/src/php-src/Zend/Optimizer/dce.c

Source
/*
   +----------------------------------------------------------------------+
   | Zend Engine, DCE - Dead Code Elimination                             |
   +----------------------------------------------------------------------+
   | Copyright © The PHP Group and Contributors.                          |
   +----------------------------------------------------------------------+
   | This source file is subject to the Modified BSD License that is      |
   | bundled with this package in the file LICENSE, and is available      |
   | through the World Wide Web at <https://www.php.net/license/>.        |
   |                                                                      |
   | SPDX-License-Identifier: BSD-3-Clause                                |
   +----------------------------------------------------------------------+
   | Authors: Nikita Popov <nikic@php.net>                                |
   |          Dmitry Stogov <dmitry@php.net>                              |
   +----------------------------------------------------------------------+
*/

#include "Optimizer/zend_optimizer_internal.h"
#include "Optimizer/zend_inference.h"
#include "Optimizer/zend_ssa.h"
#include "Optimizer/zend_func_info.h"
#include "Optimizer/zend_call_graph.h"
#include "zend_bitset.h"

/* This pass implements a form of dead code elimination (DCE). The algorithm optimistically assumes
 * that all instructions and phis are dead. Instructions with immediate side-effects are then marked
 * as live. We then recursively (using a worklist) propagate liveness to the instructions that def
 * the used operands.
 *
 * Notes:
 *  * This pass does not perform unreachable code elimination. This happens as part of the SCCP
 *    pass.
 *  * The DCE is performed without taking control-dependence into account, i.e. all conditional
 *    branches are assumed to be live. It's possible to take control-dependence into account using
 *    the DCE algorithm described by Cytron et al., however it requires the construction of a
 *    postdominator tree and of postdominance frontiers, which does not seem worthwhile at this
 *    point.
 *  * We separate intrinsic side-effects from potential side-effects in the form of notices thrown
 *    by the instruction (in case we want to make this configurable). See may_have_side_effects() and
 *    zend_may_throw().
 *  * We often cannot DCE assignments and unsets while guaranteeing that dtors run in the same
 *    order. There is an optimization option to allow reordering of dtor effects.
 *  * The algorithm is able to eliminate dead modifications of non-escaping arrays
 *    and objects as well as dead arrays and objects allocations.
 */

typedef struct {
  zend_ssa *ssa;
  zend_op_array *op_array;
  zend_bitset instr_dead;
  zend_bitset phi_dead;
  zend_bitset instr_worklist;
  zend_bitset phi_worklist;
  zend_bitset phi_worklist_no_val;
  uint32_t instr_worklist_len;
  uint32_t phi_worklist_len;
  unsigned reorder_dtor_effects : 1;
} context;

static inline bool is_bad_mod(const zend_ssa *ssa, int use, int def) {
  if (def < 0) {
    /* This modification is not tracked by SSA, assume the worst */
    return true;
  }
  if (ssa->var_info[use].type & MAY_BE_REF) {
    /* Modification of reference may have side-effect */
    return true;
  }
  return false;
}

