mesa/src/intel/compiler/elk/elk_vec4_live_variables.cpp

/*
 * Copyright © 2012 Intel Corporation
 * SPDX-License-Identifier: MIT
 */

#include "elk_vec4.h"
#include "elk_vec4_live_variables.h"

using namespace elk;

#define MAX_INSTRUCTION (1 << 30)

/** @file elk_vec4_live_variables.cpp
 *
 * Support for computing at the basic block level which variables
 * (virtual GRFs in our case) are live at entry and exit.
 *
 * See Muchnick's Advanced Compiler Design and Implementation, section
 * 14.1 (p444).
 */

/**
 * Sets up the use/def arrays and block-local approximation of the live ranges.
 *
 * The basic-block-level live variable analysis needs to know which
 * variables get used before they're completely defined, and which
 * variables are completely defined before they're used.
 *
 * We independently track each channel of a vec4.  This is because we need to
 * be able to recognize a sequence like:
 *
 * ...
 * DP4 tmp.x a b;
 * DP4 tmp.y c d;
 * MUL result.xy tmp.xy e.xy
 * ...
 *
 * as having tmp live only across that sequence (assuming it's used nowhere
 * else), because it's a common pattern.  A more conservative approach that
 * doesn't get tmp marked a deffed in this block will tend to result in
 * spilling.
 */
void
vec4_live_variables::setup_def_use()
{
   int ip = 0;

   foreach_block (block, cfg) {
      assert(ip == block->start_ip);
      if (block->num > 0)
	 assert(cfg->blocks[block->num - 1]->end_ip == ip - 1);

      foreach_inst_in_block(vec4_instruction, inst, block) {
         struct block_data *bd = &block_data[block->num];

         /* Set up the instruction uses. */
	 for (unsigned int i = 0; i < 3; i++) {
	    if (inst->src[i].file == VGRF) {
               for (unsigned j = 0; j < DIV_ROUND_UP(inst->size_read(i), 16); j++) {
                  for (int c = 0; c < 4; c++) {
                     const unsigned v = var_from_reg(alloc, inst->src[i], c, j);

                     start[v] = MIN2(start[v], ip);
                     end[v] = ip;

                     if (!BITSET_TEST(bd->def, v))
                        BITSET_SET(bd->use, v);
                  }
               }
	    }
	 }
         for (unsigned c = 0; c < 4; c++) {
            if (inst->reads_flag(c) &&
                !BITSET_TEST(bd->flag_def, c)) {
               BITSET_SET(bd->flag_use, c);
            }
         }

         /* Set up the instruction defs. */
         if (inst->dst.file == VGRF) {
            for (unsigned i = 0; i < DIV_ROUND_UP(inst->size_written, 16); i++) {
               for (int c = 0; c < 4; c++) {
                  if (inst->dst.writemask & (1 << c)) {
                     const unsigned v = var_from_reg(alloc, inst->dst, c, i);

                     start[v] = MIN2(start[v], ip);
                     end[v] = ip;

                     /* Check for unconditional register writes, these are the
                      * things that screen off preceding definitions of a
                      * variable, and thus qualify for being in def[].
                      */
                     if ((!inst->predicate || inst->opcode == ELK_OPCODE_SEL) &&
                         !BITSET_TEST(bd->use, v))
                        BITSET_SET(bd->def, v);
                  }
               }
            }
         }
         if (inst->writes_flag(devinfo)) {
            for (unsigned c = 0; c < 4; c++) {
               if ((inst->dst.writemask & (1 << c)) &&
                   !BITSET_TEST(bd->flag_use, c)) {
                  BITSET_SET(bd->flag_def, c);
               }
            }
         }

	 ip++;
      }
   }
}

/**
 * The algorithm incrementally sets bits in liveout and livein,
 * propagating it through control flow.  It will eventually terminate
 * because it only ever adds bits, and stops when no bits are added in
 * a pass.
 */
void
vec4_live_variables::compute_live_variables()
{
   bool cont = true;

   while (cont) {
      cont = false;

      foreach_block_reverse (block, cfg) {
         struct block_data *bd = &block_data[block->num];

	 /* Update liveout */
	 brw_foreach_list_typed(elk_bblock_link, child_link, link, &block->children) {
       struct block_data *child_bd = &block_data[child_link->block->num];

	    for (int i = 0; i < bitset_words; i++) {
               BITSET_WORD new_liveout = (child_bd->livein[i] &
                                          ~bd->liveout[i]);
               if (new_liveout) {
                  bd->liveout[i] |= new_liveout;
		  cont = true;
	       }
	    }
            BITSET_WORD new_liveout = (child_bd->flag_livein[0] &
                                       ~bd->flag_liveout[0]);
            if (new_liveout) {
               bd->flag_liveout[0] |= new_liveout;
               cont = true;
            }
	 }

