mesa/src/compiler/nir/nir_opt_shrink_vectors.c

/*
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 * copy of this software and associated documentation files (the "Software"),
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 *
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 * Software.
 *
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 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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/**
 * @file
 *
 * Removes unused components of SSA defs.
 *
 * Due to various optimization passes (or frontend implementations,
 * particularly prog_to_nir), we may have instructions generating vectors
 * whose components don't get read by any instruction.
 *
 * For memory loads, while it can be tricky to eliminate unused low components
 * or channels in the middle of a writemask (you might need to increment some
 * offset from a load_uniform, for example), it is trivial to just drop the
 * trailing components.
 * For vector ALU and load_const, only used by other ALU instructions,
 * this pass eliminates arbitrary channels as well as duplicate channels,
 * and reswizzles the uses.
 *
 * This pass is probably only of use to vector backends -- scalar backends
 * typically get unused def channel trimming by scalarizing and dead code
 * elimination.
 */

#include "nir.h"
#include "nir_builder.h"
#include "util/u_math.h"

/*
 * Round up a vector size to a vector size that's valid in NIR. At present, NIR
 * supports only vec2-5, vec8, and vec16. Attempting to generate other sizes
 * will fail validation.
 */
static unsigned
round_up_components(unsigned n)
{
   return (n > 5) ? util_next_power_of_two(n) : n;
}

static bool
shrink_dest_to_read_mask(nir_ssa_def *def)
{
   /* early out if there's nothing to do. */
   if (def->num_components == 1)
      return false;

   /* don't remove any channels if used by an intrinsic */
   nir_foreach_use(use_src, def) {
      if (use_src->parent_instr->type == nir_instr_type_intrinsic)
         return false;
   }

   unsigned mask = nir_ssa_def_components_read(def);
   int last_bit = util_last_bit(mask);

   /* If nothing was read, leave it up to DCE. */
   if (!mask)
      return false;

   unsigned rounded = round_up_components(last_bit);
   assert(rounded <= def->num_components);
   last_bit = rounded;

   if (def->num_components > last_bit) {
      def->num_components = last_bit;
      return true;
   }

   return false;
}

static void
reswizzle_alu_uses(nir_ssa_def *def, uint8_t *reswizzle)
{
   nir_foreach_use(use_src, def) {
      /* all uses must be ALU instructions */
      assert(use_src->parent_instr->type == nir_instr_type_alu);
      nir_alu_src *alu_src = (nir_alu_src*)use_src;

      /* reswizzle ALU sources */
      for (unsigned i = 0; i < NIR_MAX_VEC_COMPONENTS; i++)
         alu_src->swizzle[i] = reswizzle[alu_src->swizzle[i]];
   }
}

static bool
is_only_used_by_alu(nir_ssa_def *def)
{
   nir_foreach_use(use_src, def) {
      if (use_src->parent_instr->type != nir_instr_type_alu)
         return false;
   }

   return true;
}

static bool
opt_shrink_vector(nir_builder *b, nir_alu_instr *instr)
{
   nir_ssa_def *def = &instr->dest.dest.ssa;
   unsigned mask = nir_ssa_def_components_read(def);

   /* If nothing was read, leave it up to DCE. */
   if (mask == 0)
      return false;

   /* don't remove any channels if used by non-ALU */
   if (!is_only_used_by_alu(def))
      return false;

   uint8_t reswizzle[NIR_MAX_VEC_COMPONENTS] = { 0 };
   nir_ssa_scalar srcs[NIR_MAX_VEC_COMPONENTS] = { 0 };
   unsigned num_components = 0;
   for (unsigned i = 0; i < def->num_components; i++) {
      if (!((mask >> i) & 0x1))
         continue;

      nir_ssa_scalar scalar = nir_get_ssa_scalar(instr->src[i].src.ssa, instr->src[i].swizzle[0]);

      /* Try reuse a component with the same value */
      unsigned j;
      for (j = 0; j < num_components; j++) {
         if (scalar.def == srcs[j].def && scalar.comp == srcs[j].comp) {
            reswizzle[i] = j;
            break;
         }
      }

      /* Otherwise, just append the value */
      if (j == num_components) {
         srcs[num_components] = scalar;
         reswizzle[i] = num_components++;
      }
   }

   /* return if no component was removed */
   if (num_components == def->num_components)
      return false;

   /* create new vecN and replace uses */
   nir_ssa_def *new_vec = nir_vec_scalars(b, srcs, num_components);
   nir_ssa_def_rewrite_uses(def, new_vec);
   reswizzle_alu_uses(new_vec, reswizzle);

   return true;
}

static bool
opt_shrink_vectors_alu(nir_builder *b, nir_alu_instr *instr)
{
   nir_ssa_def *def = &instr->dest.dest.ssa;

   /* Nothing to shrink */
   if (def->num_components == 1)
      return false;

   switch (instr->op) {
      /* don't use nir_op_is_vec() as not all vector sizes are supported. */
      case nir_op_vec4:
      case nir_op_vec3:
      case nir_op_vec2:
         return opt_shrink_vector(b, instr);
      default:
         if (nir_op_infos[instr->op].output_size != 0)
            return false;
         break;
   }

   /* don't remove any channels if used by non-ALU */
   if (!is_only_used_by_alu(def))
      return false;

   unsigned mask = nir_ssa_def_components_read(def);
   unsigned last_bit = util_last_bit(mask);
   unsigned num_components = util_bitcount(mask);

   unsigned rounded = round_up_components(num_components);
   assert(rounded <= def->num_components);
   num_components = rounded;

