[Mesa-dev] [PATCH v2 1/2] nir: Add nir_lower_viewport_transform

[Qiang Yu]

On Mon, Apr 8, 2019 at 12:30 PM Alyssa Rosenzweig <alyssa at rosenzweig.io> wrote:
>
> 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!
>
> Signed-off-by: Alyssa Rosenzweig <alyssa at rosenzweig.io>
> Suggested-by: Qiang Yu <yuq825 at gmail.com>
> Cc: Jason Ekstrand <jason at jlekstrand.net>
> Cc: Eric Anholt <eric at anholt.net>
> ---
>  src/compiler/nir/meson.build                  |  1 +
>  src/compiler/nir/nir.h                        |  1 +
>  .../nir/nir_lower_viewport_transform.c        | 98 +++++++++++++++++++
>  3 files changed, 100 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..9646b72c053
> --- /dev/null
> +++ b/src/compiler/nir/nir_lower_viewport_transform.c
> @@ -0,0 +1,98 @@
> +/*
> + * 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);

This should be done per nir function, and I suggest to divide this function
into more sub-functions to follow conventions of other nir_lower_*
implementation.

Regards,
Qiang

> +            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);
> +   }
> +}
> --
> 2.20.1
>