nir: Add an SSA-based liveness analysis pass.

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

3 files changed, 295 insertions, 0 deletions

diff --git a/src/glsl/nir/nir.h b/src/glsl/nir/nir.h
index b9a17ed5fff..c48bde81c7b 100644
--- a/src/glsl/nir/nir.h
+++ b/src/glsl/nir/nir.h
@@ -420,6 +420,9 @@ typedef struct {
    /** generic SSA definition index. */
    unsigned index;
 
+   /** Index into the live_in and live_out bitfields */
+   unsigned live_index;
+
    nir_instr *parent_instr;
 
    struct set *uses;
@@ -999,6 +1002,10 @@ typedef struct nir_block {
    struct nir_block **dom_children;
 
    struct set *dom_frontier;
+
+   /* live in and out for this block; used for liveness analysis */
+   BITSET_WORD *live_in;
+   BITSET_WORD *live_out;
 } nir_block;
 
 #define nir_block_first_instr(block) \
@@ -1047,6 +1054,7 @@ typedef enum {
    nir_metadata_none = 0x0,
    nir_metadata_block_index = 0x1,
    nir_metadata_dominance = 0x2,
+   nir_metadata_live_variables = 0x4,
 } nir_metadata;
 
 typedef struct {
@@ -1320,6 +1328,9 @@ void nir_lower_system_values(nir_shader *shader);
 
 void nir_lower_atomics(nir_shader *shader);
 
