nir: add lowering pass for clip plane enabling

a pass which rewrites gl_ClipDistance[n] to an undef if the corresponding
clip plane is disabled in the rasterizer state

this pass is needed for zink to handle api disables of clip planes

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5529>

src/compiler/nir/meson.build

@@ -125,6 +125,7 @@ files_libnir = files(
   'nir_lower_clamp_color_outputs.c',
   'nir_lower_clip.c',
   'nir_lower_clip_cull_distance_arrays.c',
+  'nir_lower_clip_disable.c',
   'nir_lower_clip_halfz.c',
   'nir_lower_variable_initializers.c',
   'nir_lower_discard_to_demote.c',

src/compiler/nir/nir.h

@@ -4357,6 +4357,7 @@ bool nir_lower_clip_gs(nir_shader *shader, unsigned ucp_enables,
 bool nir_lower_clip_fs(nir_shader *shader, unsigned ucp_enables,
                        bool use_clipdist_array);
 bool nir_lower_clip_cull_distance_arrays(nir_shader *nir);
+bool nir_lower_clip_disable(nir_shader *shader, unsigned clip_plane_enable);
 
 void nir_lower_point_size_mov(nir_shader *shader,
                               const gl_state_index16 *pointsize_state_tokens);

src/compiler/nir/nir_lower_clip_disable.c

@@ -0,0 +1,131 @@
+/*
+ * Copyright © 2020 Mike Blumenkrantz
+ *
+ * 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.
+ * 
+ * Authors:
+ *    Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
+ */
+
+#include "nir.h"
+#include "nir_builder.h"
+
+
+/**
+ * This pass uses the enabled clip planes from the rasterizer state to rewrite
+ * vertex shader store operations and store an undef to the corresponding gl_ClipDistance[n]
+ * value if the plane is disabled
+ */
+
+/* recursively nest if/else blocks until we get to an array index,
+ * then overwrite it if that plane isn't enabled
+ */
+static void
+recursive_if_chain(nir_builder *b, nir_deref_instr *deref, nir_ssa_def *value, unsigned clip_plane_enable, nir_ssa_def *index, unsigned start, unsigned end)
+{
+   if (start == end - 1) {
+      /* store the original value again if the clip plane is enabled */
+      if (clip_plane_enable & (1 << start))
+         nir_store_deref(b, deref, value, 1 << start);
+      else
+         nir_store_deref(b, deref, nir_ssa_undef(b, 1, 32), 1 << start);
+      return;
+   }
+
+   unsigned mid = start + (end - start) / 2;
+   nir_push_if(b, nir_ilt(b, index, nir_imm_int(b, mid)));
+   recursive_if_chain(b, deref, value, clip_plane_enable, index, start, mid);
+   nir_push_else(b, NULL);
+   recursive_if_chain(b, deref, value, clip_plane_enable, index, mid, end);
+   nir_pop_if(b, NULL);
+}
+
+/* vulkan (and some drivers) provides no concept of enabling clip planes through api,
+ * so we rewrite disabled clip planes to an undefined value in order to disable them
+ */
+static bool
+lower_clip_plane_store(nir_intrinsic_instr *instr, unsigned clip_plane_enable, nir_builder *b)
+{
+   nir_variable *out;
+   unsigned plane;
+
+   if (instr->intrinsic != nir_intrinsic_store_deref)
+      return false;
+
+   nir_deref_instr *deref = nir_src_as_deref(instr->src[0]);
+
+   out = nir_deref_instr_get_variable(deref);
+   if ((out->data.location != VARYING_SLOT_CLIP_DIST0) ||
+        out->data.mode != nir_var_shader_out)
+      return false;
+
+   b->cursor = nir_after_instr(&instr->instr);
+   if (nir_src_is_const(deref->arr.index)) {
+      /* storing using a constant index */
+      plane = nir_src_as_uint(deref->arr.index);
+      /* no need to make changes if the clip plane is enabled */
+      if (clip_plane_enable & (1 << plane))
+         return false;
+
+      nir_store_deref(b, deref, nir_ssa_undef(b, 1, 32), 1 << plane);
+   } else {
+      /* storing using a variable index */
+      nir_ssa_def *index = nir_ssa_for_src(b, deref->arr.index, 1);
+      unsigned length = glsl_get_length(nir_deref_instr_parent(deref)->type);
+
+      recursive_if_chain(b, deref, instr->src[1].ssa, clip_plane_enable, index, 0, length);
+   }
+   nir_instr_remove(&instr->instr);
+   return true;
+}
+
+bool
+nir_lower_clip_disable(nir_shader *shader, unsigned clip_plane_enable)
+{
+   bool progress = false;
+
+   nir_foreach_variable(var, &shader->outputs) {
+      if (var->data.location == VARYING_SLOT_CLIP_DIST0) {
+         unsigned size = glsl_get_length(var->type);
+         /* if currently-enabled planes match used planes then no-op */
+         if (clip_plane_enable == (1u << size) - 1)
+            return false;
+      }
+   }
+
+   nir_foreach_function(function, shader) {
+      if (function->impl) {
+         nir_builder builder;
+         nir_builder_init(&builder, function->impl);
+         nir_foreach_block(block, function->impl) {
+            nir_foreach_instr_safe(instr, block) {
+               if (instr->type == nir_instr_type_intrinsic)
+                  progress |= lower_clip_plane_store(nir_instr_as_intrinsic(instr),
+                                                     clip_plane_enable,
+                                                     &builder);
+            }
+         }
+
+         nir_metadata_preserve(function->impl, nir_metadata_block_index | nir_metadata_dominance);
+      }
+   }
+
+   return progress;
+}