diff --git a/upb/decode.c b/upb/decode.c
index 32dda2f3c4..347f964a91 100644
--- a/upb/decode.c
+++ b/upb/decode.c
@@ -139,10 +139,14 @@ static const int8_t delim_ops[37] = {
 
 /* Data pertaining to the parse. */
 typedef struct {
-  const char *limit;       /* End of delimited region or end of buffer. */
-  upb_arena arena;
+  const char *end;         /* Can read up to 16 bytes slop beyond this. */
+  const char *limit_ptr;   /* = end + UPB_MIN(limit, 0) */
+  int limit;               /* Submessage limit relative to end. */
   int depth;
   uint32_t end_group; /* Set to field number of END_GROUP tag, if any. */
+  bool alias;
+  char patch[32];
+  upb_arena arena;
   jmp_buf err;
 } upb_decstate;
 
@@ -150,7 +154,7 @@ typedef union {
   bool bool_val;
   uint32_t uint32_val;
   uint64_t uint64_val;
-  upb_strview str_val;
+  uint32_t size;
 } wireval;
 
 static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
@@ -200,41 +204,48 @@ static bool decode_reserve(upb_decstate *d, upb_array *arr, size_t elem) {
   return need_realloc;
 }
 
+typedef struct {
+  const char *ptr;
+  uint64_t val;
+} decode_vret;
+
 UPB_NOINLINE
-static const char *decode_longvarint64(upb_decstate *d, const char *ptr,
-                                       const char *limit, uint64_t *val) {
-  uint8_t byte;
-  int bitpos = 0;
-  uint64_t out = 0;
-
-  do {
-    if (bitpos >= 70 || ptr == limit) decode_err(d);
-    byte = *ptr;
-    out |= (uint64_t)(byte & 0x7F) << bitpos;
-    ptr++;
-    bitpos += 7;
-  } while (byte & 0x80);
-
-  *val = out;
-  return ptr;
+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,
-                                   const char *limit, uint64_t *val) {
-  if (UPB_LIKELY(ptr < limit && (*ptr & 0x80) == 0)) {
-    *val = (uint8_t)*ptr;
+                                   uint64_t *val) {
+  uint64_t byte = (uint8_t)*ptr;
+  if (UPB_LIKELY((byte & 0x80) == 0)) {
+    *val = byte;
     return ptr + 1;
   } else {
-    return decode_longvarint64(d, ptr, limit, val);
+    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_varint32(upb_decstate *d, const char *ptr,
-                                   const char *limit, uint32_t *val) {
+                                   uint32_t *val) {
   uint64_t u64;
-  ptr = decode_varint64(d, ptr, limit, &u64);
+  ptr = decode_varint64(d, ptr, &u64);
   if (u64 > UINT32_MAX) decode_err(d);
   *val = (uint32_t)u64;
   return ptr;
@@ -287,17 +298,82 @@ static upb_msg *decode_newsubmsg(upb_decstate *d, const upb_msglayout *layout,
   return _upb_msg_new_inl(subl, &d->arena);
 }
 
-static void decode_tosubmsg(upb_decstate *d, upb_msg *submsg,
-                            const upb_msglayout *layout,
-                            const upb_msglayout_field *field, upb_strview val) {
+static int decode_pushlimit(upb_decstate *d, const char *ptr, int size) {
+  int limit = size + (int)(ptr - d->end);
+  int delta = d->limit - limit;
+  d->limit = limit;
+  d->limit_ptr = d->end + UPB_MIN(0, limit);
+  return delta;
+}
+
+static void decode_poplimit(upb_decstate *d, int saved_delta) {
+  d->limit += saved_delta;
+  d->limit_ptr = d->end + UPB_MIN(0, d->limit);
+}
+
+typedef struct {
+  bool ok;
+  const char *ptr;
+} decode_doneret;
+
+UPB_NOINLINE
+static const char *decode_isdonefallback(upb_decstate *d, const char *ptr,
+                                         int overrun) {
+  if (overrun < d->limit) {
+    /* Need to copy remaining data into patch buffer. */
+    UPB_ASSERT(overrun < 16);
+    memset(d->patch + 16, 0, 16);
+    memcpy(d->patch, d->end, 16);
+    ptr = &d->patch[0] + overrun;
+    d->end = &d->patch[16];
+    d->limit -= 16;
+    d->limit_ptr = d->end + d->limit;
+    d->alias = false;
+    UPB_ASSERT(ptr < d->limit_ptr);
+    return ptr;
+  } else {
+    decode_err(d);
+  }
+}
+
+UPB_FORCEINLINE
+static bool decode_isdone(upb_decstate *d, const char **ptr) {
+  int overrun = *ptr - d->end;
+  if (UPB_LIKELY(*ptr < d->limit_ptr)) {
+    return false;
+  } else if (UPB_LIKELY(overrun == d->limit)) {
+    return true;
+  } else {
+    *ptr = decode_isdonefallback(d, *ptr, overrun);
+    return false;
+  }
+}
+
+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;
+}
+
+static const char *decode_tosubmsg(upb_decstate *d, const char *ptr,
+                                   upb_msg *submsg, const upb_msglayout *layout,
+                                   const upb_msglayout_field *field, int size) {
   const upb_msglayout *subl = layout->submsgs[field->submsg_index];
-  const char *saved_limit = d->limit;
+  int saved_delta = decode_pushlimit(d, ptr, size);
   if (--d->depth < 0) decode_err(d);
-  d->limit = val.data + val.size;
-  decode_msg(d, val.data, submsg, subl);
-  d->limit = saved_limit;
+  ptr = decode_msg(d, ptr, submsg, subl);
+  decode_poplimit(d, saved_delta);
   if (d->end_group != 0) decode_err(d);
   d->depth++;
+  return ptr;
 }
 
