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
rewrite_emit_vertex(nir_intrinsic_instr * intrin,struct state * state)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
rewrite_end_primitive(nir_intrinsic_instr * intrin,struct state * state)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
rewrite_intrinsics(nir_block * block,struct state * state)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
append_set_vertex_count(nir_block * end_block,struct state * state)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 struct set_entry *entry;
169 set_foreach(end_block->predecessors, entry) {
170 nir_block *pred = (nir_block *) entry->key;
171 b->cursor = nir_after_block_before_jump(pred);
172
173 nir_ssa_def *count = nir_load_var(b, state->vertex_count_var);
174
175 nir_intrinsic_instr *set_vertex_count =
176 nir_intrinsic_instr_create(shader, nir_intrinsic_set_vertex_count);
177 set_vertex_count->src[0] = nir_src_for_ssa(count);
178
179 nir_builder_instr_insert(b, &set_vertex_count->instr);
180 }
181 }
182
183 bool
nir_lower_gs_intrinsics(nir_shader * shader)184 nir_lower_gs_intrinsics(nir_shader *shader)
185 {
186 struct state state;
187 state.progress = false;
188
189 nir_function_impl *impl = nir_shader_get_entrypoint(shader);
190 assert(impl);
191
192 nir_builder b;
193 nir_builder_init(&b, impl);
194 state.builder = &b;
195
196 /* Create the counter variable */
197 state.vertex_count_var =
198 nir_local_variable_create(impl, glsl_uint_type(), "vertex_count");
199 /* initialize to 0 */
200 b.cursor = nir_before_cf_list(&impl->body);
201 nir_store_var(&b, state.vertex_count_var, nir_imm_int(&b, 0), 0x1);
202
203 nir_foreach_block_safe(block, impl)
204 rewrite_intrinsics(block, &state);
205
206 /* This only works because we have a single main() function. */
207 append_set_vertex_count(impl->end_block, &state);
208
209 nir_metadata_preserve(impl, 0);
210
211 return state.progress;
212 }
213