[Mesa-dev] [PATCH 08/12] nir: Add a simple nir_lower_wpos_center() pass for Vulkan drivers.

[Matt Turner]

On Wed, May 18, 2016 at 3:00 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> nir_lower_wpos_ytransform() is great for OpenGL, which allows
> applications to choose whether their coordinate system's origin is
> upper left/lower left, and whether the pixel center should be on
> integer/half-integer boundaries.
>
> Vulkan, however, has much simpler requirements: the pixel center
> is always half-integer, and the origin is always upper left.  No
> coordinate transform is needed - we just need to add <0.5, 0.5>.
> This means that we can avoid using (and setting up) a uniform.
>
> I thought about adding more options to nir_lower_wpos_ytransform(),
> but making a new pass that never even touched uniforms seemed simpler.
>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> ---
>  src/compiler/Makefile.sources            |   1 +
>  src/compiler/nir/nir.h                   |   1 +
>  src/compiler/nir/nir_lower_wpos_center.c | 107 +++++++++++++++++++++++++++++++
>  3 files changed, 109 insertions(+)
>  create mode 100644 src/compiler/nir/nir_lower_wpos_center.c
>
> diff --git a/src/compiler/Makefile.sources b/src/compiler/Makefile.sources
> index 97f9eb4..b8f2b49 100644
> --- a/src/compiler/Makefile.sources
> +++ b/src/compiler/Makefile.sources
> @@ -212,6 +212,7 @@ 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_wpos_center.c \
>         nir/nir_lower_wpos_ytransform.c \
>         nir/nir_metadata.c \
>         nir/nir_move_vec_src_uses_to_dest.c \
> diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> index a21a7bd..78913d3 100644
> --- a/src/compiler/nir/nir.h
> +++ b/src/compiler/nir/nir.h
> @@ -2407,6 +2407,7 @@ typedef struct nir_lower_wpos_ytransform_options {
>
>  bool nir_lower_wpos_ytransform(nir_shader *shader,
>                                 const nir_lower_wpos_ytransform_options *options);
> +bool nir_lower_wpos_center(nir_shader *shader);
>
>  typedef struct nir_lower_drawpixels_options {
>     int texcoord_state_tokens[5];
> diff --git a/src/compiler/nir/nir_lower_wpos_center.c b/src/compiler/nir/nir_lower_wpos_center.c
> new file mode 100644
> index 0000000..d66109d
> --- /dev/null
> +++ b/src/compiler/nir/nir_lower_wpos_center.c
> @@ -0,0 +1,107 @@
> +/*
> + * Copyright © 2015 Red Hat
> + * Copyright © 2016 Intel Corporation
> + *
> + * 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.
> + */
> +
> +#include "nir.h"
> +#include "nir_builder.h"
> +#include "program/prog_instruction.h"
> +
> +/**
> + * This pass adds <0.5, 0.5> to all uses of gl_FragCoord.
> + *
> + * Run before nir_lower_io().
> + *
> + * For a more full featured pass, consider using nir_lower_wpos_ytransform(),
> + * which can handle pixel center integer / half integer, and origin lower
> + * left / upper left transformations.
> + *
> + * This simple pass is primarily intended for use by Vulkan drivers on
> + * hardware which provides an integer pixel center.  Vulkan mandates that
> + * the pixel center must be half-integer, and also that the coordinate
> + * system's origin must be upper left.  This means that there's no need
> + * for a uniform - we can always just add a constant.
> + */
> +
> +static void
> +add_half_to_fragcoord(nir_builder *b, nir_intrinsic_instr *intr)
> +{
> +   nir_ssa_def *wpos = &intr->dest.ssa;
> +
> +   assert(intr->dest.is_ssa);
> +
> +   b->cursor = nir_after_instr(&intr->instr);
> +
> +   wpos = nir_fadd(b, wpos, nir_imm_vec4(b, 0.5f, 0.5f, 0.0f, 0.0f));
> +
> +   nir_ssa_def_rewrite_uses_after(&intr->dest.ssa, nir_src_for_ssa(wpos),
> +                                  &intr->instr);
> +}
> +
> +static bool
> +lower_wpos_center_block(nir_builder *b, nir_block *block)
> +{
> +   bool progress = false;
> +
> +   nir_foreach_instr_safe(instr, block) {

Does this need to be _safe? If it does, why (for my own education)?