nir: Add a pass to lower vector phi nodes to scalar phi nodes

v2 Jason Ekstrand <[email protected]>:
 - Add better comments
 - Use nir_ssa_dest_init and nir_src_for_ssa more places
 - Fix some void * casts

v3 Jason Ekstrand <[email protected]>:
 - Rework the way we determine whether or not to sccalarize a phi node to
   make the recursion non-bogus
 - Treat load_const instructions as scalarizable

v4 Jason Ekstrand <[email protected]>:
 - Allow uniform and input loads to be scalarizable

v5 Jason Ekstrand <[email protected]>:
 - Also consider loads of inputs (varying, uniform, or ubo) to be
   scalarizable.  We were already doing this for load_var on uniforms and
   inputs.

Reviewed-by: Kenneth Graunke <[email protected]>

2 files changed, 292 insertions, 0 deletions

diff --git a/src/glsl/nir/nir.h b/src/glsl/nir/nir.h
index 98d26896019..866ac5d4751 100644
--- a/src/glsl/nir/nir.h
+++ b/src/glsl/nir/nir.h
@@ -1527,6 +1527,8 @@ void nir_remove_dead_variables(nir_shader *shader);
 void nir_lower_vec_to_movs(nir_shader *shader);
 void nir_lower_alu_to_scalar(nir_shader *shader);
 