static inline bool may_have_side_effects(
    const zend_op_array *op_array, const zend_ssa *ssa,
    const zend_op *opline, const zend_ssa_op *ssa_op,
    bool reorder_dtor_effects) {
  switch (opline->opcode) {
    case ZEND_NOP:
    case ZEND_IS_IDENTICAL:
    case ZEND_IS_NOT_IDENTICAL:
    case ZEND_QM_ASSIGN:
    case ZEND_FE_FREE:
    case ZEND_TYPE_CHECK:
    case ZEND_DEFINED:
    case ZEND_ADD:
    case ZEND_SUB:
    case ZEND_MUL:
    case ZEND_POW:
    case ZEND_BW_OR:
    case ZEND_BW_AND:
    case ZEND_BW_XOR:
    case ZEND_CONCAT:
    case ZEND_FAST_CONCAT:
    case ZEND_DIV:
    case ZEND_MOD:
    case ZEND_BOOL_XOR:
    case ZEND_BOOL:
    case ZEND_BOOL_NOT:
    case ZEND_BW_NOT:
    case ZEND_SL:
    case ZEND_SR:
    case ZEND_IS_EQUAL:
    case ZEND_IS_NOT_EQUAL:
    case ZEND_IS_SMALLER:
    case ZEND_IS_SMALLER_OR_EQUAL:
    case ZEND_CASE:
    case ZEND_CASE_STRICT:
    case ZEND_CAST:
    case ZEND_ROPE_INIT:
    case ZEND_ROPE_ADD:
    case ZEND_INIT_ARRAY:
    case ZEND_SPACESHIP:
    case ZEND_STRLEN:
    case ZEND_COUNT:
    case ZEND_GET_TYPE:
    case ZEND_ISSET_ISEMPTY_THIS:
    case ZEND_ISSET_ISEMPTY_DIM_OBJ:
    case ZEND_FETCH_DIM_IS:
    case ZEND_ISSET_ISEMPTY_CV:
    case ZEND_ISSET_ISEMPTY_VAR:
    case ZEND_FETCH_IS:
    case ZEND_IN_ARRAY:
    case ZEND_FUNC_NUM_ARGS:
    case ZEND_FUNC_GET_ARGS:
    case ZEND_ARRAY_KEY_EXISTS:
    case ZEND_COPY_TMP:
      /* No side effects */
      return false;
    case ZEND_FREE:
      return opline->extended_value == ZEND_FREE_VOID_CAST;
    case ZEND_ADD_ARRAY_ELEMENT:
      /* TODO: We can't free two vars. Keep instruction alive. <?php [0, "$a" => "$b"]; */
      if ((opline->op1_type & (IS_VAR|IS_TMP_VAR)) && (opline->op2_type & (IS_VAR|IS_TMP_VAR))) {
        return true;
      }
      return false;
    case ZEND_ROPE_END:
      /* TODO: Rope dce optimization, see #76446 */
      return true;
    case ZEND_JMP:
    case ZEND_JMPZ:
    case ZEND_JMPNZ:
    case ZEND_JMPZ_EX:
    case ZEND_JMPNZ_EX:
    case ZEND_JMP_SET:
    case ZEND_COALESCE:
    case ZEND_ASSERT_CHECK:
    case ZEND_JMP_NULL:
    case ZEND_BIND_INIT_STATIC_OR_JMP:
    case ZEND_JMP_FRAMELESS:
      /* For our purposes a jumps and branches are side effects. */
      return true;
    case ZEND_BEGIN_SILENCE:
    case ZEND_END_SILENCE:
    case ZEND_ECHO:
    case ZEND_INCLUDE_OR_EVAL:
    case ZEND_THROW:
    case ZEND_MATCH_ERROR:
    case ZEND_EXT_STMT:
    case ZEND_EXT_FCALL_BEGIN:
    case ZEND_EXT_FCALL_END:
    case ZEND_TICKS:
    case ZEND_YIELD:
    case ZEND_VERIFY_NEVER_TYPE:
      /* Intrinsic side effects */
      return true;
    case ZEND_YIELD_FROM: {
      uint32_t t1 = OP1_INFO();
      if ((t1 & (MAY_BE_ANY|MAY_BE_UNDEF)) == MAY_BE_ARRAY && MAY_BE_EMPTY_ONLY(t1)) {
        return false;
      }
      return true;
    }
    case ZEND_DO_FCALL:
    case ZEND_DO_FCALL_BY_NAME:
    case ZEND_DO_ICALL:
    case ZEND_DO_UCALL:
    case ZEND_FRAMELESS_ICALL_0:
    case ZEND_FRAMELESS_ICALL_1:
    case ZEND_FRAMELESS_ICALL_2:
    case ZEND_FRAMELESS_ICALL_3:
      /* For now assume all calls have side effects */
      return true;
    case ZEND_RECV:
    case ZEND_RECV_INIT:
      /* Even though RECV_INIT can be side-effect free, these cannot be simply dropped
