nir: Add a simple nir_lower_wpos_center() pass for Vulkan drivers.

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.

v2: Use normal iterator rather than _safe variant (noticed by Matt).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Rob Clark <robdclark@gmail.com>

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 \

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];

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(instr, block) {
+      if (instr->type == nir_instr_type_intrinsic) {
+         nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
+         if (intr->intrinsic == nir_intrinsic_load_var) {
+            nir_deref_var *dvar = intr->variables[0];
+            nir_variable *var = dvar->var;
+
+            if (var->data.mode == nir_var_shader_in &&
+                var->data.location == VARYING_SLOT_POS) {
+               /* gl_FragCoord should not have array/struct deref's: */
+               assert(dvar->deref.child == NULL);
+               add_half_to_fragcoord(b, intr);
+               progress = true;
+            }
+         }
+      }
+   }
+
+   return progress;
+}
+
+bool
+nir_lower_wpos_center(nir_shader *shader)
+{
+   bool progress = false;
+   nir_builder b;
+
+   assert(shader->stage == MESA_SHADER_FRAGMENT);
+
+   nir_foreach_function(function, shader) {
+      if (function->impl) {
+         nir_builder_init(&b, function->impl);
+
+         nir_foreach_block(block, function->impl) {
+            progress = lower_wpos_center_block(&b, block) || progress;
+         }
+         nir_metadata_preserve(function->impl, nir_metadata_block_index |
+                                               nir_metadata_dominance);
+      }
+   }
+
+   return progress;
+}