+void nir_lower_phis_to_scalar(nir_shader *shader);
+
 void nir_lower_samplers(nir_shader *shader,
                         struct gl_shader_program *shader_program,
                         struct gl_program *prog);
diff --git a/src/glsl/nir/nir_lower_phis_to_scalar.c b/src/glsl/nir/nir_lower_phis_to_scalar.c
new file mode 100644
index 00000000000..3bb5cc7ce34
--- /dev/null
+++ b/src/glsl/nir/nir_lower_phis_to_scalar.c
@@ -0,0 +1,290 @@
+/*
+ * Copyright © 2015 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 pass that lowers vector phi nodes to scalar phi nodes when
+ * we don't think it will hurt anything.
+ */
+
+struct lower_phis_to_scalar_state {
+   void *mem_ctx;
+   void *dead_ctx;
+
+   /* Hash table marking which phi nodes are scalarizable.  The key is
+    * pointers to phi instructions and the entry is either NULL for not
+    * scalarizable or non-null for scalarizable.
+    */
+   struct hash_table *phi_table;
+};
+
+static bool
+should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state);
+
+static bool
+is_phi_src_scalarizable(nir_phi_src *src,
+                        struct lower_phis_to_scalar_state *state)
+{
+   /* Don't know what to do with non-ssa sources */
+   if (!src->src.is_ssa)
+      return false;
+
+   nir_instr *src_instr = src->src.ssa->parent_instr;
+   switch (src_instr->type) {
+   case nir_instr_type_alu: {
+      nir_alu_instr *src_alu = nir_instr_as_alu(src_instr);
+
+      /* ALU operations with output_size == 0 should be scalarized.  We
+       * will also see a bunch of vecN operations from scalarizing ALU
+       * operations and, since they can easily be copy-propagated, they
+       * are ok too.
+       */
+      return nir_op_infos[src_alu->op].output_size == 0 ||
+             src_alu->op != nir_op_vec2 ||
+             src_alu->op != nir_op_vec3 ||
+             src_alu->op != nir_op_vec4;
+   }
+
+   case nir_instr_type_phi:
+      /* A phi is scalarizable if we're going to lower it */
+      return should_lower_phi(nir_instr_as_phi(src_instr), state);
+
+   case nir_instr_type_load_const:
+      /* These are trivially scalarizable */
+      return true;
+
+   case nir_instr_type_intrinsic: {
+      nir_intrinsic_instr *src_intrin = nir_instr_as_intrinsic(src_instr);
+
+      switch (src_intrin->intrinsic) {
+      case nir_intrinsic_load_var:
+         return src_intrin->variables[0]->var->data.mode == nir_var_shader_in ||
+                src_intrin->variables[0]->var->data.mode == nir_var_uniform;
+
+      case nir_intrinsic_interp_var_at_centroid:
+      case nir_intrinsic_interp_var_at_sample:
+      case nir_intrinsic_interp_var_at_offset:
+      case nir_intrinsic_load_uniform:
+      case nir_intrinsic_load_uniform_indirect:
+      case nir_intrinsic_load_ubo:
+      case nir_intrinsic_load_ubo_indirect:
+      case nir_intrinsic_load_input:
+      case nir_intrinsic_load_input_indirect:
+         return true;
+      default:
+         break;
+      }
+   }
+
+   default:
+      /* We can't scalarize this type of instruction */
+      return false;
+   }
+}
+
+/**
+ * Determines if the given phi node should be lowered.  The only phi nodes
+ * we will scalarize at the moment are those where all of the sources are
+ * scalarizable.
+ *
+ * The reason for this comes down to coalescing.  Since phi sources can't
+ * swizzle, swizzles on phis have to be resolved by inserting a mov right
+ * before the phi.  The choice then becomes between movs to pick off
+ * components for a scalar phi or potentially movs to recombine components
+ * for a vector phi.  The problem is that the movs generated to pick off
+ * the components are almost uncoalescable.  We can't coalesce them in NIR
+ * because we need them to pick off components and we can't coalesce them
+ * in the backend because the source register is a vector and the
+ * destination is a scalar that may be used at other places in the program.
+ * On the other hand, if we have a bunch of scalars going into a vector
+ * phi, the situation is much better.  In this case, if the SSA def is
+ * generated in the predecessor block to the corresponding phi source, the
+ * backend code will be an ALU op into a temporary and then a mov into the
+ * given vector component;  this move can almost certainly be coalesced
+ * away.
+ */
+static bool
+should_lower_phi(nir_phi_instr *phi, struct lower_phis_to_scalar_state *state)
+{
+   /* Already scalar */
+   if (phi->dest.ssa.num_components == 1)
+      return false;
+
+   struct hash_entry *entry = _mesa_hash_table_search(state->phi_table, phi);
+   if (entry)
+      return entry->data != NULL;
+
+   /* Insert an entry and mark it as scalarizable for now. That way
+    * we don't recurse forever and a cycle in the dependence graph
+    * won't automatically make us fail to scalarize.
+    */
+   entry = _mesa_hash_table_insert(state->phi_table, phi, (void *)(intptr_t)1);
+
+   bool scalarizable = true;
+
+   nir_foreach_phi_src(phi, src) {
+      scalarizable = is_phi_src_scalarizable(src, state);
