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25 #include "nir_builder.h"
26 #include "nir_xfb_info.h"
29 * \file nir_lower_gs_intrinsics.c
31 * Geometry Shaders can call EmitVertex()/EmitStreamVertex() to output an
32 * arbitrary number of vertices. However, the shader must declare the maximum
33 * number of vertices that it will ever output - further attempts to emit
34 * vertices result in undefined behavior according to the GLSL specification.
36 * Drivers might use this maximum number of vertices to allocate enough space
37 * to hold the geometry shader's output. Some drivers (such as i965) need to
38 * implement "safety checks" which ensure that the shader hasn't emitted too
39 * many vertices, to avoid overflowing that space and trashing other memory.
41 * The count of emitted vertices can also be useful in buffer offset
42 * calculations, so drivers know where to write the GS output.
44 * However, for simple geometry shaders that emit a statically determinable
45 * number of vertices, this extra bookkeeping is unnecessary and inefficient.
46 * By tracking the vertex count in NIR, we allow constant folding/propagation
47 * and dead control flow optimizations to eliminate most of it where possible.
49 * This pass introduces a new global variable which stores the current vertex
50 * count (initialized to 0), and converts emit_vertex/end_primitive intrinsics
51 * to their *_with_counter variants. emit_vertex is also wrapped in a safety
52 * check to avoid buffer overflows. Finally, it adds a set_vertex_count
53 * intrinsic at the end of the program, informing the driver of the final
59 nir_variable
*vertex_count_vars
[NIR_MAX_XFB_STREAMS
];
64 * Replace emit_vertex intrinsics with:
66 * if (vertex_count < max_vertices) {
67 * emit_vertex_with_counter vertex_count ...
72 rewrite_emit_vertex(nir_intrinsic_instr
*intrin
, struct state
*state
)
74 nir_builder
*b
= state
->builder
;
75 unsigned stream
= nir_intrinsic_stream_id(intrin
);
77 /* Load the vertex count */
78 b
->cursor
= nir_before_instr(&intrin
->instr
);
79 assert(state
->vertex_count_vars
[stream
] != NULL
);
80 nir_ssa_def
*count
= nir_load_var(b
, state
->vertex_count_vars
[stream
]);
82 nir_ssa_def
*max_vertices
=
83 nir_imm_int(b
, b
->shader
->info
.gs
.vertices_out
);
85 /* Create: if (vertex_count < max_vertices) and insert it.
87 * The new if statement needs to be hooked up to the control flow graph
88 * before we start inserting instructions into it.
90 nir_push_if(b
, nir_ilt(b
, count
, max_vertices
));
92 nir_intrinsic_instr
*lowered
=
93 nir_intrinsic_instr_create(b
->shader
,
94 nir_intrinsic_emit_vertex_with_counter
);
95 nir_intrinsic_set_stream_id(lowered
, stream
);
96 lowered
->src
[0] = nir_src_for_ssa(count
);
97 nir_builder_instr_insert(b
, &lowered
->instr
);
99 /* Increment the vertex count by 1 */
100 nir_store_var(b
, state
->vertex_count_vars
[stream
],
101 nir_iadd(b
, count
, nir_imm_int(b
, 1)),
106 nir_instr_remove(&intrin
->instr
);
108 state
->progress
= true;
112 * Replace end_primitive with end_primitive_with_counter.
115 rewrite_end_primitive(nir_intrinsic_instr
*intrin
, struct state
*state
)
117 nir_builder
*b
= state
->builder
;
118 unsigned stream
= nir_intrinsic_stream_id(intrin
);
120 b
->cursor
= nir_before_instr(&intrin
->instr
);
121 assert(state
->vertex_count_vars
[stream
] != NULL
);
122 nir_ssa_def
*count
= nir_load_var(b
, state
->vertex_count_vars
[stream
]);
124 nir_intrinsic_instr
*lowered
=
125 nir_intrinsic_instr_create(b
->shader
,
126 nir_intrinsic_end_primitive_with_counter
);
127 nir_intrinsic_set_stream_id(lowered
, stream
);
128 lowered
->src
[0] = nir_src_for_ssa(count
);
129 nir_builder_instr_insert(b
, &lowered
->instr
);
131 nir_instr_remove(&intrin
->instr
);
133 state
->progress
= true;
137 rewrite_intrinsics(nir_block
*block
, struct state
*state
)
139 nir_foreach_instr_safe(instr
, block
) {
140 if (instr
->type
!= nir_instr_type_intrinsic
)
143 nir_intrinsic_instr
*intrin
= nir_instr_as_intrinsic(instr
);
144 switch (intrin
->intrinsic
) {
145 case nir_intrinsic_emit_vertex
:
146 rewrite_emit_vertex(intrin
, state
);
148 case nir_intrinsic_end_primitive
:
149 rewrite_end_primitive(intrin
, state
);
152 /* not interesting; skip this */
161 * Add a set_vertex_count intrinsic at the end of the program
162 * (representing the final vertex count).
165 append_set_vertex_count(nir_block
*end_block
, struct state
*state
)
167 nir_builder
*b
= state
->builder
;
168 nir_shader
*shader
= state
->builder
->shader
;
170 /* Insert the new intrinsic in all of the predecessors of the end block,
171 * but before any jump instructions (return).
173 set_foreach(end_block
->predecessors
, entry
) {
174 nir_block
*pred
= (nir_block
*) entry
->key
;
175 b
->cursor
= nir_after_block_before_jump(pred
);
177 nir_ssa_def
*count
= nir_load_var(b
, state
->vertex_count_vars
[0]);
179 nir_intrinsic_instr
*set_vertex_count
=
180 nir_intrinsic_instr_create(shader
, nir_intrinsic_set_vertex_count
);
181 set_vertex_count
->src
[0] = nir_src_for_ssa(count
);
183 nir_builder_instr_insert(b
, &set_vertex_count
->instr
);
188 nir_lower_gs_intrinsics(nir_shader
*shader
, bool per_stream
)
191 state
.progress
= false;
193 nir_function_impl
*impl
= nir_shader_get_entrypoint(shader
);
197 nir_builder_init(&b
, impl
);
200 /* Create the counter variables */
201 b
.cursor
= nir_before_cf_list(&impl
->body
);
202 for (unsigned i
= 0; i
< NIR_MAX_XFB_STREAMS
; i
++) {
203 if (per_stream
&& !(shader
->info
.gs
.active_stream_mask
& (1 << i
)))
206 if (i
== 0 || per_stream
) {
207 state
.vertex_count_vars
[i
] =
208 nir_local_variable_create(impl
, glsl_uint_type(), "vertex_count");
209 /* initialize to 0 */
210 nir_store_var(&b
, state
.vertex_count_vars
[i
], nir_imm_int(&b
, 0), 0x1);
212 /* If per_stream is false, we only have one counter which we want to use
213 * for all streams. Duplicate the counter pointer so all streams use the
216 state
.vertex_count_vars
[i
] = state
.vertex_count_vars
[0];
220 nir_foreach_block_safe(block
, impl
)
221 rewrite_intrinsics(block
, &state
);
223 /* This only works because we have a single main() function. */
225 append_set_vertex_count(impl
->end_block
, &state
);
227 nir_metadata_preserve(impl
, 0);
229 return state
.progress
;