1 /*
2 * Copyright 2019 Advanced Micro Devices, Inc.
3 *
4 * Permission is hereby granted, free of charge, to any person obtaining a
5 * copy of this software and associated documentation files (the
6 * "Software"), to deal in the Software without restriction, including
7 * without limitation the rights to use, copy, modify, merge, publish,
8 * distribute, sub license, and/or sell copies of the Software, and to
9 * permit persons to whom the Software is furnished to do so, subject to
10 * the following conditions:
11 *
12 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
13 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
14 * FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL
15 * THE COPYRIGHT HOLDERS, AUTHORS AND/OR ITS SUPPLIERS BE LIABLE FOR ANY CLAIM,
16 * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
17 * OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
18 * USE OR OTHER DEALINGS IN THE SOFTWARE.
19 *
20 * The above copyright notice and this permission notice (including the
21 * next paragraph) shall be included in all copies or substantial portions
22 * of the Software.
23 *
24 */
25
26 #include "ac_llvm_cull.h"
27
28 #include <llvm-c/Core.h>
29
30 struct ac_position_w_info {
31 /* If a primitive intersects the W=0 plane, it causes a reflection
32 * of the determinant used for face culling. Every vertex behind
33 * the W=0 plane negates the determinant, so having 2 vertices behind
34 * the plane has no effect. This is i1 true if the determinant should be
35 * negated.
36 */
37 LLVMValueRef w_reflection;
38
39 /* If we simplify the "-w <= p <= w" view culling equation, we get
40 * "-w <= w", which can't be satisfied when w is negative.
41 * In perspective projection, a negative W means that the primitive
42 * is behind the viewer, but the equation is independent of the type
43 * of projection.
44 *
45 * w_accepted is false when all W are negative and therefore
46 * the primitive is invisible.
47 */
48 LLVMValueRef w_accepted;
49
50 LLVMValueRef all_w_positive;
51 LLVMValueRef any_w_negative;
52 };
53
ac_analyze_position_w(struct ac_llvm_context * ctx,LLVMValueRef pos[3][4],struct ac_position_w_info * w)54 static void ac_analyze_position_w(struct ac_llvm_context *ctx, LLVMValueRef pos[3][4],
55 struct ac_position_w_info *w)
56 {
57 LLVMBuilderRef builder = ctx->builder;
58 LLVMValueRef all_w_negative = ctx->i1true;
59
60 w->w_reflection = ctx->i1false;
61 w->any_w_negative = ctx->i1false;
62
63 for (unsigned i = 0; i < 3; i++) {
64 LLVMValueRef neg_w;
65
66 neg_w = LLVMBuildFCmp(builder, LLVMRealOLT, pos[i][3], ctx->f32_0, "");
67 /* If neg_w is true, negate w_reflection. */
68 w->w_reflection = LLVMBuildXor(builder, w->w_reflection, neg_w, "");
69 w->any_w_negative = LLVMBuildOr(builder, w->any_w_negative, neg_w, "");
70 all_w_negative = LLVMBuildAnd(builder, all_w_negative, neg_w, "");
71 }
72 w->all_w_positive = LLVMBuildNot(builder, w->any_w_negative, "");
73 w->w_accepted = LLVMBuildNot(builder, all_w_negative, "");
74 }
75
76 /* Perform front/back face culling and return true if the primitive is accepted. */
ac_cull_face(struct ac_llvm_context * ctx,LLVMValueRef pos[3][4],struct ac_position_w_info * w,bool cull_front,bool cull_back,bool cull_zero_area)77 static LLVMValueRef ac_cull_face(struct ac_llvm_context *ctx, LLVMValueRef pos[3][4],
78 struct ac_position_w_info *w, bool cull_front, bool cull_back,
79 bool cull_zero_area)
80 {
81 LLVMBuilderRef builder = ctx->builder;
82
83 if (cull_front && cull_back)
84 return ctx->i1false;
85
86 if (!cull_front && !cull_back && !cull_zero_area)
87 return ctx->i1true;
88
89 /* Front/back face culling. Also if the determinant == 0, the triangle
90 * area is 0.
