protobuf/upb/decode.c

/*
 * Copyright (c) 2009-2021, Google LLC
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *     * Redistributions of source code must retain the above copyright
 *       notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above copyright
 *       notice, this list of conditions and the following disclaimer in the
 *       documentation and/or other materials provided with the distribution.
 *     * Neither the name of Google LLC nor the
 *       names of its contributors may be used to endorse or promote products
 *       derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL Google LLC BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include "upb/decode.h"

#include <setjmp.h>
#include <string.h>

#include "upb/decode_internal.h"
#include "upb/upb.h"
#include "upb/upb_internal.h"

/* Must be last. */
#include "upb/port_def.inc"

/* Maps descriptor type -> elem_size_lg2.  */
static const uint8_t desctype_to_elem_size_lg2[] = {
    -1,               /* invalid descriptor type */
    3,  /* DOUBLE */
    2,   /* FLOAT */
    3,   /* INT64 */
    3,  /* UINT64 */
    2,   /* INT32 */
    3,  /* FIXED64 */
    2,  /* FIXED32 */
    0,    /* BOOL */
    UPB_SIZE(3, 4),  /* STRING */
    UPB_SIZE(2, 3),  /* GROUP */
    UPB_SIZE(2, 3),  /* MESSAGE */
    UPB_SIZE(3, 4),  /* BYTES */
    2,  /* UINT32 */
    2,    /* ENUM */
    2,   /* SFIXED32 */
    3,   /* SFIXED64 */
    2,   /* SINT32 */
    3,   /* SINT64 */
};

/* Maps descriptor type -> upb map size.  */
static const uint8_t desctype_to_mapsize[] = {
    -1,                 /* invalid descriptor type */
    8,                  /* DOUBLE */
    4,                  /* FLOAT */
    8,                  /* INT64 */
    8,                  /* UINT64 */
    4,                  /* INT32 */
    8,                  /* FIXED64 */
    4,                  /* FIXED32 */
    1,                  /* BOOL */
    UPB_MAPTYPE_STRING, /* STRING */
    sizeof(void *),     /* GROUP */
    sizeof(void *),     /* MESSAGE */
    UPB_MAPTYPE_STRING, /* BYTES */
    4,                  /* UINT32 */
    4,                  /* ENUM */
    4,                  /* SFIXED32 */
    8,                  /* SFIXED64 */
    4,                  /* SINT32 */
    8,                  /* SINT64 */
};

static const unsigned fixed32_ok = (1 << UPB_DTYPE_FLOAT) |
                                   (1 << UPB_DTYPE_FIXED32) |
                                   (1 << UPB_DTYPE_SFIXED32);

static const unsigned fixed64_ok = (1 << UPB_DTYPE_DOUBLE) |
                                   (1 << UPB_DTYPE_FIXED64) |
                                   (1 << UPB_DTYPE_SFIXED64);

/* Op: an action to be performed for a wire-type/field-type combination. */
#define OP_SCALAR_LG2(n) (n)      /* n in [0, 2, 3] => op in [0, 2, 3] */
#define OP_STRING 4
#define OP_BYTES 5
#define OP_SUBMSG 6
/* Ops above are scalar-only. Repeated fields can use any op.  */
#define OP_FIXPCK_LG2(n) (n + 5)  /* n in [2, 3] => op in [7, 8] */
#define OP_VARPCK_LG2(n) (n + 9)  /* n in [0, 2, 3] => op in [9, 11, 12] */

static const int8_t varint_ops[19] = {
    -1,               /* field not found */
    -1,               /* DOUBLE */
    -1,               /* FLOAT */
    OP_SCALAR_LG2(3), /* INT64 */
    OP_SCALAR_LG2(3), /* UINT64 */
    OP_SCALAR_LG2(2), /* INT32 */
    -1,               /* FIXED64 */
    -1,               /* FIXED32 */
    OP_SCALAR_LG2(0), /* BOOL */
    -1,               /* STRING */
    -1,               /* GROUP */
    -1,               /* MESSAGE */
    -1,               /* BYTES */
    OP_SCALAR_LG2(2), /* UINT32 */
    OP_SCALAR_LG2(2), /* ENUM */
    -1,               /* SFIXED32 */
    -1,               /* SFIXED64 */
    OP_SCALAR_LG2(2), /* SINT32 */
    OP_SCALAR_LG2(3), /* SINT64 */
};

