nir: Add a peephole select optimization

Reviewed-by: Connor Abbott <[email protected]>

1 files changed, 214 insertions, 0 deletions

diff --git a/src/glsl/nir/nir_opt_peephole_select.c b/src/glsl/nir/nir_opt_peephole_select.c
new file mode 100644
index 00000000000..b3b54c1a1cb
--- /dev/null
+++ b/src/glsl/nir/nir_opt_peephole_select.c
@@ -0,0 +1,214 @@
+/*
+ * Copyright © 2014 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.
+ *
+ * Authors:
+ *    Jason Ekstrand ([email protected])
+ *
+ */
+
+#include "nir.h"
+
+/*
+ * Implements a small peephole optimization that looks for
+ *
+ * if (cond) {
+ *    <empty>
+ * } else {
+ *    <empty>
+ * }
+ * phi
+ * ...
+ * phi
+ *
+ * and replaces it with a series of selects.  It can also handle the case
+ * where, instead of being empty, the if may contain some move operations
+ * whose only use is one of the following phi nodes.  This happens all the
+ * time when the SSA form comes from a conditional assignment with a
+ * swizzle.
+ */
+
+struct peephole_select_state {
+   void *mem_ctx;
+   nir_function_impl *impl;
+   bool progress;
+};
+
+static bool
+are_all_move_to_phi(nir_block *block)
+{
+   nir_foreach_instr(block, instr) {
+      if (instr->type != nir_instr_type_alu)
+         return false;
+
+      /* It must be a move operation */
+      nir_alu_instr *mov = nir_instr_as_alu(instr);
+      if (mov->op != nir_op_fmov && mov->op != nir_op_imov)
+         return false;
+
+      /* Can't handle saturate */
+      if (mov->dest.saturate)
+         return false;
+
+      /* It must be SSA */
+      if (!mov->dest.dest.is_ssa)
+         return false;
+
+      /* It cannot have any if-uses */
+      if (mov->dest.dest.ssa.if_uses->entries != 0)
+         return false;
+
+      /* The only uses of this definition must be phi's in the successor */
+      struct set_entry *entry;
+      set_foreach(mov->dest.dest.ssa.uses, entry) {
+         const nir_instr *dest_instr = entry->key;
+         if (dest_instr->type != nir_instr_type_phi ||
+             dest_instr->block != block->successors[0])
+            return false;
+      }
+   }
+
+   return true;
+}
+
+static bool
+nir_opt_peephole_select_block(nir_block *block, void *void_state)
+{
+   struct peephole_select_state *state = void_state;
+
+   /* If the block is empty, then it certainly doesn't have any phi nodes,
+    * so we can skip it.  This also ensures that we do an early skip on the
+    * end block of the function which isn't actually attached to the CFG.
+    */
+   if (exec_list_is_empty(&block->instr_list))
+      return true;
+
+   if (nir_cf_node_is_first(&block->cf_node))
+      return true;
+
+   nir_cf_node *prev_node = nir_cf_node_prev(&block->cf_node);
+   if (prev_node->type != nir_cf_node_if)
+      return true;
+
+   nir_if *if_stmt = nir_cf_node_as_if(prev_node);
+   nir_cf_node *then_node = nir_if_first_then_node(if_stmt);
+   nir_cf_node *else_node = nir_if_first_else_node(if_stmt);
+
+   /* We can only have one block in each side ... */
+   if (nir_if_last_then_node(if_stmt) != then_node ||
+       nir_if_last_else_node(if_stmt) != else_node)
+      return true;
+
+   nir_block *then_block = nir_cf_node_as_block(then_node);
+   nir_block *else_block = nir_cf_node_as_block(else_node);
+
+   /* ... and those blocks must only contain move-to-phi. */
+   if (!are_all_move_to_phi(then_block) || !are_all_move_to_phi(else_block))
+      return true;
+
+   /* At this point, we know that the previous CFG node is an if-then
+    * statement containing only moves to phi nodes in this block.  We can
+    * just remove that entire CF node and replace all of the phi nodes with
+    * selects.
+    */
+
+   nir_foreach_instr_safe(block, instr) {
+      if (instr->type != nir_instr_type_phi)
+         break;
+
+      nir_phi_instr *phi = nir_instr_as_phi(instr);
+      nir_alu_instr *sel = nir_alu_instr_create(state->mem_ctx, nir_op_bcsel);
+      sel->src[0].src = nir_src_copy(if_stmt->condition, state->mem_ctx);
+
+      assert(exec_list_length(&phi->srcs) == 2);
+      foreach_list_typed(nir_phi_src, src, node, &phi->srcs) {
+         assert(src->pred == then_block || src->pred == else_block);
+         assert(src->src.is_ssa);
+
+         unsigned idx = src->pred == then_block ? 1 : 2;
+
+         if (src->src.ssa->parent_instr->block == src->pred) {
+            /* We already know that this instruction must be a move with
+             * this phi's in this block as its only users.
+             */
+            nir_alu_instr *mov = nir_instr_as_alu(src->src.ssa->parent_instr);
+            assert(mov->instr.type == nir_instr_type_alu);
+            assert(mov->op == nir_op_fmov || mov->op == nir_op_imov);
+
+            sel->src[idx].src = nir_src_copy(mov->src[0].src, state->mem_ctx);
+            sel->src[idx].negate = mov->src[0].negate;
+            sel->src[idx].abs = mov->src[0].abs;
+            memcpy(sel->src[idx].swizzle, mov->src[0].swizzle, 4);
+         } else {
+            sel->src[idx].src = nir_src_copy(src->src, state->mem_ctx);
+         }
+      }
+
+      sel->dest.dest.is_ssa = true;
+      nir_ssa_def_init(state->impl, &sel->instr, &sel->dest.dest.ssa,
+                       phi->dest.ssa.num_components, phi->dest.ssa.name);
+      sel->dest.write_mask = (1 << phi->dest.ssa.num_components) - 1;
+
+      nir_src sel_dest_src = {
+         .is_ssa = true,
+         .ssa = &sel->dest.dest.ssa,
+      };
+      nir_ssa_def_rewrite_uses(&phi->dest.ssa, sel_dest_src, state->mem_ctx);
+
+      nir_instr_insert_before(&phi->instr, &sel->instr);
+      nir_instr_remove(&phi->instr);
+   }
+
+   nir_cf_node_remove(&if_stmt->cf_node);
+   state->progress = true;
+
+   return true;
+}
+
+static bool
+nir_opt_peephole_select_impl(nir_function_impl *impl)
+{
+   struct peephole_select_state state;
+
+   state.mem_ctx = ralloc_parent(impl);
+   state.progress = false;
+   state.impl = impl;
+
+   nir_foreach_block(impl, nir_opt_peephole_select_block, &state);
+
+   if (state.progress)
+      nir_metadata_dirty(impl, nir_metadata_none);
+
+   return state.progress;
+}
+
+bool
+nir_opt_peephole_select(nir_shader *shader)
+{
+   bool progress = false;
+
+   nir_foreach_overload(shader, overload) {
+      if (overload->impl)
+         progress |= nir_opt_peephole_select_impl(overload->impl);
+   }
+
+   return progress;
+}