         /* Update livein */
         for (int i = 0; i < bitset_words; i++) {
            BITSET_WORD new_livein = (bd->use[i] |
                                      (bd->liveout[i] &
                                       ~bd->def[i]));
            if (new_livein & ~bd->livein[i]) {
               bd->livein[i] |= new_livein;
               cont = true;
            }
         }
         BITSET_WORD new_livein = (bd->flag_use[0] |
                                   (bd->flag_liveout[0] &
                                    ~bd->flag_def[0]));
         if (new_livein & ~bd->flag_livein[0]) {
            bd->flag_livein[0] |= new_livein;
            cont = true;
         }
      }
   }
}

/**
 * Extend the start/end ranges for each variable to account for the
 * new information calculated from control flow.
 */
void
vec4_live_variables::compute_start_end()
{
   foreach_block (block, cfg) {
      const struct block_data &bd = block_data[block->num];

      for (int i = 0; i < num_vars; i++) {
         if (BITSET_TEST(bd.livein, i)) {
            start[i] = MIN2(start[i], block->start_ip);
            end[i] = MAX2(end[i], block->start_ip);
         }

         if (BITSET_TEST(bd.liveout, i)) {
            start[i] = MIN2(start[i], block->end_ip);
            end[i] = MAX2(end[i], block->end_ip);
         }
      }
   }
}

vec4_live_variables::vec4_live_variables(const elk_backend_shader *s)
   : alloc(s->alloc), cfg(s->cfg)
{
   mem_ctx = ralloc_context(NULL);

   num_vars = alloc.total_size * 8;
   start = ralloc_array(mem_ctx, int, num_vars);
   end = ralloc_array(mem_ctx, int, num_vars);

   for (int i = 0; i < num_vars; i++) {
      start[i] = MAX_INSTRUCTION;
      end[i] = -1;
   }

   devinfo = s->compiler->devinfo;

   block_data = rzalloc_array(mem_ctx, struct block_data, cfg->num_blocks);

   bitset_words = BITSET_WORDS(num_vars);
   for (int i = 0; i < cfg->num_blocks; i++) {
      block_data[i].def = rzalloc_array(mem_ctx, BITSET_WORD, bitset_words);
      block_data[i].use = rzalloc_array(mem_ctx, BITSET_WORD, bitset_words);
      block_data[i].livein = rzalloc_array(mem_ctx, BITSET_WORD, bitset_words);
      block_data[i].liveout = rzalloc_array(mem_ctx, BITSET_WORD, bitset_words);

      block_data[i].flag_def[0] = 0;
      block_data[i].flag_use[0] = 0;
      block_data[i].flag_livein[0] = 0;
      block_data[i].flag_liveout[0] = 0;
   }

   setup_def_use();
   compute_live_variables();
   compute_start_end();
}

vec4_live_variables::~vec4_live_variables()
{
   ralloc_free(mem_ctx);
}

static bool
check_register_live_range(const vec4_live_variables *live, int ip,
                          unsigned var, unsigned n)
{
   for (unsigned j = 0; j < n; j += 4) {
      if (var + j >= unsigned(live->num_vars) ||
          live->start[var + j] > ip || live->end[var + j] < ip)
         return false;
   }

   return true;
}

bool
vec4_live_variables::validate(const elk_backend_shader *s) const
{
   unsigned ip = 0;

   foreach_block_and_inst(block, vec4_instruction, inst, s->cfg) {
      for (unsigned c = 0; c < 4; c++) {
         if (inst->dst.writemask & (1 << c)) {
            for (unsigned i = 0; i < 3; i++) {
               if (inst->src[i].file == VGRF &&
                   !check_register_live_range(this, ip,
                                              var_from_reg(alloc, inst->src[i], c),
                                              regs_read(inst, i)))
                  return false;
            }

            if (inst->dst.file == VGRF &&
                !check_register_live_range(this, ip,
                                           var_from_reg(alloc, inst->dst, c),
                                           regs_written(inst)))
               return false;
         }
      }

      ip++;
   }

   return true;
}

int
vec4_live_variables::var_range_start(unsigned v, unsigned n) const
{
   int ip = INT_MAX;

   for (unsigned i = 0; i < n; i++)
      ip = MIN2(ip, start[v + i]);

   return ip;
}

int
vec4_live_variables::var_range_end(unsigned v, unsigned n) const
{
   int ip = INT_MIN;

   for (unsigned i = 0; i < n; i++)
      ip = MAX2(ip, end[v + i]);

   return ip;
}

bool
vec4_live_variables::vgrfs_interfere(int a, int b) const
{
   return !((var_range_end(8 * alloc.offsets[a], 8 * alloc.sizes[a]) <=
             var_range_start(8 * alloc.offsets[b], 8 * alloc.sizes[b])) ||
            (var_range_end(8 * alloc.offsets[b], 8 * alloc.sizes[b]) <=
             var_range_start(8 * alloc.offsets[a], 8 * alloc.sizes[a])));
}