nir: Add an optimization pass to remove trivial continues

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>

src/compiler/Makefile.sources

@@ -244,6 +244,7 @@ NIR_FILES = \
 	nir/nir_opt_loop_unroll.c \
 	nir/nir_opt_peephole_select.c \
 	nir/nir_opt_remove_phis.c \
+	nir/nir_opt_trivial_continues.c \
 	nir/nir_opt_undef.c \
 	nir/nir_phi_builder.c \
 	nir/nir_phi_builder.h \

src/compiler/nir/nir.h

@@ -2559,6 +2559,8 @@ bool nir_opt_peephole_select(nir_shader *shader, unsigned limit);
 
 bool nir_opt_remove_phis(nir_shader *shader);
 
+bool nir_opt_trivial_continues(nir_shader *shader);
+
 bool nir_opt_undef(nir_shader *shader);
 
 bool nir_opt_conditional_discard(nir_shader *shader);

src/compiler/nir/nir_opt_trivial_continues.c

@@ -0,0 +1,137 @@
+/*
+ * 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"
+
+static bool
+instr_is_continue(nir_instr *instr)
+{
+   if (instr->type != nir_instr_type_jump)
+      return false;
+
+   return nir_instr_as_jump(instr)->type == nir_jump_continue;
+}
+
+static bool
+lower_trivial_continues_block(nir_block *block, nir_loop *loop)
+{
+   bool progress = false;
+   nir_instr *first_instr = nir_block_first_instr(block);
+   if (!first_instr || instr_is_continue(first_instr)) {
+      /* The block contains only a continue if anything */
+      nir_cf_node *prev_node = nir_cf_node_prev(&block->cf_node);
+      if (prev_node && prev_node->type == nir_cf_node_if) {
+         nir_if *prev_if = nir_cf_node_as_if(prev_node);
+         progress |= lower_trivial_continues_block(
+            nir_if_last_then_block(prev_if), loop);
+         progress |= lower_trivial_continues_block(
+            nir_if_last_else_block(prev_if), loop);
+      }
+
+      /* The lower_phis_to_regs helper is smart enough to push phi sources as
+       * far up if-ladders as it possibly can.  In other words, the recursive
+       * calls above shouldn't be adding instructions to this block since it
+       * follows an if statement.
+       */
+      first_instr = nir_block_first_instr(block);
+      assert(!first_instr || instr_is_continue(first_instr));
+   }
+
+   nir_instr *last_instr = nir_block_last_instr(block);
+   if (!last_instr || !instr_is_continue(last_instr))
+      return progress;
+
+   /* We're at the end of a block that goes straight through to the end of the
+    * loop and continues to the top.  We can exit normally instead of jump.
+    */
+   nir_lower_phis_to_regs_block(nir_loop_first_block(loop));
+   nir_instr_remove(last_instr);
+   return true;
+}
+
+static bool
+lower_trivial_continues_list(struct exec_list *cf_list,
+                             bool list_ends_at_loop_tail,
+                             nir_loop *loop)
+{
+   bool progress = false;
+   foreach_list_typed(nir_cf_node, cf_node, node, cf_list) {
+      bool at_loop_tail = list_ends_at_loop_tail &&
+                          &cf_node->node == exec_list_get_tail(cf_list);
+      switch (cf_node->type) {
+      case nir_cf_node_block:
+         break;
+
+      case nir_cf_node_if: {
+         nir_if *nif = nir_cf_node_as_if(cf_node);
+         if (lower_trivial_continues_list(&nif->then_list, at_loop_tail, loop))
+            progress = true;
+         if (lower_trivial_continues_list(&nif->else_list, at_loop_tail, loop))
+            progress = true;
+         break;
+      }
+
+      case nir_cf_node_loop: {
+         nir_loop *loop = nir_cf_node_as_loop(cf_node);
+         if (lower_trivial_continues_list(&loop->body, true, loop))
+            progress = true;
+         if (lower_trivial_continues_block(nir_loop_last_block(loop), loop))
+            progress = true;
+         break;
+      }
+
+      case nir_cf_node_function:
+         unreachable("Invalid cf type");
+      }
+   }
+
+   return progress;
+}
+
+/**
+ * This simple pass gets rid of any "trivial" continues, i.e. those that are
+ * at the tail of a loop where we can just delete the continue instruction and
+ * control-flow will naturally and harmlessly take us back to the top of the
+ * loop.
+ */
+bool
+nir_opt_trivial_continues(nir_shader *shader)
+{
+   bool progress = false;
+
+   nir_foreach_function(function, shader) {
+      if (function->impl == NULL)
+         continue;
+
+      /* First we run the simple pass to get rid of pesky continues */
+      if (lower_trivial_continues_list(&function->impl->body, false, NULL)) {
+         nir_metadata_preserve(function->impl, nir_metadata_none);
+
+         /* If that made progress, we're no longer really in SSA form. */
+         nir_convert_to_ssa_impl(function->impl);
+         progress = true;
+      }
+   }
+
+   return progress;
+}