nir: Add a pass to rewrite uses of vecN sources to the vecN destination

v2 (Jason Ekstrand):
 - Handle non-SSA sources and destinations

Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>

src/glsl/Makefile.sources

@@ -50,6 +50,7 @@ NIR_FILES = \
 	nir/nir_lower_var_copies.c \
 	nir/nir_lower_vec_to_movs.c \
 	nir/nir_metadata.c \
+	nir/nir_move_vec_src_uses_to_dest.c \
 	nir/nir_normalize_cubemap_coords.c \
 	nir/nir_opt_constant_folding.c \
 	nir/nir_opt_copy_propagate.c \

src/glsl/nir/nir.h

@@ -1825,6 +1825,7 @@ void nir_lower_vars_to_ssa(nir_shader *shader);
 
 void nir_remove_dead_variables(nir_shader *shader);
 
+void nir_move_vec_src_uses_to_dest(nir_shader *shader);
 void nir_lower_vec_to_movs(nir_shader *shader);
 void nir_lower_alu_to_scalar(nir_shader *shader);
 void nir_lower_load_const_to_scalar(nir_shader *shader);

src/glsl/nir/nir_move_vec_src_uses_to_dest.c

@@ -0,0 +1,197 @@
+/*
+ * 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 (jason@jlekstrand.net)
+ *
+ */
+
+#include "nir.h"
+
+/*
+ * Implements a pass that tries to move uses vecN sources to their
+ * destinations.  This is kind of like an inverse copy-propagation pass.
+ * For instance, if you have
+ *
+ * ssa_1 = vec4(a, b, c, d)
+ * ssa_2 = fadd(a, b)
+ *
+ * This will be turned into
+ *
+ * ssa_1 = vec4(a, b, c, d)
+ * ssa_2 = fadd(ssa_1.x, ssa_1.y)
+ *
+ * While this is "worse" because it adds a bunch of unneeded dependencies, it
+ * actually makes it much easier for vec4-based backends to coalesce the MOV's
+ * that result from the vec4 operation because it doesn't have to worry about
+ * quite as many reads.
+ */
+
+/* Returns true if the given SSA def dominates the instruction.  An SSA def is
+ * considered to *not* dominate the instruction that defines it.
+ */
+static bool
+ssa_def_dominates_instr(nir_ssa_def *def, nir_instr *instr)
+{
+   if (instr->index <= def->parent_instr->index) {
+      return false;
+   } else if (def->parent_instr->block == instr->block) {
+      return def->parent_instr->index < instr->index;
+   } else {
+      return nir_block_dominates(def->parent_instr->block, instr->block);
+   }
+}
+
+static bool
+move_vec_src_uses_to_dest_block(nir_block *block, void *shader)
+{
+   nir_foreach_instr(block, instr) {
+      if (instr->type != nir_instr_type_alu)
+         continue;
+
+      nir_alu_instr *vec = nir_instr_as_alu(instr);
+
+      switch (vec->op) {
+      case nir_op_vec2:
+      case nir_op_vec3:
+      case nir_op_vec4:
+         break;
+      default:
+         continue; /* The loop */
+      }
+
+      /* Can't handle non-SSA vec operations */
+      if (!vec->dest.dest.is_ssa)
+         continue;
+
+      /* Can't handle saturation */
+      if (vec->dest.saturate)
+         continue;
+
+      /* First, mark all of the sources we are going to consider for rewriting
+       * to the destination
+       */
+      int srcs_remaining = 0;
+      for (unsigned i = 0; i < nir_op_infos[vec->op].num_inputs; i++) {
+         /* We can't rewrite a source if it's not in SSA form */
+         if (!vec->src[i].src.is_ssa)
+            continue;
+
+         /* We can't rewrite a source if it has modifiers */
+         if (vec->src[i].abs || vec->src[i].negate)
+            continue;
+
+         srcs_remaining |= 1 << i;
+      }
+
+      /* We can't actually do anything with this instruction */
+      if (srcs_remaining == 0)
+         continue;
+
+      for (unsigned i; i = ffs(srcs_remaining) - 1, srcs_remaining;) {
+         int8_t swizzle[4] = { -1, -1, -1, -1 };
+
+         for (unsigned j = i; j < nir_op_infos[vec->op].num_inputs; j++) {
+            if (vec->src[j].src.ssa != vec->src[i].src.ssa)
+               continue;
+
+            /* Mark the given chanle as having been handled */
+            srcs_remaining &= ~(1 << j);
+
+            /* Mark the appropreate channel as coming from src j */
+            swizzle[vec->src[j].swizzle[0]] = j;
+         }
+
+         nir_foreach_use_safe(vec->src[i].src.ssa, use) {
+            if (use->parent_instr == &vec->instr)
+               continue;
+
+            /* We need to dominate the use if we are going to rewrite it */
+            if (!ssa_def_dominates_instr(&vec->dest.dest.ssa, use->parent_instr))
+               continue;
+
+            /* For now, we'll just rewrite ALU instructions */
+            if (use->parent_instr->type != nir_instr_type_alu)
+               continue;
+
+            assert(use->is_ssa);
+
+            nir_alu_instr *use_alu = nir_instr_as_alu(use->parent_instr);
+
+            /* Figure out which source we're actually looking at */
+            nir_alu_src *use_alu_src = exec_node_data(nir_alu_src, use, src);
+            unsigned src_idx = use_alu_src - use_alu->src;
+            assert(src_idx < nir_op_infos[use_alu->op].num_inputs);
+
+            bool can_reswizzle = true;
+            for (unsigned j = 0; j < 4; j++) {
+               if (!nir_alu_instr_channel_used(use_alu, src_idx, j))
+                  continue;
+
+               if (swizzle[use_alu_src->swizzle[j]] == -1) {
+                  can_reswizzle = false;
+                  break;
+               }
+            }
+
+            if (!can_reswizzle)
+               continue;
+
+            /* At this point, we have determined that the given use can be
+             * reswizzled to actually use the destination of the vecN operation.
+             * Go ahead and rewrite it as needed.
+             */
+            nir_instr_rewrite_src(use->parent_instr, use,
+                                  nir_src_for_ssa(&vec->dest.dest.ssa));
+            for (unsigned j = 0; j < 4; j++) {
+               if (!nir_alu_instr_channel_used(use_alu, src_idx, j))
+                  continue;
+
+               use_alu_src->swizzle[j] = swizzle[use_alu_src->swizzle[j]];
+            }
+         }
+      }
+   }
+
+   return true;
+}
+
+static void
+nir_move_vec_src_uses_to_dest_impl(nir_shader *shader, nir_function_impl *impl)
+{
+   nir_metadata_require(impl, nir_metadata_dominance);
+
+   nir_index_instrs(impl);
+   nir_foreach_block(impl, move_vec_src_uses_to_dest_block, shader);
+
+   nir_metadata_preserve(impl, nir_metadata_block_index |
+                               nir_metadata_dominance);
+}
+
+void
+nir_move_vec_src_uses_to_dest(nir_shader *shader)
+{
+   nir_foreach_overload(shader, overload) {
+      if (overload->impl)
+         nir_move_vec_src_uses_to_dest_impl(shader, overload->impl);
+   }
+}