[Mesa-dev] [PATCH 05/13] nir: Add a LCSAA-pass

[Timothy Arceri]

From: Thomas Helland <thomashelland90 at gmail.com>

V2: Do a "depth first search" to convert to LCSSA

V3: Small comment fixup

V4: Rebase, adapt to removal of function overloads

V5: Rebase, adapt to relocation of nir to compiler/nir
    Still need to adapt to potential if-uses
    Work around nir_validate issue

V6 (Timothy):
 - tidy lcssa and stop leaking memory
 - dont rewrite the src for the lcssa phi node
 - validate lcssa phi srcs to avoid postvalidate assert
 - don't add new phi if one already exists
 - more lcssa phi validation fixes
 - Rather than marking ssa defs inside a loop just mark blocks inside
   a loop. This is simpler and fixes lcssa for intrinsics which do
   not have a destination.
 - don't create LCSSA phis for loops we won't unroll
 - require loop metadata for lcssa pass
 - handle case were the ssa defs use outside the loop is already a phi

V7: (Timothy)
- pass indirect mask to metadata call

v8: (Timothy)
- make convert to lcssa a helper function rather than a nir pass
- replace inside loop bitset with on the fly block index logic.
- remove lcssa phi validation special cases
- inline code from useless helpers, suggested by Jason.
- always do lcssa on loops, suggested by Jason.
- stop making lcssa phis special. Add as many source as the block
  has predecessors, suggested by Jason.

V9: (Timothy)
- fix regression with the is_lcssa_phi field not being initialised
  to false now that ralloc() doesn't zero out memory.

V10: (Timothy)
- remove extra braces in SSA example, pointed out by Topi

V11: (Timothy)
- add missing support for LCSSA phis in if conditions.

V12:
- small tidy up suggested by Jason.
- always create lcssa phi even if it just points to an lcssa
  phi from an inner loop
---
 src/compiler/Makefile.sources   |   1 +
 src/compiler/nir/nir.c          |   1 +
 src/compiler/nir/nir.h          |   4 +
 src/compiler/nir/nir_to_lcssa.c | 204 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 210 insertions(+)
 create mode 100644 src/compiler/nir/nir_to_lcssa.c

diff --git a/src/compiler/Makefile.sources b/src/compiler/Makefile.sources
index edc70f7..f551449 100644
--- a/src/compiler/Makefile.sources
+++ b/src/compiler/Makefile.sources
@@ -256,6 +256,7 @@ NIR_FILES = \
 	nir/nir_split_var_copies.c \
 	nir/nir_sweep.c \
 	nir/nir_to_ssa.c \
+	nir/nir_to_lcssa.c \
 	nir/nir_validate.c \
 	nir/nir_vla.h \
 	nir/nir_worklist.c \
diff --git a/src/compiler/nir/nir.c b/src/compiler/nir/nir.c
index 2c3531c..e522a67 100644
--- a/src/compiler/nir/nir.c
+++ b/src/compiler/nir/nir.c
@@ -561,6 +561,7 @@ nir_phi_instr_create(nir_shader *shader)
 {
    nir_phi_instr *instr = ralloc(shader, nir_phi_instr);
    instr_init(&instr->instr, nir_instr_type_phi);
+   instr->is_lcssa_phi = false;
 
    dest_init(&instr->dest);
    exec_list_make_empty(&instr->srcs);
diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
index c7da1e7..fd35bd0 100644
--- a/src/compiler/nir/nir.h
+++ b/src/compiler/nir/nir.h
@@ -1360,6 +1360,8 @@ typedef struct {
    struct exec_list srcs; /** < list of nir_phi_src */
 
    nir_dest dest;
+
+   bool is_lcssa_phi;
 } nir_phi_instr;
 
 typedef struct {
@@ -2526,6 +2528,8 @@ void nir_convert_to_ssa(nir_shader *shader);
 bool nir_repair_ssa_impl(nir_function_impl *impl);
 bool nir_repair_ssa(nir_shader *shader);
 