91 */
92 LLVMValueRef det_t0 = LLVMBuildFSub(builder, pos[2][0], pos[0][0], "");
93 LLVMValueRef det_t1 = LLVMBuildFSub(builder, pos[1][1], pos[0][1], "");
94 LLVMValueRef det_t2 = LLVMBuildFSub(builder, pos[0][0], pos[1][0], "");
95 LLVMValueRef det_t3 = LLVMBuildFSub(builder, pos[0][1], pos[2][1], "");
96 LLVMValueRef det_p0 = LLVMBuildFMul(builder, det_t0, det_t1, "");
97 LLVMValueRef det_p1 = LLVMBuildFMul(builder, det_t2, det_t3, "");
98 LLVMValueRef det = LLVMBuildFSub(builder, det_p0, det_p1, "");
99
100 /* Negative W negates the determinant. */
101 det = LLVMBuildSelect(builder, w->w_reflection, LLVMBuildFNeg(builder, det, ""), det, "");
102
103 LLVMValueRef accepted = NULL;
104 if (cull_front) {
105 LLVMRealPredicate cond = cull_zero_area ? LLVMRealOGT : LLVMRealOGE;
106 accepted = LLVMBuildFCmp(builder, cond, det, ctx->f32_0, "");
107 } else if (cull_back) {
108 LLVMRealPredicate cond = cull_zero_area ? LLVMRealOLT : LLVMRealOLE;
109 accepted = LLVMBuildFCmp(builder, cond, det, ctx->f32_0, "");
110 } else if (cull_zero_area) {
111 accepted = LLVMBuildFCmp(builder, LLVMRealONE, det, ctx->f32_0, "");
112 }
113 return accepted;
114 }
115
116 /* Perform view culling and small primitive elimination and return true
117 * if the primitive is accepted and initially_accepted == true. */
cull_bbox(struct ac_llvm_context * ctx,LLVMValueRef pos[3][4],LLVMValueRef initially_accepted,struct ac_position_w_info * w,LLVMValueRef vp_scale[2],LLVMValueRef vp_translate[2],LLVMValueRef small_prim_precision,bool cull_view_xy,bool cull_view_near_z,bool cull_view_far_z,bool cull_small_prims,bool use_halfz_clip_space)118 static LLVMValueRef cull_bbox(struct ac_llvm_context *ctx, LLVMValueRef pos[3][4],
119 LLVMValueRef initially_accepted, struct ac_position_w_info *w,
120 LLVMValueRef vp_scale[2], LLVMValueRef vp_translate[2],
121 LLVMValueRef small_prim_precision, bool cull_view_xy,
122 bool cull_view_near_z, bool cull_view_far_z, bool cull_small_prims,
123 bool use_halfz_clip_space)
124 {
125 LLVMBuilderRef builder = ctx->builder;
126
127 if (!cull_view_xy && !cull_view_near_z && !cull_view_far_z && !cull_small_prims)
128 return initially_accepted;
129
130 /* Skip the culling if the primitive has already been rejected or
131 * if any W is negative. The bounding box culling doesn't work when
132 * W is negative.
133 */
134 LLVMValueRef cond = LLVMBuildAnd(builder, initially_accepted, w->all_w_positive, "");
135 LLVMValueRef accepted_var = ac_build_alloca_undef(ctx, ctx->i1, "");
136 LLVMBuildStore(builder, initially_accepted, accepted_var);
137
138 ac_build_ifcc(ctx, cond, 10000000 /* does this matter? */);
139 {
140 LLVMValueRef bbox_min[3], bbox_max[3];
141 LLVMValueRef accepted = initially_accepted;
142
143 /* Compute the primitive bounding box for easy culling. */
144 for (unsigned chan = 0; chan < (cull_view_near_z || cull_view_far_z ? 3 : 2); chan++) {
145 bbox_min[chan] = ac_build_fmin(ctx, pos[0][chan], pos[1][chan]);
146 bbox_min[chan] = ac_build_fmin(ctx, bbox_min[chan], pos[2][chan]);
147
148 bbox_max[chan] = ac_build_fmax(ctx, pos[0][chan], pos[1][chan]);
149 bbox_max[chan] = ac_build_fmax(ctx, bbox_max[chan], pos[2][chan]);
150 }
151
152 /* View culling. */
153 if (cull_view_xy || cull_view_near_z || cull_view_far_z) {
154 for (unsigned chan = 0; chan < 3; chan++) {
155 LLVMValueRef visible;
156
157 if ((cull_view_xy && chan <= 1) || (cull_view_near_z && chan == 2)) {
158 float t = chan == 2 && use_halfz_clip_space ? 0 : -1;
159 visible = LLVMBuildFCmp(builder, LLVMRealOGE, bbox_max[chan],
160 LLVMConstReal(ctx->f32, t), "");
161 accepted = LLVMBuildAnd(builder, accepted, visible, "");
162 }
163
164 if ((cull_view_xy && chan <= 1) || (cull_view_far_z && chan == 2)) {
165 visible = LLVMBuildFCmp(builder, LLVMRealOLE, bbox_min[chan], ctx->f32_1, "");
166 accepted = LLVMBuildAnd(builder, accepted, visible, "");
167 }
168 }
169 }
170
171 /* Small primitive elimination. */
172 if (cull_small_prims) {
173 /* Assuming a sample position at (0.5, 0.5), if we round
174 * the bounding box min/max extents and the results of
175 * the rounding are equal in either the X or Y direction,
176 * the bounding box does not intersect the sample.