static const int8_t delim_ops[37] = {
    /* For non-repeated field type. */
    -1,        /* field not found */
    -1,        /* DOUBLE */
    -1,        /* FLOAT */
    -1,        /* INT64 */
    -1,        /* UINT64 */
    -1,        /* INT32 */
    -1,        /* FIXED64 */
    -1,        /* FIXED32 */
    -1,        /* BOOL */
    OP_STRING, /* STRING */
    -1,        /* GROUP */
    OP_SUBMSG, /* MESSAGE */
    OP_BYTES,  /* BYTES */
    -1,        /* UINT32 */
    -1,        /* ENUM */
    -1,        /* SFIXED32 */
    -1,        /* SFIXED64 */
    -1,        /* SINT32 */
    -1,        /* SINT64 */
    /* For repeated field type. */
    OP_FIXPCK_LG2(3), /* REPEATED DOUBLE */
    OP_FIXPCK_LG2(2), /* REPEATED FLOAT */
    OP_VARPCK_LG2(3), /* REPEATED INT64 */
    OP_VARPCK_LG2(3), /* REPEATED UINT64 */
    OP_VARPCK_LG2(2), /* REPEATED INT32 */
    OP_FIXPCK_LG2(3), /* REPEATED FIXED64 */
    OP_FIXPCK_LG2(2), /* REPEATED FIXED32 */
    OP_VARPCK_LG2(0), /* REPEATED BOOL */
    OP_STRING,        /* REPEATED STRING */
    OP_SUBMSG,        /* REPEATED GROUP */
    OP_SUBMSG,        /* REPEATED MESSAGE */
    OP_BYTES,         /* REPEATED BYTES */
    OP_VARPCK_LG2(2), /* REPEATED UINT32 */
    OP_VARPCK_LG2(2), /* REPEATED ENUM */
    OP_FIXPCK_LG2(2), /* REPEATED SFIXED32 */
    OP_FIXPCK_LG2(3), /* REPEATED SFIXED64 */
    OP_VARPCK_LG2(2), /* REPEATED SINT32 */
    OP_VARPCK_LG2(3), /* REPEATED SINT64 */
};

typedef union {
  bool bool_val;
  uint32_t uint32_val;
  uint64_t uint64_val;
  uint32_t size;
} wireval;

static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
                              const upb_msglayout *layout);

UPB_NORETURN static void decode_err(upb_decstate *d) { UPB_LONGJMP(d->err, 1); }

// We don't want to mark this NORETURN, see comment in .h.
// Unfortunately this code to suppress the warning doesn't appear to be working.
#ifdef __clang__
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunknown-warning-option"
#pragma clang diagnostic ignored "-Wsuggest-attribute"
#endif

const char *fastdecode_err(upb_decstate *d) {
  longjmp(d->err, 1);
  return NULL;
}

#ifdef __clang__
#pragma clang diagnostic pop
#endif

const uint8_t upb_utf8_offsets[] = {
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
    2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
    4, 4, 4, 4, 4, 4, 4, 4, 0, 0, 0, 0, 0, 0, 0, 0,
};

static void decode_verifyutf8(upb_decstate *d, const char *buf, int len) {
  if (!decode_verifyutf8_inl(buf, len)) decode_err(d);
}

static bool decode_reserve(upb_decstate *d, upb_array *arr, size_t elem) {
  bool need_realloc = arr->size - arr->len < elem;
  if (need_realloc && !_upb_array_realloc(arr, arr->len + elem, &d->arena)) {
    decode_err(d);
  }
  return need_realloc;
}

typedef struct {
  const char *ptr;
  uint64_t val;
} decode_vret;