       * due to the prologue skipping code. */
      return true;
    case ZEND_ASSIGN_REF:
      return true;
    case ZEND_ASSIGN:
    {
      if (is_bad_mod(ssa, ssa_op->op1_use, ssa_op->op1_def)) {
        return true;
      }
      if (!reorder_dtor_effects) {
        if (opline->op2_type != IS_CONST
          && (OP2_INFO() & MAY_HAVE_DTOR)
          && ssa->vars[ssa_op->op2_use].escape_state != ESCAPE_STATE_NO_ESCAPE) {
          /* DCE might shorten lifetime */
          return true;
        }
      }
      return false;
    }
    case ZEND_UNSET_VAR:
      return true;
    case ZEND_UNSET_CV:
    {
      uint32_t t1 = OP1_INFO();
      if (t1 & MAY_BE_REF) {
        /* We don't consider uses as the LHS of an assignment as real uses during DCE, so
         * an unset may be considered dead even if there is a later assignment to the
         * variable. Removing the unset in this case would not be correct if the variable
         * is a reference, because unset breaks references. */
        return true;
      }
      return false;
    }
    case ZEND_PRE_INC:
    case ZEND_POST_INC:
    case ZEND_PRE_DEC:
    case ZEND_POST_DEC:
      return is_bad_mod(ssa, ssa_op->op1_use, ssa_op->op1_def);
    case ZEND_ASSIGN_OP:
      return is_bad_mod(ssa, ssa_op->op1_use, ssa_op->op1_def)
        || ssa->vars[ssa_op->op1_def].escape_state != ESCAPE_STATE_NO_ESCAPE;
    case ZEND_ASSIGN_DIM:
    case ZEND_ASSIGN_OBJ:
      if (is_bad_mod(ssa, ssa_op->op1_use, ssa_op->op1_def)
        || ssa->vars[ssa_op->op1_def].escape_state != ESCAPE_STATE_NO_ESCAPE) {
        return true;
      }
      if (!reorder_dtor_effects) {
        opline++;
        ssa_op++;
        if (opline->op1_type != IS_CONST
          && (OP1_INFO() & MAY_HAVE_DTOR)) {
          /* DCE might shorten lifetime */
          return true;
        }
      }
      return false;
    case ZEND_PRE_INC_OBJ:
    case ZEND_PRE_DEC_OBJ:
    case ZEND_POST_INC_OBJ:
    case ZEND_POST_DEC_OBJ:
      if (is_bad_mod(ssa, ssa_op->op1_use, ssa_op->op1_def)
        || ssa->vars[ssa_op->op1_def].escape_state != ESCAPE_STATE_NO_ESCAPE) {
        return true;
      }
      return false;
    case ZEND_BIND_STATIC:
      if (op_array->static_variables) {
        /* Implicit and Explicit bind static is effectively prologue of closure so
           report it has side effects like RECV, RECV_INIT; This allows us to
           reflect on the closure and discover used variable at runtime */
        if ((opline->extended_value & (ZEND_BIND_IMPLICIT|ZEND_BIND_EXPLICIT))) {
          return true;
        }
        /* Modifies static variables which are observable through reflection */
        if ((opline->extended_value & ZEND_BIND_REF) && opline->op2_type != IS_UNUSED) {
          return true;
        }
      }
      return false;
    case ZEND_CHECK_VAR:
      return (OP1_INFO() & MAY_BE_UNDEF) != 0;
    case ZEND_FE_RESET_R:
    case ZEND_FE_RESET_RW:
      /* Model as not having side-effects -- let the side-effect be introduced by
       * FE_FETCH if the array is not known to be non-empty. */
      return (OP1_INFO() & MAY_BE_ANY) != MAY_BE_ARRAY;
    default:
      /* For everything we didn't handle, assume a side-effect */
      return true;
  }
}

