src/compiler/nir/nir_lower_viewport_transform.c

/*
 * 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.mode != nir_var_shader_out ||
                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);
   }
}