UPB_NOINLINE
static decode_vret decode_longvarint64(const char *ptr, uint64_t val) {
  decode_vret ret = {NULL, 0};
  uint64_t byte;
  int i;
  for (i = 1; i < 10; i++) {
    byte = (uint8_t)ptr[i];
    val += (byte - 1) << (i * 7);
    if (!(byte & 0x80)) {
      ret.ptr = ptr + i + 1;
      ret.val = val;
      return ret;
    }
  }
  return ret;
}

UPB_FORCEINLINE
static const char *decode_varint64(upb_decstate *d, const char *ptr,
                                   uint64_t *val) {
  uint64_t byte = (uint8_t)*ptr;
  if (UPB_LIKELY((byte & 0x80) == 0)) {
    *val = byte;
    return ptr + 1;
  } else {
    decode_vret res = decode_longvarint64(ptr, byte);
    if (!res.ptr) decode_err(d);
    *val = res.val;
    return res.ptr;
  }
}

UPB_FORCEINLINE
static const char *decode_tag(upb_decstate *d, const char *ptr,
                                   uint32_t *val) {
  uint64_t byte = (uint8_t)*ptr;
  if (UPB_LIKELY((byte & 0x80) == 0)) {
    *val = byte;
    return ptr + 1;
  } else {
    const char *start = ptr;
    decode_vret res = decode_longvarint64(ptr, byte);
    ptr = res.ptr;
    *val = res.val;
    if (!ptr || *val > UINT32_MAX || ptr - start > 5) decode_err(d);
    return ptr;
  }
}

static void decode_munge(int type, wireval *val) {
  switch (type) {
    case UPB_DESCRIPTOR_TYPE_BOOL:
      val->bool_val = val->uint64_val != 0;
      break;
    case UPB_DESCRIPTOR_TYPE_SINT32: {
      uint32_t n = val->uint32_val;
      val->uint32_val = (n >> 1) ^ -(int32_t)(n & 1);
      break;
    }
    case UPB_DESCRIPTOR_TYPE_SINT64: {
      uint64_t n = val->uint64_val;
      val->uint64_val = (n >> 1) ^ -(int64_t)(n & 1);
      break;
    }
    case UPB_DESCRIPTOR_TYPE_INT32:
    case UPB_DESCRIPTOR_TYPE_UINT32:
      if (!_upb_isle()) {
        /* The next stage will memcpy(dst, &val, 4) */
        val->uint32_val = val->uint64_val;
      }
      break;
  }
}

static const upb_msglayout_field *upb_find_field(const upb_msglayout *l,
                                                 uint32_t field_number,
                                                 int *last_field_index) {
  static upb_msglayout_field none = {0, 0, 0, 0, 0, 0};

  if (l == NULL) return &none;

  size_t idx = (size_t)field_number - 1;  // 0 wraps to SIZE_MAX
  if (idx < l->dense_below) {
    goto found;
  }

  int last = *last_field_index;
  for (idx = last; idx < l->field_count; idx++) {
    if (l->fields[idx].number == field_number) {
      goto found;
    }
  }

  for (idx = 0; idx < last; idx++) {
    if (l->fields[idx].number == field_number) {
      goto found;
    }
  }

  return &none; /* Unknown field. */

 found:
  UPB_ASSERT(l->fields[idx].number == field_number);
  *last_field_index = idx;
  return &l->fields[idx];
}

static upb_msg *decode_newsubmsg(upb_decstate *d,
                                 const upb_msglayout *const *submsgs,
                                 const upb_msglayout_field *field) {
  const upb_msglayout *subl = submsgs[field->submsg_index];
  return _upb_msg_new_inl(subl, &d->arena);
}