   /* return, if there is nothing to do */
   if (mask == 0 || num_components == def->num_components)
      return false;

   const bool is_bitfield_mask = last_bit == num_components;
   if (is_bitfield_mask) {
      /* just reduce the number of components and return */
      def->num_components = num_components;
      instr->dest.write_mask = mask;
      return true;
   }

   uint8_t reswizzle[NIR_MAX_VEC_COMPONENTS] = { 0 };
   unsigned index = 0;
   for (unsigned i = 0; i < last_bit; i++) {
      /* skip unused components */
      if (!((mask >> i) & 0x1))
         continue;

      /* reswizzle the sources */
      for (int k = 0; k < nir_op_infos[instr->op].num_inputs; k++) {
         instr->src[k].swizzle[index] = instr->src[k].swizzle[i];
         reswizzle[i] = index;
      }
      index++;
   }
   assert(index == num_components);

   /* update dest */
   def->num_components = num_components;
   instr->dest.write_mask = BITFIELD_MASK(num_components);

   /* update uses */
   reswizzle_alu_uses(def, reswizzle);

   return true;
}

static bool
opt_shrink_vectors_intrinsic(nir_builder *b, nir_intrinsic_instr *instr)
{
   switch (instr->intrinsic) {
   case nir_intrinsic_load_uniform:
   case nir_intrinsic_load_ubo:
   case nir_intrinsic_load_input:
   case nir_intrinsic_load_input_vertex:
   case nir_intrinsic_load_per_vertex_input:
   case nir_intrinsic_load_interpolated_input:
   case nir_intrinsic_load_ssbo:
   case nir_intrinsic_load_push_constant:
   case nir_intrinsic_load_constant:
   case nir_intrinsic_load_shared:
   case nir_intrinsic_load_global:
   case nir_intrinsic_load_global_constant:
   case nir_intrinsic_load_kernel_input:
   case nir_intrinsic_load_scratch:
      break;
   default:
      return false;
   }

   /* Must be a vectorized intrinsic that we can resize. */
   assert(instr->num_components != 0);

   /* Trim the dest to the used channels */
   if (shrink_dest_to_read_mask(&instr->dest.ssa)) {
      instr->num_components = instr->dest.ssa.num_components;
      return true;
   }

   return false;
}

static bool
opt_shrink_vectors_load_const(nir_load_const_instr *instr)
{
   nir_ssa_def *def = &instr->def;

   /* early out if there's nothing to do. */
   if (def->num_components == 1)
      return false;

   /* don't remove any channels if used by non-ALU */
   if (!is_only_used_by_alu(def))
      return false;

   unsigned mask = nir_ssa_def_components_read(def);

   /* If nothing was read, leave it up to DCE. */
   if (!mask)
      return false;

   uint8_t reswizzle[NIR_MAX_VEC_COMPONENTS] = { 0 };
   unsigned num_components = 0;
   for (unsigned i = 0; i < def->num_components; i++) {
      if (!((mask >> i) & 0x1))
         continue;

      /* Try reuse a component with the same constant */
      unsigned j;
      for (j = 0; j < num_components; j++) {
         if (instr->value[i].u64 == instr->value[j].u64) {
            reswizzle[i] = j;
            break;
         }
      }

      /* Otherwise, just append the value */
      if (j == num_components) {
         instr->value[num_components] = instr->value[i];
         reswizzle[i] = num_components++;
      }
   }

   unsigned rounded = round_up_components(num_components);
   assert(rounded <= def->num_components);
   num_components = rounded;

   if (num_components == def->num_components)
      return false;

   def->num_components = num_components;
   reswizzle_alu_uses(def, reswizzle);

   return true;
}

static bool
opt_shrink_vectors_ssa_undef(nir_ssa_undef_instr *instr)
{
   return shrink_dest_to_read_mask(&instr->def);
}

static bool
opt_shrink_vectors_instr(nir_builder *b, nir_instr *instr)
{
   b->cursor = nir_before_instr(instr);

   switch (instr->type) {
   case nir_instr_type_alu:
      return opt_shrink_vectors_alu(b, nir_instr_as_alu(instr));

   case nir_instr_type_intrinsic:
      return opt_shrink_vectors_intrinsic(b, nir_instr_as_intrinsic(instr));

   case nir_instr_type_load_const:
      return opt_shrink_vectors_load_const(nir_instr_as_load_const(instr));

   case nir_instr_type_ssa_undef:
      return opt_shrink_vectors_ssa_undef(nir_instr_as_ssa_undef(instr));

   default:
      return false;
   }

   return true;
}

bool
nir_opt_shrink_vectors(nir_shader *shader)
{
   bool progress = false;

   nir_foreach_function(function, shader) {
      if (!function->impl)
         continue;

      nir_builder b;
      nir_builder_init(&b, function->impl);

      nir_foreach_block_reverse(block, function->impl) {
         nir_foreach_instr_reverse(instr, block) {
            progress |= opt_shrink_vectors_instr(&b, instr);
         }
      }

      if (progress) {
         nir_metadata_preserve(function->impl,
                               nir_metadata_block_index |
                               nir_metadata_dominance);
      } else {
         nir_metadata_preserve(function->impl, nir_metadata_all);
      }
   }

   return progress;
}