static zend_always_inline void add_to_worklists(const context *ctx, int var_num, int check) {
  const zend_ssa_var *var = &ctx->ssa->vars[var_num];
  if (var->definition >= 0) {
    if (!check || zend_bitset_in(ctx->instr_dead, var->definition)) {
      zend_bitset_incl(ctx->instr_worklist, var->definition);
    }
  } else if (var->definition_phi) {
    if (!check || zend_bitset_in(ctx->phi_dead, var_num)) {
      zend_bitset_incl(ctx->phi_worklist, var_num);
    }
  }
}

static inline void add_to_phi_worklist_no_val(const context *ctx, int var_num) {
  const zend_ssa_var *var = &ctx->ssa->vars[var_num];
  if (var->definition_phi && zend_bitset_in(ctx->phi_dead, var_num)) {
    zend_bitset_incl(ctx->phi_worklist_no_val, var_num);
  }
}

static zend_always_inline void add_operands_to_worklists(const context *ctx, const zend_op *opline, const zend_ssa_op *ssa_op, const zend_ssa *ssa, int check) {
  if (ssa_op->result_use >= 0) {
    add_to_worklists(ctx, ssa_op->result_use, check);
  }
  if (ssa_op->op1_use >= 0) {
    if (!zend_ssa_is_no_val_use(opline, ssa_op, ssa_op->op1_use)
     || (opline->opcode == ZEND_ASSIGN
      && (ssa->var_info[ssa_op->op1_use].type & MAY_BE_REF) != 0)) {
      add_to_worklists(ctx, ssa_op->op1_use, check);
    } else {
      add_to_phi_worklist_no_val(ctx, ssa_op->op1_use);
    }
  }
  if (ssa_op->op2_use >= 0) {
    if (!zend_ssa_is_no_val_use(opline, ssa_op, ssa_op->op2_use)
     || (opline->opcode == ZEND_FE_FETCH_R
      && (ssa->var_info[ssa_op->op2_use].type & MAY_BE_REF) != 0)) {
      add_to_worklists(ctx, ssa_op->op2_use, check);
    } else {
      add_to_phi_worklist_no_val(ctx, ssa_op->op2_use);
    }
  }
}

static zend_always_inline void add_phi_sources_to_worklists(const context *ctx, zend_ssa_phi *phi, int check) {
  const zend_ssa *ssa = ctx->ssa;
  int source;
  FOREACH_PHI_SOURCE(phi, source) {
    add_to_worklists(ctx, source, check);
  } FOREACH_PHI_SOURCE_END();
}

static inline bool is_var_dead(const context *ctx, int var_num) {
  const zend_ssa_var *var = &ctx->ssa->vars[var_num];
  if (var->definition_phi) {
    return zend_bitset_in(ctx->phi_dead, var_num);
  } else if (var->definition >= 0) {
    return zend_bitset_in(ctx->instr_dead, var->definition);
  } else {
    /* Variable has no definition, so either the definition has already been removed (var is
     * dead) or this is one of the implicit variables at the start of the function (for our
     * purposes live) */
    return var_num >= ctx->op_array->last_var;
  }
}

// Sometimes we can mark the var as EXT_UNUSED
static bool try_remove_var_def(const context *ctx, int free_var, int use_chain, const zend_op *opline) {
  if (use_chain >= 0) {
    return false;
  }
  zend_ssa_var *var = &ctx->ssa->vars[free_var];
  int def = var->definition;

  if (def >= 0) {
    zend_ssa_op *def_op = &ctx->ssa->ops[def];

    if (def_op->result_def == free_var
        && var->phi_use_chain == NULL
        && var->use_chain == (opline - ctx->op_array->opcodes)) {
      zend_op *def_opline = &ctx->op_array->opcodes[def];

      switch (def_opline->opcode) {
        case ZEND_ASSIGN:
        case ZEND_ASSIGN_REF:
        case ZEND_ASSIGN_DIM:
        case ZEND_ASSIGN_OBJ:
        case ZEND_ASSIGN_OBJ_REF:
        case ZEND_ASSIGN_STATIC_PROP:
        case ZEND_ASSIGN_STATIC_PROP_REF:
        case ZEND_ASSIGN_OP:
        case ZEND_ASSIGN_DIM_OP:
        case ZEND_ASSIGN_OBJ_OP:
        case ZEND_ASSIGN_STATIC_PROP_OP:
        case ZEND_PRE_INC:
        case ZEND_PRE_DEC:
        case ZEND_PRE_INC_OBJ:
        case ZEND_PRE_DEC_OBJ:
        case ZEND_DO_ICALL:
        case ZEND_DO_UCALL:
        case ZEND_DO_FCALL_BY_NAME:
        case ZEND_DO_FCALL:
        case ZEND_INCLUDE_OR_EVAL:
        case ZEND_YIELD:
        case ZEND_YIELD_FROM:
        case ZEND_ASSERT_CHECK:
          def_opline->result_type = IS_UNUSED;
          def_opline->result.var = 0;
          def_op->result_def = -1;
          var->definition = -1;
          return true;
        default:
          break;
      }
    }
  }
  return false;
}

static zend_always_inline bool may_be_refcounted(uint32_t type) {
  return (type & (MAY_BE_STRING|MAY_BE_ARRAY|MAY_BE_OBJECT|MAY_BE_RESOURCE|MAY_BE_REF)) != 0;
}

static inline bool is_free_of_live_var(const context *ctx, const zend_op *opline, const zend_ssa_op *ssa_op) {
  switch (opline->opcode) {
    case ZEND_FREE:
      /* It is always safe to remove FREEs of non-refcounted values, even if they are live. */
      if ((ctx->ssa->var_info[ssa_op->op1_use].type & (MAY_BE_REF|MAY_BE_ANY|MAY_BE_UNDEF)) != 0
       && !may_be_refcounted(ctx->ssa->var_info[ssa_op->op1_use].type)) {
        return false;
      }
      ZEND_FALLTHROUGH;
    case ZEND_FE_FREE:
      return !is_var_dead(ctx, ssa_op->op1_use);
    default:
      return false;
  }
}