UPB_NOINLINE
const char *decode_isdonefallback(upb_decstate *d, const char *ptr,
                                  int overrun) {
  ptr = decode_isdonefallback_inl(d, ptr, overrun);
  if (ptr == NULL) {
    decode_err(d);
  }
  return ptr;
}

static const char *decode_readstr(upb_decstate *d, const char *ptr, int size,
                                  upb_strview *str) {
  if (d->alias) {
    str->data = ptr;
  } else {
    char *data =  upb_arena_malloc(&d->arena, size);
    if (!data) decode_err(d);
    memcpy(data, ptr, size);
    str->data = data;
  }
  str->size = size;
  return ptr + size;
}

UPB_FORCEINLINE
static const char *decode_tosubmsg(upb_decstate *d, const char *ptr,
                                   upb_msg *submsg, 
                                   const upb_msglayout *const *submsgs,
                                   const upb_msglayout_field *field, int size) {
  const upb_msglayout *subl = submsgs[field->submsg_index];
  int saved_delta = decode_pushlimit(d, ptr, size);
  if (--d->depth < 0) decode_err(d);
  if (!decode_isdone(d, &ptr)) {
    ptr = decode_msg(d, ptr, submsg, subl);
  }
  if (d->end_group != DECODE_NOGROUP) decode_err(d);
  decode_poplimit(d, ptr, saved_delta);
  d->depth++;
  return ptr;
}

UPB_FORCEINLINE
static const char *decode_group(upb_decstate *d, const char *ptr,
                                upb_msg *submsg, const upb_msglayout *subl,
                                uint32_t number) {
  if (--d->depth < 0) decode_err(d);
  if (decode_isdone(d, &ptr)) {
    decode_err(d);
  }
  ptr = decode_msg(d, ptr, submsg, subl);
  if (d->end_group != number) decode_err(d);
  d->end_group = DECODE_NOGROUP;
  d->depth++;
  return ptr;
}

UPB_FORCEINLINE
static const char *decode_togroup(upb_decstate *d, const char *ptr,
                                  upb_msg *submsg,
                                  const upb_msglayout *const *submsgs,
                                  const upb_msglayout_field *field) {
  const upb_msglayout *subl = submsgs[field->submsg_index];
  return decode_group(d, ptr, submsg, subl, field->number);
}

static const char *decode_toarray(upb_decstate *d, const char *ptr,
                                  upb_msg *msg,
                                  const upb_msglayout *const *submsgs,
                                  const upb_msglayout_field *field, wireval *val,
                                  int op) {
  upb_array **arrp = UPB_PTR_AT(msg, field->offset, void);
  upb_array *arr = *arrp;
  void *mem;

  if (arr) {
    decode_reserve(d, arr, 1);
  } else {
    size_t lg2 = desctype_to_elem_size_lg2[field->descriptortype];
    arr = _upb_array_new(&d->arena, 4, lg2);
    if (!arr) decode_err(d);
    *arrp = arr;
  }

