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

[Kenneth Graunke]

On Thursday, May 19, 2016 1:21:16 PM PDT Matt Turner wrote:
> 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)?
> 

I don't think it does - I just copied that from Rob's code.

We do add instructions after the current instruction...so using safe
will skip over those newly emitted instruction (which don't need
processing).  Otherwise, we'll actually iterate over them (and not
touch them anyway).

Dropping _safe seems pretty reasonable.  Want me to do that?
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