/* Returns whether the instruction has been DCEd */
static bool dce_instr(const context *ctx, zend_op *opline, zend_ssa_op *ssa_op) {
  const zend_ssa *ssa = ctx->ssa;
  int free_var = -1;
  uint8_t free_var_type;

  if (opline->opcode == ZEND_NOP) {
    return false;
  }

  /* We mark FREEs as dead, but they're only really dead if the destroyed var is dead */
  if (is_free_of_live_var(ctx, opline, ssa_op)) {
    return false;
  }

  if ((opline->op1_type & (IS_VAR|IS_TMP_VAR)) && !is_var_dead(ctx, ssa_op->op1_use)) {
    if (!try_remove_var_def(ctx, ssa_op->op1_use, ssa_op->op1_use_chain, opline)) {
      if (may_be_refcounted(ssa->var_info[ssa_op->op1_use].type)
          && opline->opcode != ZEND_CASE
          && opline->opcode != ZEND_CASE_STRICT
          && opline->opcode != ZEND_COPY_TMP) {
        free_var = ssa_op->op1_use;
        free_var_type = opline->op1_type;
      }
    }
  }
  if ((opline->op2_type & (IS_VAR|IS_TMP_VAR)) && !is_var_dead(ctx, ssa_op->op2_use)) {
    if (!try_remove_var_def(ctx, ssa_op->op2_use, ssa_op->op2_use_chain, opline)) {
      if (may_be_refcounted(ssa->var_info[ssa_op->op2_use].type)) {
        if (free_var >= 0) {
          // TODO: We can't free two vars. Keep instruction alive.
          zend_bitset_excl(ctx->instr_dead, opline - ctx->op_array->opcodes);
          return false;
        }
        free_var = ssa_op->op2_use;
        free_var_type = opline->op2_type;
      }
    }
  }

  zend_ssa_rename_defs_of_instr(ctx->ssa, ssa_op);
  zend_ssa_remove_instr(ctx->ssa, opline, ssa_op);

  if (free_var >= 0) {
    opline->opcode = ZEND_FREE;
    opline->op1.var = EX_NUM_TO_VAR(ssa->vars[free_var].var);
    opline->op1_type = free_var_type;

    ssa_op->op1_use = free_var;
    ssa_op->op1_use_chain = ssa->vars[free_var].use_chain;
    ssa->vars[free_var].use_chain = ssa_op - ssa->ops;
    return false;
  }
  return true;
}

static inline int get_common_phi_source(const zend_ssa *ssa, zend_ssa_phi *phi) {
  int common_source = -1;
  int source;
  FOREACH_PHI_SOURCE(phi, source) {
    if (source == phi->ssa_var) {
      continue;
    }
    if (common_source == -1) {
      common_source = source;
    } else if (common_source != source) {
      return -1;
    }
  } FOREACH_PHI_SOURCE_END();

  /* If all sources are phi->ssa_var this phi must be in an unreachable cycle.
   * We can't easily drop the phi in that case, as we don't have something to replace it with.
   * Ideally SCCP would eliminate the whole cycle. */
  return common_source;
}

static void try_remove_trivial_phi(const context *ctx, zend_ssa_phi *phi) {
  zend_ssa *ssa = ctx->ssa;
  if (phi->pi < 0) {
    /* Phi assignment with identical source operands */
    int common_source = get_common_phi_source(ssa, phi);
    if (common_source >= 0) {
      zend_ssa_rename_var_uses(ssa, phi->ssa_var, common_source, 1);
      zend_ssa_remove_phi(ssa, phi);
    }
  } else {
    /* Pi assignment that is only used in Phi/Pi assignments */
    // TODO What if we want to rerun type inference after DCE? Maybe separate this?
    /*ZEND_ASSERT(phi->sources[0] != -1);
    if (ssa->vars[phi->ssa_var].use_chain < 0) {
      zend_ssa_rename_var_uses_keep_types(ssa, phi->ssa_var, phi->sources[0], 1);
      zend_ssa_remove_phi(ssa, phi);
    }*/
  }
}