  switch (op) {
    case OP_SCALAR_LG2(0):
    case OP_SCALAR_LG2(2):
    case OP_SCALAR_LG2(3):
      /* Append scalar value. */
      mem = UPB_PTR_AT(_upb_array_ptr(arr), arr->len << op, void);
      arr->len++;
      memcpy(mem, val, 1 << op);
      return ptr;
    case OP_STRING:
      decode_verifyutf8(d, ptr, val->size);
      /* Fallthrough. */
    case OP_BYTES: {
      /* Append bytes. */
      upb_strview *str = (upb_strview*)_upb_array_ptr(arr) + arr->len;
      arr->len++;
      return decode_readstr(d, ptr, val->size, str);
    }
    case OP_SUBMSG: {
      /* Append submessage / group. */
      upb_msg *submsg = decode_newsubmsg(d, submsgs, field);
      *UPB_PTR_AT(_upb_array_ptr(arr), arr->len * sizeof(void *), upb_msg *) =
          submsg;
      arr->len++;
      if (UPB_UNLIKELY(field->descriptortype == UPB_DTYPE_GROUP)) {
        return decode_togroup(d, ptr, submsg, submsgs, field);
      } else {
        return decode_tosubmsg(d, ptr, submsg, submsgs, field, val->size);
      }
    }
    case OP_FIXPCK_LG2(2):
    case OP_FIXPCK_LG2(3): {
      /* Fixed packed. */
      int lg2 = op - OP_FIXPCK_LG2(0);
      int mask = (1 << lg2) - 1;
      size_t count = val->size >> lg2;
      if ((val->size & mask) != 0) {
        decode_err(d); /* Length isn't a round multiple of elem size. */
      }
      decode_reserve(d, arr, count);
      mem = UPB_PTR_AT(_upb_array_ptr(arr), arr->len << lg2, void);
      arr->len += count;
      memcpy(mem, ptr, val->size);  /* XXX: ptr boundary. */
      return ptr + val->size;
    }
    case OP_VARPCK_LG2(0):
    case OP_VARPCK_LG2(2):
    case OP_VARPCK_LG2(3): {
      /* Varint packed. */
      int lg2 = op - OP_VARPCK_LG2(0);
      int scale = 1 << lg2;
      int saved_limit = decode_pushlimit(d, ptr, val->size);
      char *out = UPB_PTR_AT(_upb_array_ptr(arr), arr->len << lg2, void);
      while (!decode_isdone(d, &ptr)) {
        wireval elem;
        ptr = decode_varint64(d, ptr, &elem.uint64_val);
        decode_munge(field->descriptortype, &elem);
        if (decode_reserve(d, arr, 1)) {
          out = UPB_PTR_AT(_upb_array_ptr(arr), arr->len << lg2, void);
        }
        arr->len++;
        memcpy(out, &elem, scale);
        out += scale;
      }
      decode_poplimit(d, ptr, saved_limit);
      return ptr;
    }
    default:
      UPB_UNREACHABLE();
  }
}

static const char *decode_tomap(upb_decstate *d, const char *ptr, upb_msg *msg,
                                const upb_msglayout *const *submsgs,
                                const upb_msglayout_field *field, wireval *val) {
  upb_map **map_p = UPB_PTR_AT(msg, field->offset, upb_map *);
  upb_map *map = *map_p;
  upb_map_entry ent;
  const upb_msglayout *entry = submsgs[field->submsg_index];

  if (!map) {
    /* Lazily create map. */
    const upb_msglayout_field *key_field = &entry->fields[0];
    const upb_msglayout_field *val_field = &entry->fields[1];
    char key_size = desctype_to_mapsize[key_field->descriptortype];
    char val_size = desctype_to_mapsize[val_field->descriptortype];
    UPB_ASSERT(key_field->offset == 0);
    UPB_ASSERT(val_field->offset == sizeof(upb_strview));
    map = _upb_map_new(&d->arena, key_size, val_size);
    *map_p = map;
  }

  /* Parse map entry. */
  memset(&ent, 0, sizeof(ent));

  if (entry->fields[1].descriptortype == UPB_DESCRIPTOR_TYPE_MESSAGE ||
      entry->fields[1].descriptortype == UPB_DESCRIPTOR_TYPE_GROUP) {
    /* Create proactively to handle the case where it doesn't appear. */
    ent.v.val = upb_value_ptr(_upb_msg_new(entry->submsgs[0], &d->arena));
  }

  ptr = decode_tosubmsg(d, ptr, &ent.k, submsgs, field, val->size);
  _upb_map_set(map, &ent.k, map->key_size, &ent.v, map->val_size, &d->arena);
  return ptr;
}

static const char *decode_tomsg(upb_decstate *d, const char *ptr, upb_msg *msg,
                                const upb_msglayout *const *submsgs,
                                const upb_msglayout_field *field, wireval *val,
                                int op) {
  void *mem = UPB_PTR_AT(msg, field->offset, void);
  int type = field->descriptortype;

