nir: Add a phi node placement helper

Right now, we have phi placement code in two places and there are other
places where it would be nice to be able to do this analysis.  Instead of
repeating it all over the place, this commit adds a helper for placing all
of the needed phi nodes for a value.

v2: Add better documentation

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

1 files changed, 115 insertions, 0 deletions

diff --git a/src/compiler/nir/nir_phi_builder.h b/src/compiler/nir/nir_phi_builder.h
new file mode 100644
index 0000000..edc5302
--- /dev/null
+++ b/src/compiler/nir/nir_phi_builder.h
@@ -0,0 +1,115 @@
+/*
+ * 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.
+ */
+
+#pragma once
+
+#include "nir.h"
+
+/** A helper for placing phi nodes in a NIR shader
+ *
+ * Basic usage goes something like this:
+ *
+ *     each variable, var, has:
+ *         a bitset var.defs of blocks where the variable is defined
+ *         a struct nir_phi_builder_value *pb_val
+ *
+ *     // initialize bitsets
+ *     foreach block:
+ *         foreach def of variable var:
+ *             var.defs[def.block] = true;
+ *
+ *     // initialize phi builder
+ *     pb = nir_phi_builder_create()
+ *     foreach var:
+ *         var.pb_val = nir_phi_builder_add_value(pb, var.defs)
+ *
+ *     // Visit each block.  This needs to visit dominators first;
+ *     // nir_for_each_block() will be ok.
+ *     foreach block:
+ *         foreach instruction:
+ *             foreach use of variable var:
+ *                 replace use with nir_phi_builder_get_block_def(var.pb_val)
+ *             foreach def of variable var:
+ *                 create ssa def, register with
+ *     nir_phi_builder_set_block_def(var.pb_val)
+ *
+ *     nir_phi_builder_finish(pb)
+ */
+struct nir_phi_builder;
+
+struct nir_phi_builder_value;
+
+/* Create a new phi builder.
+ *
+ * While this is fairly cheap, it does allocate some memory and walk the list
+ * of blocks so it's recommended that you only call it once and use it to
+ * build phis for several values.
+ */
+struct nir_phi_builder *nir_phi_builder_create(nir_function_impl *impl);
+
+/* Register a value with the builder.
+ *
+ * The 'defs' parameter specifies a bitset of blocks in which the given value
+ * is defined.  This is used to determine where to place the phi nodes.
+ */
+struct nir_phi_builder_value *
+nir_phi_builder_add_value(struct nir_phi_builder *pb, unsigned num_components,
+                          unsigned bit_size, const BITSET_WORD *defs);
+
+/* Register a definition for the given value and block.
+ *
+ * It is safe to call this function as many times as you wish for any given
+ * block/value pair.  However, it always replaces whatever was there
+ * previously even if that definition is from a phi node.  The phi builder
+ * always uses the latest information it has, so you must be careful about the
+ * order in which you register definitions.  The final value at the end of the
+ * block must be the last value registered.
+ */
+void
+nir_phi_builder_value_set_block_def(struct nir_phi_builder_value *val,
+                                    nir_block *block, nir_ssa_def *def);
+
+/* Get the definition for the given value in the given block.
+ *
+ * This definition will always be the latest definition known for the given
+ * block.  If no definition is immediately available, it will crawl up the
+ * dominance tree and insert phi nodes as needed until it finds one.  In the
+ * case that no suitable definition is found, it will return the result of a
+ * nir_ssa_undef_instr with the correct number of components.
+ *
+ * Because this function only uses the latest available information for any
+ * given block, you must have already finished registering definitions for any
+ * blocks that dominate the current block in order to get the correct result.
+ */
+nir_ssa_def *
+nir_phi_builder_value_get_block_def(struct nir_phi_builder_value *val,
+                                    nir_block *block);
+
+/* Finish building phi nodes and free the builder.
+ *
+ * This function does far more than just free memory.  Prior to calling
+ * nir_phi_builder_finish, no phi nodes have actually been inserted in the
+ * program.  This function is what finishes setting up phi node sources and
+ * adds the phi nodes to the program.
+ */
+void nir_phi_builder_finish(struct nir_phi_builder *pb);