static inline bool may_break_varargs(const zend_op_array *op_array, const zend_ssa *ssa, const zend_ssa_op *ssa_op) {
  if (ssa_op->op1_def >= 0
      && ssa->vars[ssa_op->op1_def].var < op_array->num_args) {
    return true;
  }
  if (ssa_op->op2_def >= 0
      && ssa->vars[ssa_op->op2_def].var < op_array->num_args) {
    return true;
  }
  if (ssa_op->result_def >= 0
      && ssa->vars[ssa_op->result_def].var < op_array->num_args) {
    return true;
  }
  return false;
}

static inline bool may_throw_dce_exception(const zend_op *opline) {
  return opline->opcode == ZEND_ADD_ARRAY_ELEMENT && opline->op2_type == IS_UNUSED;
}

int dce_optimize_op_array(zend_op_array *op_array, zend_optimizer_ctx *optimizer_ctx, zend_ssa *ssa, bool reorder_dtor_effects) {
  int i;
  zend_ssa_phi *phi;
  int removed_ops = 0;

  /* DCE of CV operations that changes arguments may affect vararg functions. */
  bool has_varargs = (ssa->cfg.flags & ZEND_FUNC_VARARG) != 0;

  context ctx;
  ctx.ssa = ssa;
  ctx.op_array = op_array;
  ctx.reorder_dtor_effects = reorder_dtor_effects;

  void *checkpoint = zend_arena_checkpoint(optimizer_ctx->arena);
  /* We have no dedicated phi vector, so we use the whole ssa var vector instead */
  ctx.instr_worklist_len = zend_bitset_len(op_array->last);
  ctx.instr_worklist = zend_arena_calloc(&optimizer_ctx->arena, ctx.instr_worklist_len, sizeof(zend_ulong));
  ctx.phi_worklist_len = zend_bitset_len(ssa->vars_count);
  ctx.phi_worklist = zend_arena_calloc(&optimizer_ctx->arena, ctx.phi_worklist_len, sizeof(zend_ulong));
  ctx.phi_worklist_no_val = zend_arena_calloc(&optimizer_ctx->arena, ctx.phi_worklist_len, sizeof(zend_ulong));

  /* Optimistically assume all instructions and phis to be dead */
  ctx.instr_dead = zend_arena_calloc(&optimizer_ctx->arena, ctx.instr_worklist_len, sizeof(zend_ulong));
  ctx.phi_dead = zend_arena_alloc(&optimizer_ctx->arena, ctx.phi_worklist_len * sizeof(zend_ulong));
  memset(ctx.phi_dead, 0xff, sizeof(zend_ulong) * ctx.phi_worklist_len);

  /* Mark non-CV phis as live. Even if the result is unused, we generally cannot remove one
   * of the producing instructions, as it combines producing the result with control flow.
   * This can be made more precise if there are any cases where this is not the case. */
  FOREACH_PHI(phi) {
    if (phi->var >= op_array->last_var
        && may_be_refcounted(ssa->var_info[phi->ssa_var].type)) {
      zend_bitset_excl(ctx.phi_dead, phi->ssa_var);
      add_phi_sources_to_worklists(&ctx, phi, 0);
    }
  } FOREACH_PHI_END();

  /* Mark reachable instruction without side effects as dead */
  uint32_t b = ssa->cfg.blocks_count;
  while (b > 0) {
    int op_data = -1;

    b--;
    const zend_basic_block *block = &ssa->cfg.blocks[b];
    if (!(block->flags & ZEND_BB_REACHABLE)) {
      continue;
    }
    i = block->start + block->len;
    while (i > block->start) {
      i--;

      if (op_array->opcodes[i].opcode == ZEND_OP_DATA) {
        op_data = i;
        continue;
      }