  /* Set presence if necessary. */
  if (field->presence > 0) {
    _upb_sethas_field(msg, field);
  } else if (field->presence < 0) {
    /* Oneof case */
    uint32_t *oneof_case = _upb_oneofcase_field(msg, field);
    if (op == OP_SUBMSG && *oneof_case != field->number) {
      memset(mem, 0, sizeof(void*));
    }
    *oneof_case = field->number;
  }

  /* Store into message. */
  switch (op) {
    case OP_SUBMSG: {
      upb_msg **submsgp = mem;
      upb_msg *submsg = *submsgp;
      if (!submsg) {
        submsg = decode_newsubmsg(d, submsgs, field);
        *submsgp = submsg;
      }
      if (UPB_UNLIKELY(type == UPB_DTYPE_GROUP)) {
        ptr = decode_togroup(d, ptr, submsg, submsgs, field);
      } else {
        ptr = decode_tosubmsg(d, ptr, submsg, submsgs, field, val->size);
      }
      break;
    }
    case OP_STRING:
      decode_verifyutf8(d, ptr, val->size);
      /* Fallthrough. */
    case OP_BYTES:
      return decode_readstr(d, ptr, val->size, mem);
    case OP_SCALAR_LG2(3):
      memcpy(mem, val, 8);
      break;
    case OP_SCALAR_LG2(2):
      memcpy(mem, val, 4);
      break;
    case OP_SCALAR_LG2(0):
      memcpy(mem, val, 1);
      break;
    default:
      UPB_UNREACHABLE();
  }

  return ptr;
}

UPB_FORCEINLINE
static bool decode_tryfastdispatch(upb_decstate *d, const char **ptr,
                                   upb_msg *msg, const upb_msglayout *layout) {
#if UPB_FASTTABLE
  if (layout && layout->table_mask != (unsigned char)-1) {
    uint16_t tag = fastdecode_loadtag(*ptr);
    intptr_t table = decode_totable(layout);
    *ptr = fastdecode_tagdispatch(d, *ptr, msg, table, 0, tag);
    return true;
  }
#endif
  return false;
}

UPB_NOINLINE
static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
                              const upb_msglayout *layout) {
  int last_field_index = 0;
  while (true) {
    uint32_t tag;
    const upb_msglayout_field *field;
    int field_number;
    int wire_type;
    const char *field_start = ptr;
    wireval val;
    int op;

    UPB_ASSERT(ptr < d->limit_ptr);
    ptr = decode_tag(d, ptr, &tag);
    field_number = tag >> 3;
    wire_type = tag & 7;

    field = upb_find_field(layout, field_number, &last_field_index);

    switch (wire_type) {
      case UPB_WIRE_TYPE_VARINT:
        ptr = decode_varint64(d, ptr, &val.uint64_val);
        op = varint_ops[field->descriptortype];
        decode_munge(field->descriptortype, &val);
        break;
      case UPB_WIRE_TYPE_32BIT:
        memcpy(&val.uint32_val, ptr, 4);
        val.uint32_val = _upb_be_swap32(val.uint32_val);
        ptr += 4;
        op = OP_SCALAR_LG2(2);
        if (((1 << field->descriptortype) & fixed32_ok) == 0) goto unknown;
        break;
      case UPB_WIRE_TYPE_64BIT:
        memcpy(&val.uint64_val, ptr, 8);
        val.uint64_val = _upb_be_swap64(val.uint64_val);
        ptr += 8;
        op = OP_SCALAR_LG2(3);
        if (((1 << field->descriptortype) & fixed64_ok) == 0) goto unknown;
        break;
      case UPB_WIRE_TYPE_DELIMITED: {
        int ndx = field->descriptortype;
        uint64_t size;
        if (_upb_isrepeated(field)) ndx += 18;
        ptr = decode_varint64(d, ptr, &size);
        if (size >= INT32_MAX ||
            ptr - d->end + (int32_t)size > d->limit) {
          decode_err(d); /* Length overflow. */
        }
        op = delim_ops[ndx];
        val.size = size;
        break;
      }
      case UPB_WIRE_TYPE_START_GROUP:
        val.uint32_val = field_number;
        op = OP_SUBMSG;
        if (field->descriptortype != UPB_DTYPE_GROUP) goto unknown;
        break;
      case UPB_WIRE_TYPE_END_GROUP:
        d->end_group = field_number;
        return ptr;
      default:
        decode_err(d);
    }