 static const char *decode_group(upb_decstate *d, const char *ptr,
@@ -345,15 +421,14 @@ static const char *decode_toarray(upb_decstate *d, const char *ptr,
       memcpy(mem, &val, 1 << op);
       return ptr;
     case OP_STRING:
-      decode_verifyutf8(d, val.str_val.data, val.str_val.size);
+      decode_verifyutf8(d, ptr, val.size);
       /* Fallthrough. */
-    case OP_BYTES:
+    case OP_BYTES: {
       /* Append bytes. */
-      mem =
-          UPB_PTR_AT(_upb_array_ptr(arr), arr->len * sizeof(upb_strview), void);
+      upb_strview *str = (upb_strview*)_upb_array_ptr(arr) + arr->len;
       arr->len++;
-      memcpy(mem, &val, sizeof(upb_strview));
-      return ptr;
+      return decode_readstr(d, ptr, val.size, str);
+    }
     case OP_SUBMSG: {
       /* Append submessage / group. */
       upb_msg *submsg = decode_newsubmsg(d, layout, field);
@@ -361,26 +436,25 @@ static const char *decode_toarray(upb_decstate *d, const char *ptr,
           submsg;
       arr->len++;
       if (UPB_UNLIKELY(field->descriptortype == UPB_DTYPE_GROUP)) {
-        ptr = decode_togroup(d, ptr, submsg, layout, field);
+        return decode_togroup(d, ptr, submsg, layout, field);
       } else {
-        decode_tosubmsg(d, submsg, layout, field, val.str_val);
+        return decode_tosubmsg(d, ptr, submsg, layout, field, val.size);
       }
-      return ptr;
     }
     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.str_val.size >> lg2;
-      if ((val.str_val.size & mask) != 0) {
+      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, val.str_val.data, val.str_val.size);
-      return ptr;
+      memcpy(mem, ptr, val.size);  /* XXX: ptr boundary. */
+      return ptr + val.size;
     }
     case OP_VARPCK_LG2(0):
     case OP_VARPCK_LG2(2):
@@ -388,12 +462,11 @@ static const char *decode_toarray(upb_decstate *d, const char *ptr,
       /* Varint packed. */
       int lg2 = op - OP_VARPCK_LG2(0);
       int scale = 1 << lg2;
-      const char *ptr = val.str_val.data;
-      const char *end = ptr + val.str_val.size;
+      int saved_limit = decode_pushlimit(d, ptr, val.size);
       char *out = UPB_PTR_AT(_upb_array_ptr(arr), arr->len << lg2, void);
-      while (ptr < end) {
+      while (!decode_isdone(d, &ptr)) {
         wireval elem;
-        ptr = decode_varint64(d, ptr, end, &elem.uint64_val);
+        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);
@@ -402,7 +475,7 @@ static const char *decode_toarray(upb_decstate *d, const char *ptr,
         memcpy(out, &elem, scale);
         out += scale;
       }
-      if (ptr != end) decode_err(d);
+      decode_poplimit(d, saved_limit);
       return ptr;
     }
     default:
@@ -410,9 +483,9 @@ static const char *decode_toarray(upb_decstate *d, const char *ptr,
   }
 }
 
-static void decode_tomap(upb_decstate *d, upb_msg *msg,
-                         const upb_msglayout *layout,
-                         const upb_msglayout_field *field, wireval val) {
+static const char *decode_tomap(upb_decstate *d, const char *ptr, upb_msg *msg,
+                                const upb_msglayout *layout,
+                                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;
@@ -440,10 +513,9 @@ static void decode_tomap(upb_decstate *d, upb_msg *msg,
     ent.v.val = upb_value_ptr(_upb_msg_new(entry->submsgs[0], &d->arena));
   }
 
