nir: Add nir_lower_viewport_transform

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.

v2: Run directly on derefs before io/vars are lowered to cleanup the
code substantially. Thank you to Qiang for this suggestion!

v3: Bikeshed continues.

v4: Add to Makefile.sources (per Jason's comment). Bikeshed comment.

Ian and Qiang's reviews are from v3, but no real functional changes from
v4. Rob's review is from v4.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Suggested-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>

src/compiler/Makefile.sources

@@ -273,6 +273,7 @@ NIR_FILES = \
 	nir/nir_lower_vars_to_ssa.c \
 	nir/nir_lower_var_copies.c \
 	nir/nir_lower_vec_to_movs.c \
+	nir/nir_lower_viewport_transform.c \
 	nir/nir_lower_wpos_center.c \
 	nir/nir_lower_wpos_ytransform.c \
 	nir/nir_metadata.c \

src/compiler/nir/meson.build

@@ -152,6 +152,7 @@ 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',

src/compiler/nir/nir.h

@@ -3114,6 +3114,7 @@ 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 {

src/compiler/nir/nir_lower_viewport_transform.c

@@ -0,0 +1,102 @@
+/*
+ * 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.
+ *
+ * This pass must run before lower_vars/lower_io such that derefs are
+ * still in place.
+ */
+
+#include "nir/nir.h"
+#include "nir/nir_builder.h"
+
+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);
+            if (intr->intrinsic != nir_intrinsic_store_deref)
+               continue;
+
+            nir_variable *var = nir_intrinsic_get_var(intr, 0);
+            if (var->data.location != VARYING_SLOT_POS)
+               continue;
+
+            nir_builder b;
+            nir_builder_init(&b, func->impl);
+            b.cursor = nir_before_instr(instr);
+
+            /* Grab the source and viewport */
+            nir_ssa_def *input_point = nir_ssa_for_src(&b, intr->src[1], 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);
+
+            nir_instr_rewrite_src(instr, &intr->src[1],
+                                  nir_src_for_ssa(screen_space));
+         }
+      }
+
+      nir_metadata_preserve(func->impl, nir_metadata_block_index |
+                            nir_metadata_dominance);
+   }
+}