+      if (!scalarizable)
+         break;
+   }
+
+   entry->data = (void *)(intptr_t)scalarizable;
+
+   return scalarizable;
+}
+
+static bool
+lower_phis_to_scalar_block(nir_block *block, void *void_state)
+{
+   struct lower_phis_to_scalar_state *state = void_state;
+
+   /* Find the last phi node in the block */
+   nir_phi_instr *last_phi = NULL;
+   nir_foreach_instr(block, instr) {
+      if (instr->type != nir_instr_type_phi)
+         break;
+
+      last_phi = nir_instr_as_phi(instr);
+   }
+
+   /* We have to handle the phi nodes in their own pass due to the way
+    * we're modifying the linked list of instructions.
+    */
+   nir_foreach_instr_safe(block, instr) {
+      if (instr->type != nir_instr_type_phi)
+         break;
+
+      nir_phi_instr *phi = nir_instr_as_phi(instr);
+
+      if (!should_lower_phi(phi, state))
+         continue;
+
+      /* Create a vecN operation to combine the results.  Most of these
+       * will be redundant, but copy propagation should clean them up for
+       * us.  No need to add the complexity here.
+       */
+      nir_op vec_op;
+      switch (phi->dest.ssa.num_components) {
+      case 2: vec_op = nir_op_vec2; break;
+      case 3: vec_op = nir_op_vec3; break;
+      case 4: vec_op = nir_op_vec4; break;
+      default: unreachable("Invalid number of components");
+      }
+
+      nir_alu_instr *vec = nir_alu_instr_create(state->mem_ctx, vec_op);
+      nir_ssa_dest_init(&vec->instr, &vec->dest.dest,
+                        phi->dest.ssa.num_components, NULL);
+      vec->dest.write_mask = (1 << phi->dest.ssa.num_components) - 1;
+
+      for (unsigned i = 0; i < phi->dest.ssa.num_components; i++) {
+         nir_phi_instr *new_phi = nir_phi_instr_create(state->mem_ctx);
+         nir_ssa_dest_init(&new_phi->instr, &new_phi->dest, 1, NULL);
+
+         vec->src[i].src = nir_src_for_ssa(&new_phi->dest.ssa);
+
+         nir_foreach_phi_src(phi, src) {
+            /* We need to insert a mov to grab the i'th component of src */
+            nir_alu_instr *mov = nir_alu_instr_create(state->mem_ctx,
+                                                      nir_op_imov);
+            nir_ssa_dest_init(&mov->instr, &mov->dest.dest, 1, NULL);
+            mov->dest.write_mask = 1;
+            nir_src_copy(&mov->src[0].src, &src->src, state->mem_ctx);
+            mov->src[0].swizzle[0] = i;
+
+            /* Insert at the end of the predecessor but before the jump */
+            nir_instr *pred_last_instr = nir_block_last_instr(src->pred);
+            if (pred_last_instr && pred_last_instr->type == nir_instr_type_jump)
+               nir_instr_insert_before(pred_last_instr, &mov->instr);
+            else
+               nir_instr_insert_after_block(src->pred, &mov->instr);
+
+            nir_phi_src *new_src = ralloc(state->mem_ctx, nir_phi_src);
+            new_src->pred = src->pred;
+            new_src->src = nir_src_for_ssa(&mov->dest.dest.ssa);
+
+            exec_list_push_tail(&new_phi->srcs, &new_src->node);
+         }
+
+         nir_instr_insert_before(&phi->instr, &new_phi->instr);
+      }
+
+      nir_instr_insert_after(&last_phi->instr, &vec->instr);
+
+      nir_ssa_def_rewrite_uses(&phi->dest.ssa,
+                               nir_src_for_ssa(&vec->dest.dest.ssa),
+                               state->mem_ctx);
+
+      ralloc_steal(state->dead_ctx, phi);
+      nir_instr_remove(&phi->instr);
+
+      /* We're using the safe iterator and inserting all the newly
+       * scalarized phi nodes before their non-scalarized version so that's
+       * ok.  However, we are also inserting vec operations after all of
+       * the last phi node so once we get here, we can't trust even the
+       * safe iterator to stop properly.  We have to break manually.
+       */
+      if (instr == &last_phi->instr)
+         break;
+   }
+
+   return true;
+}
+
+static void
+lower_phis_to_scalar_impl(nir_function_impl *impl)
+{
+   struct lower_phis_to_scalar_state state;
+
+   state.mem_ctx = ralloc_parent(impl);
+   state.dead_ctx = ralloc_context(NULL);
+   state.phi_table = _mesa_hash_table_create(state.dead_ctx, _mesa_hash_pointer,
+                                             _mesa_key_pointer_equal);
+
+   nir_foreach_block(impl, lower_phis_to_scalar_block, &state);
+
+   nir_metadata_preserve(impl, nir_metadata_block_index |
+                               nir_metadata_dominance);
+
+   ralloc_free(state.dead_ctx);
+}
+
+/** A pass that lowers vector phi nodes to scalar
+ *
+ * This pass loops through the blocks and lowers looks for vector phi nodes
+ * it can lower to scalar phi nodes.  Not all phi nodes are lowered.  For
+ * instance, if one of the sources is a non-scalarizable vector, then we
+ * don't bother lowering because that would generate hard-to-coalesce movs.
+ */
+void
+nir_lower_phis_to_scalar(nir_shader *shader)
+{
+   nir_foreach_overload(shader, overload) {
+      if (overload->impl)
+         lower_phis_to_scalar_impl(overload->impl);
+   }
+}