-  decode_tosubmsg(d, &ent.k, layout, field, val.str_val);
-
-  /* Insert into map. */
+  ptr = decode_tosubmsg(d, ptr, &ent.k, layout, 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,
@@ -477,16 +549,15 @@ static const char *decode_tomsg(upb_decstate *d, const char *ptr, upb_msg *msg,
       if (UPB_UNLIKELY(type == UPB_DTYPE_GROUP)) {
         ptr = decode_togroup(d, ptr, submsg, layout, field);
       } else {
-        decode_tosubmsg(d, submsg, layout, field, val.str_val);
+        ptr = decode_tosubmsg(d, ptr, submsg, layout, field, val.size);
       }
       break;
     }
     case OP_STRING:
-      decode_verifyutf8(d, val.str_val.data, val.str_val.size);
+      decode_verifyutf8(d, ptr, val.size);
       /* Fallthrough. */
     case OP_BYTES:
-      memcpy(mem, &val, sizeof(upb_strview));
-      break;
+      return decode_readstr(d, ptr, val.size, mem);
     case OP_SCALAR_LG2(3):
       memcpy(mem, &val, 8);
       break;
@@ -505,7 +576,7 @@ static const char *decode_tomsg(upb_decstate *d, const char *ptr, upb_msg *msg,
 
 static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
                               const upb_msglayout *layout) {
-  while (ptr < d->limit) {
+  while (!decode_isdone(d, &ptr)) {
     uint32_t tag;
     const upb_msglayout_field *field;
     int field_number;
@@ -514,7 +585,7 @@ static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
     wireval val;
     int op;
 
-    ptr = decode_varint32(d, ptr, d->limit, &tag);
+    ptr = decode_varint32(d, ptr, &tag);
     field_number = tag >> 3;
     wire_type = tag & 7;
 
@@ -522,12 +593,11 @@ static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
 
     switch (wire_type) {
       case UPB_WIRE_TYPE_VARINT:
-        ptr = decode_varint64(d, ptr, d->limit, &val.uint64_val);
+        ptr = decode_varint64(d, ptr, &val.uint64_val);
         op = varint_ops[field->descriptortype];
         decode_munge(field->descriptortype, &val);
         break;
       case UPB_WIRE_TYPE_32BIT:
-        if (d->limit - ptr < 4) decode_err(d);
         memcpy(&val.uint32_val, ptr, 4);
         val.uint32_val = _upb_be_swap32(val.uint32_val);
         ptr += 4;
@@ -535,7 +605,6 @@ static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
         if (((1 << field->descriptortype) & fixed32_ok) == 0) goto unknown;
         break;
       case UPB_WIRE_TYPE_64BIT:
-        if (d->limit - ptr < 8) decode_err(d);
         memcpy(&val.uint64_val, ptr, 8);
         val.uint64_val = _upb_be_swap64(val.uint64_val);
         ptr += 8;
@@ -543,16 +612,12 @@ static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
         if (((1 << field->descriptortype) & fixed64_ok) == 0) goto unknown;
         break;
       case UPB_WIRE_TYPE_DELIMITED: {
-        uint32_t size;
         int ndx = field->descriptortype;
         if (_upb_isrepeated(field)) ndx += 18;
-        ptr = decode_varint32(d, ptr, d->limit, &size);
-        if (size >= INT32_MAX || (size_t)(d->limit - ptr) < size) {
+        ptr = decode_varint32(d, ptr, &val.size);
+        if (val.size >= INT32_MAX || ptr - d->end + val.size > d->limit) {
           decode_err(d); /* Length overflow. */
         }
-        val.str_val.data = ptr;
-        val.str_val.size = size;
-        ptr += size;
         op = delim_ops[ndx];
         break;
       }
@@ -576,7 +641,7 @@ static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
           ptr = decode_toarray(d, ptr, msg, layout, field, val, op);
           break;
         case _UPB_LABEL_MAP:
-          decode_tomap(d, msg, layout, field, val);
+          ptr = decode_tomap(d, ptr, msg, layout, field, val);
           break;
         default:
           ptr = decode_tomsg(d, ptr, msg, layout, field, val, op);
@@ -590,6 +655,7 @@ static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
         ptr = decode_group(d, ptr, NULL, NULL, field_number);
       }
       if (msg) {
+        if (wire_type == UPB_WIRE_TYPE_DELIMITED) ptr += val.size;
         if (!_upb_msg_addunknown(msg, field_start, ptr - field_start,
                                  &d->arena)) {
           decode_err(d);
@@ -598,7 +664,6 @@ static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
     }
   }
 
-  if (ptr != d->limit) decode_err(d);
   return ptr;
 }
 
@@ -607,9 +672,22 @@ bool upb_decode(const char *buf, size_t size, void *msg, const upb_msglayout *l,
   bool ok;
   upb_decstate state;
 
-  if (size == 0) return true;
+  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 = true;
+  }
 
-  state.limit = buf + size;
+  state.limit_ptr = state.end;
   state.depth = 64;
   state.end_group = 0;
   state.arena.head = arena->head;