    if (op >= 0) {
      /* Parse, using op for dispatch. */
      switch (field->label) {
        case UPB_LABEL_REPEATED:
        case _UPB_LABEL_PACKED:
          ptr = decode_toarray(d, ptr, msg, layout->submsgs, field, &val, op);
          break;
        case _UPB_LABEL_MAP:
          ptr = decode_tomap(d, ptr, msg, layout->submsgs, field, &val);
          break;
        default:
          ptr = decode_tomsg(d, ptr, msg, layout->submsgs, field, &val, op);
          break;
      }
    } else {
    unknown:
      /* Skip unknown field. */
      if (field_number == 0) decode_err(d);
      if (wire_type == UPB_WIRE_TYPE_DELIMITED) ptr += val.size;
      if (msg) {
        if (wire_type == UPB_WIRE_TYPE_START_GROUP) {
          d->unknown = field_start;
          d->unknown_msg = msg;
          ptr = decode_group(d, ptr, NULL, NULL, field_number);
          d->unknown_msg = NULL;
          field_start = d->unknown;
        }
        if (!_upb_msg_addunknown(msg, field_start, ptr - field_start,
                                 &d->arena)) {
          decode_err(d);
        }
      } else if (wire_type == UPB_WIRE_TYPE_START_GROUP) {
        ptr = decode_group(d, ptr, NULL, NULL, field_number);
      }
    }

    if (decode_isdone(d, &ptr)) return ptr;
    if (decode_tryfastdispatch(d, &ptr, msg, layout)) return ptr;
  }
}

const char *fastdecode_generic(struct upb_decstate *d, const char *ptr,
                               upb_msg *msg, intptr_t table, uint64_t hasbits,
                               uint64_t data) {
  (void)data;
  *(uint32_t*)msg |= hasbits;
  return decode_msg(d, ptr, msg, decode_totablep(table));
}

static bool decode_top(struct upb_decstate *d, const char *buf, void *msg,
                       const upb_msglayout *l) {
  if (!decode_tryfastdispatch(d, &buf, msg, l)) {
    decode_msg(d, buf, msg, l);
  }
  return d->end_group == DECODE_NOGROUP;
}

bool _upb_decode(const char *buf, size_t size, void *msg,
                 const upb_msglayout *l, upb_arena *arena, int options) {
  bool ok;
  upb_decstate state;
  unsigned depth = (unsigned)options >> 16;

  if (size == 0) {
    return true;
  } else if (size <= 16) {
    memset(&state.patch, 0, 32);
    memcpy(&state.patch, buf, size);
    buf = state.patch;
    state.end = buf + size;
    state.limit = 0;
    state.alias = false;
  } else {
    state.end = buf + size - 16;
    state.limit = 16;
    state.alias = options & UPB_DECODE_ALIAS;
  }

  state.limit_ptr = state.end;
  state.unknown_msg = NULL;
  state.depth = depth ? depth : 64;
  state.end_group = DECODE_NOGROUP;
  state.arena.head = arena->head;
  state.arena.last_size = arena->last_size;
  state.arena.cleanup_metadata = arena->cleanup_metadata;
  state.arena.parent = arena;

  if (UPB_UNLIKELY(UPB_SETJMP(state.err))) {
    ok = false;
  } else {
    ok = decode_top(&state, buf, msg, l);
  }

  arena->head.ptr = state.arena.head.ptr;
  arena->head.end = state.arena.head.end;
  arena->cleanup_metadata = state.arena.cleanup_metadata;
  return ok;
}

#undef OP_SCALAR_LG2
#undef OP_FIXPCK_LG2
#undef OP_VARPCK_LG2
#undef OP_STRING
#undef OP_SUBMSG