+void nir_live_variables_impl(nir_function_impl *impl);
+bool nir_ssa_defs_interfere(nir_ssa_def *a, nir_ssa_def *b);
+
 void nir_convert_to_ssa_impl(nir_function_impl *impl);
 void nir_convert_to_ssa(nir_shader *shader);
 void nir_convert_from_ssa(nir_shader *shader);
diff --git a/src/glsl/nir/nir_live_variables.c b/src/glsl/nir/nir_live_variables.c
new file mode 100644
index 00000000000..fd076899de8
--- /dev/null
+++ b/src/glsl/nir/nir_live_variables.c
@@ -0,0 +1,282 @@
+/*
+ * 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"
+
+/*
+ * Basic liveness analysis.  This works only in SSA form.
+ *
+ * This liveness pass treats phi nodes as being melded to the space between
+ * blocks so that the destinations of a phi are in the livein of the block
+ * in which it resides and the sources are in the liveout of the
+ * corresponding block.  By formulating the liveness information in this
+ * way, we ensure that the definition of any variable dominates its entire
+ * live range.  This is true because the only way that the definition of an
+ * SSA value may not dominate a use is if the use is in a phi node and the
+ * uses in phi no are in the live-out of the corresponding predecessor
+ * block but not in the live-in of the block containing the phi node.
+ */
+
+struct live_variables_state {
+   unsigned num_ssa_defs;
+   unsigned bitset_words;
+   bool progress;
+};
+
+static bool
+index_dest(nir_dest *dest, void *void_state)
+{
+   struct live_variables_state *state = void_state;
+
+   if (dest->is_ssa)
+      dest->ssa.live_index = state->num_ssa_defs++;
+
+   return true;
+}
+
+static bool
+index_ssa_definitions_block(nir_block *block, void *void_state)
+{
+   struct live_variables_state *state = void_state;
+
+   nir_foreach_instr(block, instr) {
+      if (instr->type == nir_instr_type_ssa_undef) {
+         nir_ssa_undef_instr *undef = nir_instr_as_ssa_undef(instr);
+         undef->def.live_index = 0;
+      } else {
+         nir_foreach_dest(instr, index_dest, state);
+      }
+   }
+
+   return true;
+}
+
+static bool
+init_liveness_block(nir_block *block, void *void_state)
+{
+   struct live_variables_state *state = void_state;
+
+   block->live_in = reralloc(block, block->live_in, BITSET_WORD,
+                             state->bitset_words);
+   memset(block->live_in, 0, state->bitset_words * sizeof(BITSET_WORD));
+
+   block->live_out = reralloc(block, block->live_out, BITSET_WORD,
+                              state->bitset_words);
+   memset(block->live_out, 0, state->bitset_words * sizeof(BITSET_WORD));
+
+   return true;
+}
+
+static bool
+set_src_live(nir_src *src, void *void_live)
+{
+   BITSET_WORD *live = void_live;
+
+   if (!src->is_ssa)
+      return true;
+
+   if (src->ssa->live_index == 0)
+      return true;   /* undefined variables are never live */
+
+   BITSET_SET(live, src->ssa->live_index);
+
+   return true;
+}
+
+static bool
+set_dest_dead(nir_dest *dest, void *void_live)
+{
+   BITSET_WORD *live = void_live;
+
+   if (dest->is_ssa)
+      BITSET_CLEAR(live, dest->ssa.live_index);
+
+   return true;
+}
+
+/* Phi nodes exist "between" blocks and all the phi nodes at the start of a
+ * block act "in parallel".  When we propagate from the live_in of one
+ * block to the live out of the other, we have to kill any writes from phis
+ * and make live any sources.
+ */
+static void
+propagate_across_edge(nir_block *pred, nir_block *succ,
+                      struct live_variables_state *state)
+{
+   BITSET_WORD live[state->bitset_words];
+   memcpy(live, succ->live_in, sizeof live);
+
+   nir_foreach_instr(succ, instr) {
+      if (instr->type != nir_instr_type_phi)
+         break;
+      nir_phi_instr *phi = nir_instr_as_phi(instr);
+
+      set_dest_dead(&phi->dest, live);
+   }
+
+   nir_foreach_instr(succ, instr) {
+      if (instr->type != nir_instr_type_phi)
+         break;
+      nir_phi_instr *phi = nir_instr_as_phi(instr);
+
+      foreach_list_typed(nir_phi_src, src, node, &phi->srcs) {
+         if (src->pred == pred) {
+            set_src_live(&src->src, live);
+            break;
+         }
+      }
+   }
+
+   for (unsigned i = 0; i < state->bitset_words; ++i) {
+      state->progress = state->progress || (live[i] & ~pred->live_out[i]) != 0;
+      pred->live_out[i] |= live[i];
+   }
+}
+
+static bool
+walk_instructions_block(nir_block *block, void *void_state)
+{
+   struct live_variables_state *state = void_state;
+
+   /* The live out is the union (modulo phi nodes) of the live ins of its
+    * successors */
+   if (block->successors[0])
+      propagate_across_edge(block, block->successors[0], state);
+   if (block->successors[1])
+      propagate_across_edge(block, block->successors[1], state);
+
+   memcpy(block->live_in, block->live_out,
+          state->bitset_words * sizeof(BITSET_WORD));
+
+   nir_if *following_if = nir_block_following_if(block);
+   if (following_if)
+      set_src_live(&following_if->condition, block->live_in);
+
+   nir_foreach_instr_reverse(block, instr) {
+      /* Phi nodes are handled seperately so we want to skip them.  Since
+       * we are going backwards and they are at the beginning, we can just
+       * break as soon as we see one.
+       */
+      if (instr->type == nir_instr_type_phi)
+         break;
+
+      nir_foreach_dest(instr, set_dest_dead, block->live_in);
+      nir_foreach_src(instr, set_src_live, block->live_in);
+   }
+
+   return true;
+}
+
+static bool
+src_does_not_use_def(nir_src *src, void *def)
+{
+   return !src->is_ssa || src->ssa != (nir_ssa_def *)def;
+}
+
+static bool
+search_for_use_after_instr(nir_instr *start, nir_ssa_def *def)
+{
+   /* Only look for a use strictly after the given instruction */
+   struct exec_node *node = start->node.next;
+   while (!exec_node_is_tail_sentinel(node)) {
+      nir_instr *instr = exec_node_data(nir_instr, node, node);
+      if (!nir_foreach_src(instr, src_does_not_use_def, def))
+         return true;
+      node = node->next;
+   }
+   return false;
+}
+
+/* Returns true if def is live at instr assuming that def comes before
+ * instr in a pre DFS search of the dominance tree.
+ */
+static bool
+nir_ssa_def_is_live_at(nir_ssa_def *def, nir_instr *instr)
+{
+   if (BITSET_TEST(instr->block->live_out, def->live_index)) {
+      /* Since def dominates instr, if def is in the liveout of the block,
+       * it's live at instr
+       */
+      return true;
+   } else {
+      if (BITSET_TEST(instr->block->live_in, def->live_index) ||
+          def->parent_instr->block == instr->block) {
+         /* In this case it is either live coming into instr's block or it
+          * is defined in the same block.  In this case, we simply need to
+          * see if it is used after instr.
+          */
+         return search_for_use_after_instr(instr, def);
+      } else {
+         return false;
+      }
+   }
+}
+
+bool
+nir_ssa_defs_interfere(nir_ssa_def *a, nir_ssa_def *b)
+{
+   if (a->parent_instr == b->parent_instr) {
+      /* Two variables defined at the same time interfere assuming at
+       * least one isn't dead.
+       */
+      return true;
+   } else if (a->live_index == 0 || b->live_index == 0) {
+      /* If either variable is an ssa_undef, then there's no interference */
+      return false;
+   } else if (a->live_index < b->live_index) {
+      return nir_ssa_def_is_live_at(a, b->parent_instr);
+   } else {
+      return nir_ssa_def_is_live_at(b, a->parent_instr);
+   }
+}
+
+void
+nir_live_variables_impl(nir_function_impl *impl)
+{
+   struct live_variables_state state;
+
+   /* We start at 1 because we reserve the index value of 0 for ssa_undef
+    * instructions.  Those are never live, so their liveness information
+    * can be compacted into a single bit.
+    */
+   state.num_ssa_defs = 1;
+   nir_foreach_block(impl, index_ssa_definitions_block, &state);
+
+   /* We now know how many unique ssa definitions we have and we can go
+    * ahead and allocate live_in and live_out sets
+    */
+   state.bitset_words = BITSET_WORDS(state.num_ssa_defs);
+   nir_foreach_block(impl, init_liveness_block, &state);
+
+   /* We need to propagate the liveness back through the CFG.  Thanks to
+    * the wonders of SSA, this will run no more times than the depth of the
+    * deepest loop + 1.
+    */
+   do {
+      state.progress = false;
+      nir_foreach_block_reverse(impl, walk_instructions_block, &state);
+   } while (state.progress);
+}
diff --git a/src/glsl/nir/nir_metadata.c b/src/glsl/nir/nir_metadata.c
index 070cb74d6d6..a4d618ccc05 100644
--- a/src/glsl/nir/nir_metadata.c
+++ b/src/glsl/nir/nir_metadata.c
@@ -39,6 +39,8 @@ nir_metadata_require(nir_function_impl *impl, nir_metadata required)
       nir_index_blocks(impl);
    if (NEEDS_UPDATE(nir_metadata_dominance))
       nir_calc_dominance_impl(impl);
+   if (NEEDS_UPDATE(nir_metadata_live_variables))
+      nir_live_variables_impl(impl);
 
 #undef NEEDS_UPDATE