+void nir_convert_loop_to_lcssa(nir_loop *loop);
+
 /* If phi_webs_only is true, only convert SSA values involved in phi nodes to
  * registers.  If false, convert all values (even those not involved in a phi
  * node) to registers.
diff --git a/src/compiler/nir/nir_to_lcssa.c b/src/compiler/nir/nir_to_lcssa.c
new file mode 100644
index 0000000..f4bb57b
--- /dev/null
+++ b/src/compiler/nir/nir_to_lcssa.c
@@ -0,0 +1,204 @@
+/*
+ * Copyright © 2015 Thomas Helland
+ *
+ * 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.
+ */
+
+/*
+ * This pass converts the ssa-graph into "Loop Closed SSA form". This is
+ * done by placing phi nodes at the exits of the loop for all values
+ * that are used outside the loop. The result is it transforms:
+ *
+ * loop {                    ->      loop {
+ *    ssa2 = ....            ->          ssa2 = ...
+ *    if (cond)              ->          if (cond)
+ *       break;              ->             break;
+ *    ssa3 = ssa2 * ssa4     ->          ssa3 = ssa2 * ssa4
+ * }                         ->       }
+ * ssa6 = ssa2 + 4           ->       ssa5 = lcssa_phi(ssa2)
+ *                                    ssa6 = ssa5 + 4
+ */
+
+#include "nir.h"
+
+typedef struct {
+   /* The nir_shader we are transforming */
+   nir_shader *shader;
+
+   /* The loop we store information for */
+   nir_loop *loop;
+
+} lcssa_state;
+
+static bool
+is_if_use_inside_loop(nir_src *use, nir_loop* loop)
+{
+   nir_block *block_before_loop =
+      nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
+   nir_block *block_after_loop =
+      nir_cf_node_as_block(nir_cf_node_next(&loop->cf_node));
+
+   nir_block *prev_block =
+      nir_cf_node_as_block(nir_cf_node_prev(&use->parent_if->cf_node));
+   if (prev_block->index <= block_before_loop->index ||
+       prev_block->index >= block_after_loop->index) {
+      return false;
+   }
+
+   return true;
+}
+
+static bool
+is_use_inside_loop(nir_src *use, nir_loop* loop)
+{
+   nir_block *block_before_loop =
+      nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
+   nir_block *block_after_loop =
+      nir_cf_node_as_block(nir_cf_node_next(&loop->cf_node));
+
+   if (use->parent_instr->block->index <= block_before_loop->index ||
+       use->parent_instr->block->index >= block_after_loop->index) {
+      return false;
+   }
+
+   return true;
+}
+
+static bool
+convert_loop_exit_for_ssa(nir_ssa_def *def, void *void_state)
+{
+   lcssa_state *state = void_state;
+   bool all_uses_inside_loop = true;
+
+   nir_block *block_after_loop =
+      nir_cf_node_as_block(nir_cf_node_next(&state->loop->cf_node));
+
+   nir_foreach_use(use, def) {
+      if (use->parent_instr->type == nir_instr_type_phi &&
+          use->parent_instr->block == block_after_loop) {
+         continue;
+      }
+
+      if (!is_use_inside_loop(use, state->loop)) {
+         all_uses_inside_loop = false;
+      }
+   }
+
+   nir_foreach_if_use(use, def) {
+      if (!is_if_use_inside_loop(use, state->loop)) {
+         all_uses_inside_loop = false;
+      }
+   }
+
+   /* There where no sources that had defs outside the loop */
+   if (all_uses_inside_loop)
+      return true;
+
+   /* Initialize a phi-instruction */
+   nir_phi_instr *phi = nir_phi_instr_create(state->shader);
+   nir_ssa_dest_init(&phi->instr, &phi->dest,
+                     def->num_components, def->bit_size, "LCSSA-phi");
+   phi->is_lcssa_phi = true;
+
+   /* Create a phi node with as many sources pointing to the same ssa_def as
+    * the block has predecessors.
+    */
+   struct set_entry *entry;
+   set_foreach(block_after_loop->predecessors, entry) {
+      nir_phi_src *phi_src = ralloc(phi, nir_phi_src);
+      phi_src->src = nir_src_for_ssa(def);
+      phi_src->pred = (nir_block *) entry->key;
+
+      exec_list_push_tail(&phi->srcs, &phi_src->node);
+   }
+
+   nir_instr_insert_before_block(block_after_loop, &phi->instr);
+
+   /* Run through all uses and rewrite those outside the loop to point to
+    * the phi instead of pointing to the ssa-def.
+    */
+   nir_foreach_use_safe(use, def) {
+      if (use->parent_instr->type == nir_instr_type_phi &&
+          block_after_loop == use->parent_instr->block) {
+         continue;
+      }
+
+      if (!is_use_inside_loop(use, state->loop)) {
+         nir_instr_rewrite_src(use->parent_instr, use,
+                               nir_src_for_ssa(&phi->dest.ssa));
+      }
+   }
+
+   nir_foreach_if_use_safe(use, def) {
+      if (!is_if_use_inside_loop(use, state->loop)) {
+         nir_if_rewrite_condition(use->parent_if,
+                                  nir_src_for_ssa(&phi->dest.ssa));
+      }
+   }
+
+   return true;
+}
+
+static void
+convert_to_lcssa(nir_cf_node *cf_node, lcssa_state *state)
+{
+   switch (cf_node->type) {
+   case nir_cf_node_block:
+      nir_foreach_instr(instr, nir_cf_node_as_block(cf_node))
+         nir_foreach_ssa_def(instr, convert_loop_exit_for_ssa, state);
+      return;
+   case nir_cf_node_if: {
+      nir_if *if_stmt = nir_cf_node_as_if(cf_node);
+      foreach_list_typed(nir_cf_node, nested_node, node, &if_stmt->then_list)
+         convert_to_lcssa(nested_node, state);
+      foreach_list_typed(nir_cf_node, nested_node, node, &if_stmt->else_list)
+         convert_to_lcssa(nested_node, state);
+      return;
+   }
+   case nir_cf_node_loop: {
+      nir_loop *parent_loop = state->loop;
+      state->loop = nir_cf_node_as_loop(cf_node);
+
+      foreach_list_typed(nir_cf_node, nested_node, node, &state->loop->body)
+         convert_to_lcssa(nested_node, state);
+
+      state->loop = parent_loop;
+      return;
+   }
+   default:
+      unreachable("unknown cf node type");
+   }
+}
+
+void
+nir_convert_loop_to_lcssa(nir_loop *loop) {
+   nir_function_impl *impl = nir_cf_node_get_function(&loop->cf_node);
+
+   nir_metadata_require(impl, nir_metadata_block_index);
+
+   lcssa_state *state = rzalloc(NULL, lcssa_state);
+   state->loop = loop;
+   state->shader = impl->function->shader;
+
+   foreach_list_typed(nir_cf_node, node, node, &state->loop->body)
+      convert_to_lcssa(node, state);
+
+   ralloc_free(state);
+}
-- 
2.9.3