On Mali hardware (supported by Panfrost and Lima), the fixed-function
transformation from world-space to screen-space coordinates is done in
the vertex shader prior to writing out the gl_Position varying, rather
than in dedicated hardware. This commit adds a shared NIR pass for
implementing coordinate transformation and lowering gl_Position writes
into screen-space gl_Position writes.
Signed-off-by: Alyssa Rosenzweig <aly...@rosenzweig.io>
Suggested-by: Qiang Yu <yuq...@gmail.com>
Cc: Jason Ekstrand <ja...@jlekstrand.net>
Cc: Eric Anholt <e...@anholt.net>
---
src/compiler/nir/meson.build | 1 +
src/compiler/nir/nir.h | 1 +
.../nir/nir_lower_viewport_transform.c | 126 ++++++++++++++++++
3 files changed, 128 insertions(+)
create mode 100644 src/compiler/nir/nir_lower_viewport_transform.c
diff --git a/src/compiler/nir/meson.build b/src/compiler/nir/meson.build
index c65f2ff62ff..c274361bdc4 100644
--- a/src/compiler/nir/meson.build
+++ b/src/compiler/nir/meson.build
@@ -151,6 +151,7 @@ files_libnir = files(
'nir_lower_vars_to_ssa.c',
'nir_lower_var_copies.c',
'nir_lower_vec_to_movs.c',
+ 'nir_lower_viewport_transform.c',
'nir_lower_wpos_center.c',
'nir_lower_wpos_ytransform.c',
'nir_lower_bit_size.c',
diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
index bc72d8f83f5..0f6ed734efa 100644
--- a/src/compiler/nir/nir.h
+++ b/src/compiler/nir/nir.h
@@ -3124,6 +3124,7 @@ void nir_lower_io_to_scalar(nir_shader *shader,
nir_variable_mode mask);
void nir_lower_io_to_scalar_early(nir_shader *shader, nir_variable_mode mask);
bool nir_lower_io_to_vector(nir_shader *shader, nir_variable_mode mask);
+void nir_lower_viewport_transform(nir_shader *shader);
bool nir_lower_uniforms_to_ubo(nir_shader *shader, int multiplier);
typedef struct nir_lower_subgroups_options {
diff --git a/src/compiler/nir/nir_lower_viewport_transform.c
b/src/compiler/nir/nir_lower_viewport_transform.c
new file mode 100644
index 00000000000..34a248d5871
--- /dev/null
+++ b/src/compiler/nir/nir_lower_viewport_transform.c
@@ -0,0 +1,126 @@
+/*
+ * Copyright (C) 2019 Alyssa Rosenzweig
+ *
+ * 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.
+ */
+
+/* On some hardware (particularly, all current versions of Mali GPUs),
+ * vertex shaders do not output gl_Position in world-space. Instead, they
+ * output gl_Position in transformed screen space via the "pseudo"
+ * position varying. Thus, this pass finds writes to gl_Position and
+ * changes them to transformed writes, still to gl_Position. The
+ * outputted screen space is still written back to VARYING_SLOT_POS,
+ * which is semantically ambiguous but nevertheless a good match for
+ * Gallium/NIR/Mali.
+ *
+ * Implements coordinate transformation as defined in section 12.5
+ * "Coordinate Transformation" of the OpenGL ES 3.2 full specification.
+ */
+
+#include "nir/nir.h"
+#include "nir/nir_builder.h"
+
+static void write_transformed_position(nir_builder *b, nir_src
+ input_point_src)
+{
+ nir_ssa_def *input_point = nir_ssa_for_src(b, input_point_src, 4);
+ nir_ssa_def *scale = nir_load_viewport_scale(b);
+ nir_ssa_def *offset = nir_load_viewport_offset(b);
+
+ /* World space to normalised device coordinates to screen space */
+
+ nir_ssa_def *w_recip = nir_frcp(b, nir_channel(b, input_point, 3));
+ nir_ssa_def *ndc_point = nir_fmul(b, nir_channels(b, input_point, 0x7),
w_recip);
+ nir_ssa_def *screen = nir_fadd(b, nir_fmul(b, ndc_point, scale), offset);
+
+ /* gl_Position will be written out in screenspace xyz, with w set to
+ * the reciprocal we computed earlier. The transformed w component is
+ * then used for perspective-correct varying interpolation. The
+ * transformed w component must preserve its original sign; this is
+ * used in depth clipping computations */
+
+ nir_ssa_def *screen_space = nir_vec4(b,
+ nir_channel(b, screen, 0),
+ nir_channel(b, screen, 1),
+ nir_channel(b, screen, 2),
+ w_recip);
+
+ /* Finally, write out the transformed values to the varying */
+
+ nir_intrinsic_instr *store;
+ store = nir_intrinsic_instr_create(b->shader, nir_intrinsic_store_output);
+ store->num_components = 4;
+ nir_intrinsic_set_base(store, 0);
+ nir_intrinsic_set_write_mask(store, 0xf);
+ store->src[0].ssa = screen_space;
+ store->src[0].is_ssa = true;
+ store->src[1] = nir_src_for_ssa(nir_imm_int(b, 0));
+ nir_builder_instr_insert(b, &store->instr);
+}
+
+void
+nir_lower_viewport_transform(nir_shader *shader)
+{
+ assert(shader->info.stage == MESA_SHADER_VERTEX);
+
+ nir_foreach_function(func, shader) {
+ nir_foreach_block(block, func->impl) {
+ nir_foreach_instr_safe(instr, block) {
+ if (instr->type != nir_instr_type_intrinsic) continue;
+
+ nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
+ nir_variable *out = NULL;
+
+ switch (intr->intrinsic) {
+ case nir_intrinsic_store_output:
+ /* already had i/o lowered.. lookup the matching output var: */
+ nir_foreach_variable(var, &shader->outputs) {
+ int drvloc = var->data.driver_location;
+
+ if (nir_intrinsic_base(intr) == drvloc) {
+ out = var;
+ break;
+ }
+ }
+
+ break;
+
+ default:
+ break;
+ }
+
+ if (!out) continue;
+
+ if (out->data.mode != nir_var_shader_out)
+ continue;
+
+ if (out->data.location != VARYING_SLOT_POS)
+ continue;
+
+ nir_builder b;
+ nir_builder_init(&b, func->impl);
+ b.cursor = nir_before_instr(instr);
+
+ write_transformed_position(&b, intr->src[0]);
+ nir_instr_remove(instr);
+ }
+ }
+ }
+}
--
2.20.1
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