rewrite_emit_vertex(nir_intrinsic_instr *intrin, struct state *state)
{
   nir_builder *b = state->builder;

   /* Load the vertex count */
   b->cursor = nir_before_instr(&intrin->instr);
   nir_ssa_def *count = nir_load_var(b, state->vertex_count_var);

   nir_ssa_def *max_vertices =
      nir_imm_int(b, b->shader->info.gs.vertices_out);

   /* Create: if (vertex_count < max_vertices) and insert it.
    *
    * The new if statement needs to be hooked up to the control flow graph
    * before we start inserting instructions into it.
    */
   nir_push_if(b, nir_ilt(b, count, max_vertices));

   nir_intrinsic_instr *lowered =
      nir_intrinsic_instr_create(b->shader,
                                 nir_intrinsic_emit_vertex_with_counter);
   nir_intrinsic_set_stream_id(lowered, nir_intrinsic_stream_id(intrin));
   lowered->src[0] = nir_src_for_ssa(count);
   nir_builder_instr_insert(b, &lowered->instr);

   /* Increment the vertex count by 1 */
   nir_store_var(b, state->vertex_count_var,
                 nir_iadd(b, count, nir_imm_int(b, 1)),
                 0x1); /* .x */

   nir_pop_if(b, NULL);

   nir_instr_remove(&intrin->instr);

   state->progress = true;
}

/**
 * Replace end_primitive with end_primitive_with_counter.
 */
static void
rewrite_end_primitive(nir_intrinsic_instr *intrin, struct state *state)
{
   nir_builder *b = state->builder;

   b->cursor = nir_before_instr(&intrin->instr);
   nir_ssa_def *count = nir_load_var(b, state->vertex_count_var);

   nir_intrinsic_instr *lowered =
      nir_intrinsic_instr_create(b->shader,
                                 nir_intrinsic_end_primitive_with_counter);
   nir_intrinsic_set_stream_id(lowered, nir_intrinsic_stream_id(intrin));
   lowered->src[0] = nir_src_for_ssa(count);
   nir_builder_instr_insert(b, &lowered->instr);

   nir_instr_remove(&intrin->instr);

   state->progress = true;
}

static bool
rewrite_intrinsics(nir_block *block, struct state *state)
{
   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_emit_vertex:
         rewrite_emit_vertex(intrin, state);
         break;
      case nir_intrinsic_end_primitive:
         rewrite_end_primitive(intrin, state);
         break;
      default:
         /* not interesting; skip this */
         break;
      }
   }

   return true;
}

/**
 * Add a set_vertex_count intrinsic at the end of the program
 * (representing the final vertex count).
 */
static void
append_set_vertex_count(nir_block *end_block, struct state *state)
{
   nir_builder *b = state->builder;
   nir_shader *shader = state->builder->shader;

   /* Insert the new intrinsic in all of the predecessors of the end block,
    * but before any jump instructions (return).
    */
   set_foreach(end_block->predecessors, entry) {
      nir_block *pred = (nir_block *) entry->key;
      b->cursor = nir_after_block_before_jump(pred);

      nir_ssa_def *count = nir_load_var(b, state->vertex_count_var);

      nir_intrinsic_instr *set_vertex_count =
         nir_intrinsic_instr_create(shader, nir_intrinsic_set_vertex_count);
      set_vertex_count->src[0] = nir_src_for_ssa(count);

      nir_builder_instr_insert(b, &set_vertex_count->instr);
   }
}

bool
nir_lower_gs_intrinsics(nir_shader *shader)
{
   struct state state;
   state.progress = false;

   nir_function_impl *impl = nir_shader_get_entrypoint(shader);
   assert(impl);

   nir_builder b;
   nir_builder_init(&b, impl);
   state.builder = &b;

   /* Create the counter variable */
   state.vertex_count_var =
      nir_local_variable_create(impl, glsl_uint_type(), "vertex_count");
   /* initialize to 0 */
   b.cursor = nir_before_cf_list(&impl->body);
   nir_store_var(&b, state.vertex_count_var, nir_imm_int(&b, 0), 0x1);

   nir_foreach_block_safe(block, impl)
      rewrite_intrinsics(block, &state);

   /* This only works because we have a single main() function. */
   append_set_vertex_count(impl->end_block, &state);

   nir_metadata_preserve(impl, 0);

   return state.progress;
}