177 *
178 * See these GDC slides for pictures:
179 * https://frostbite-wp-prd.s3.amazonaws.com/wp-content/uploads/2016/03/29204330/GDC_2016_Compute.pdf
180 */
181 LLVMValueRef min, max, not_equal[2], visible;
182
183 for (unsigned chan = 0; chan < 2; chan++) {
184 /* Convert the position to screen-space coordinates. */
185 min = ac_build_fmad(ctx, bbox_min[chan], vp_scale[chan], vp_translate[chan]);
186 max = ac_build_fmad(ctx, bbox_max[chan], vp_scale[chan], vp_translate[chan]);
187 /* Scale the bounding box according to the precision of
188 * the rasterizer and the number of MSAA samples. */
189 min = LLVMBuildFSub(builder, min, small_prim_precision, "");
190 max = LLVMBuildFAdd(builder, max, small_prim_precision, "");
191
192 /* Determine if the bbox intersects the sample point.
193 * It also works for MSAA, but vp_scale, vp_translate,
194 * and small_prim_precision are computed differently.
195 */
196 min = ac_build_round(ctx, min);
197 max = ac_build_round(ctx, max);
198 not_equal[chan] = LLVMBuildFCmp(builder, LLVMRealONE, min, max, "");
199 }
200 visible = LLVMBuildAnd(builder, not_equal[0], not_equal[1], "");
201 accepted = LLVMBuildAnd(builder, accepted, visible, "");
202 }
203
204 LLVMBuildStore(builder, accepted, accepted_var);
205 }
206 ac_build_endif(ctx, 10000000);
207
208 return LLVMBuildLoad(builder, accepted_var, "");
209 }
210
211 /**
212 * Return i1 true if the primitive is accepted (not culled).
213 *
214 * \param pos Vertex positions 3x vec4
215 * \param initially_accepted AND'ed with the result. Some computations can be
216 * skipped if this is false.
217 * \param vp_scale Viewport scale XY.
218 * For MSAA, multiply them by the number of samples.
219 * \param vp_translate Viewport translation XY.
220 * For MSAA, multiply them by the number of samples.
221 * \param small_prim_precision Precision of small primitive culling. This should
222 * be the same as or greater than the precision of
223 * the rasterizer. Set to num_samples / 2^subpixel_bits.
224 * subpixel_bits are defined by the quantization mode.
225 * \param options See ac_cull_options.
226 */
ac_cull_triangle(struct ac_llvm_context * ctx,LLVMValueRef pos[3][4],LLVMValueRef initially_accepted,LLVMValueRef vp_scale[2],LLVMValueRef vp_translate[2],LLVMValueRef small_prim_precision,struct ac_cull_options * options)227 LLVMValueRef ac_cull_triangle(struct ac_llvm_context *ctx, LLVMValueRef pos[3][4],
228 LLVMValueRef initially_accepted, LLVMValueRef vp_scale[2],
229 LLVMValueRef vp_translate[2], LLVMValueRef small_prim_precision,
230 struct ac_cull_options *options)
231 {
232 struct ac_position_w_info w;
233 ac_analyze_position_w(ctx, pos, &w);
234
235 /* W culling. */
236 LLVMValueRef accepted = options->cull_w ? w.w_accepted : ctx->i1true;
237 accepted = LLVMBuildAnd(ctx->builder, accepted, initially_accepted, "");
238
239 /* Face culling. */
240 accepted = LLVMBuildAnd(
241 ctx->builder, accepted,
242 ac_cull_face(ctx, pos, &w, options->cull_front, options->cull_back, options->cull_zero_area),
243 "");
244
245 /* View culling and small primitive elimination. */
246 accepted = cull_bbox(ctx, pos, accepted, &w, vp_scale, vp_translate, small_prim_precision,
247 options->cull_view_xy, options->cull_view_near_z, options->cull_view_far_z,
248 options->cull_small_prims, options->use_halfz_clip_space);
249 return accepted;
250 }
251