      if (zend_bitset_in(ctx.instr_worklist, i)) {
        zend_bitset_excl(ctx.instr_worklist, i);
        add_operands_to_worklists(&ctx, &op_array->opcodes[i], &ssa->ops[i], ssa, 0);
        if (op_data >= 0) {
          add_operands_to_worklists(&ctx, &op_array->opcodes[op_data], &ssa->ops[op_data], ssa, 0);
        }
      } else if (may_have_side_effects(op_array, ssa, &op_array->opcodes[i], &ssa->ops[i], ctx.reorder_dtor_effects)
          || (zend_may_throw(&op_array->opcodes[i], &ssa->ops[i], op_array, ssa)
            && !may_throw_dce_exception(&op_array->opcodes[i]))
          || (has_varargs && may_break_varargs(op_array, ssa, &ssa->ops[i]))) {
        if (op_array->opcodes[i].opcode == ZEND_NEW
            && op_array->opcodes[i+1].opcode == ZEND_DO_FCALL
            && ssa->ops[i].result_def >= 0
            && ssa->vars[ssa->ops[i].result_def].escape_state == ESCAPE_STATE_NO_ESCAPE) {
          zend_bitset_incl(ctx.instr_dead, i);
          zend_bitset_incl(ctx.instr_dead, i+1);
        } else {
          add_operands_to_worklists(&ctx, &op_array->opcodes[i], &ssa->ops[i], ssa, 0);
          if (op_data >= 0) {
            add_operands_to_worklists(&ctx, &op_array->opcodes[op_data], &ssa->ops[op_data], ssa, 0);
          }
        }
      } else {
        zend_bitset_incl(ctx.instr_dead, i);
        if (op_data >= 0) {
          zend_bitset_incl(ctx.instr_dead, op_data);
        }
      }
      op_data = -1;
    }
  }

  /* Propagate liveness backwards to all definitions of used vars */
  while (!zend_bitset_empty(ctx.instr_worklist, ctx.instr_worklist_len)
      || !zend_bitset_empty(ctx.phi_worklist, ctx.phi_worklist_len)) {
    while ((i = zend_bitset_pop_first(ctx.instr_worklist, ctx.instr_worklist_len)) >= 0) {
      zend_bitset_excl(ctx.instr_dead, i);
      add_operands_to_worklists(&ctx, &op_array->opcodes[i], &ssa->ops[i], ssa, 1);
      if (i < op_array->last
       && (op_array->opcodes[i+1].opcode == ZEND_OP_DATA
        || (op_array->opcodes[i].opcode == ZEND_NEW
         && op_array->opcodes[i+1].opcode == ZEND_DO_FCALL))) {
        zend_bitset_excl(ctx.instr_dead, i+1);
        add_operands_to_worklists(&ctx, &op_array->opcodes[i+1], &ssa->ops[i+1], ssa, 1);
      }
    }
    while ((i = zend_bitset_pop_first(ctx.phi_worklist, ctx.phi_worklist_len)) >= 0) {
      zend_bitset_excl(ctx.phi_dead, i);
      zend_bitset_excl(ctx.phi_worklist_no_val, i);
      add_phi_sources_to_worklists(&ctx, ssa->vars[i].definition_phi, 1);
    }
  }

  /* Eliminate dead instructions */
  ZEND_BITSET_FOREACH(ctx.instr_dead, ctx.instr_worklist_len, i) {
    removed_ops += dce_instr(&ctx, &op_array->opcodes[i], &ssa->ops[i]);
  } ZEND_BITSET_FOREACH_END();

  /* Improper uses don't count as "uses" for the purpose of instruction elimination,
   * but we have to retain phis defining them.
   * Propagate this information backwards, marking any phi with an improperly used
   * target as non-dead. */
  while ((i = zend_bitset_pop_first(ctx.phi_worklist_no_val, ctx.phi_worklist_len)) >= 0) {
    zend_ssa_phi *phi = ssa->vars[i].definition_phi;
    int source;
    zend_bitset_excl(ctx.phi_dead, i);
    FOREACH_PHI_SOURCE(phi, source) {
      add_to_phi_worklist_no_val(&ctx, source);
    } FOREACH_PHI_SOURCE_END();
  }

  /* Now collect the actually dead phis */
  FOREACH_PHI(phi) {
    if (zend_bitset_in(ctx.phi_dead, phi->ssa_var)) {
      zend_ssa_remove_uses_of_var(ssa, phi->ssa_var);
      zend_ssa_remove_phi(ssa, phi);
    } else {
      /* Remove trivial phis (phis with identical source operands) */
      try_remove_trivial_phi(&ctx, phi);
    }
  } FOREACH_PHI_END();

  zend_arena_release(&optimizer_ctx->arena, checkpoint);

  return removed_ops;
}

Coverage Report

Created: 2026-06-02 06:36