list_to_atom与list_to_existing_atom区别

之前在程序中看到这样一段代码

list_to_atom(List) when is_list(List) ->    case catch(erlang:list_to_existing_atom(List)) of        {'EXIT', _} -> erlang:list_to_atom(List);        Atom when is_atom(Atom) -> Atom    end.

一直没有去深究其中的问题，知道有个同事提到这个函数存在问题，里面的list_to_existing_atom是多余的，才关注到这点。

直接看源码
list_to_existing_atom

BIF_RETTYPE list_to_existing_atom_1(BIF_ALIST_1){    int i;    char *buf = (char *) erts_alloc(ERTS_ALC_T_TMP, MAX_ATOM_CHARACTERS);    if ((i = intlist_to_buf(BIF_ARG_1, buf, MAX_ATOM_CHARACTERS)) < 0) {    error:    erts_free(ERTS_ALC_T_TMP, (void *) buf);    BIF_ERROR(BIF_P, BADARG);    } else {    Eterm a;    if (erts_atom_get(buf, i, &a, ERTS_ATOM_ENC_LATIN1)) {        erts_free(ERTS_ALC_T_TMP, (void *) buf);        BIF_RET(a);    } else {        goto error;    }    }}

再看看list_to_atom的实现

BIF_RETTYPE list_to_atom_1(BIF_ALIST_1){    Eterm res;    char *buf = (char *) erts_alloc(ERTS_ALC_T_TMP, MAX_ATOM_CHARACTERS);    int i = intlist_to_buf(BIF_ARG_1, buf, MAX_ATOM_CHARACTERS);    if (i < 0) {    erts_free(ERTS_ALC_T_TMP, (void *) buf);    i = erts_list_length(BIF_ARG_1);    if (i > MAX_ATOM_CHARACTERS) {        BIF_ERROR(BIF_P, SYSTEM_LIMIT);    }    BIF_ERROR(BIF_P, BADARG);    }    res = erts_atom_put((byte *) buf, i, ERTS_ATOM_ENC_LATIN1, 1);    ASSERT(is_atom(res));    erts_free(ERTS_ALC_T_TMP, (void *) buf);    BIF_RET(res);}

几乎是一样的，差别在于list_to_atom多了参数长度的判断，并且list_to_existing_atom使用了erts_atom_get，而list_to_atom使用了erts_atom_put
继续看这两个函数的实现

interts_atom_get(const char *name, int len, Eterm* ap, ErtsAtomEncoding enc){    byte utf8_copy[MAX_ATOM_SZ_FROM_LATIN1];    Atom a;    int i;    int res;    a.len = (Sint16) len;    a.name = (byte *)name;    if (enc == ERTS_ATOM_ENC_LATIN1) {    latin1_to_utf8(utf8_copy, (const byte**)&a.name, &len);    a.len = (Sint16) len;    }    atom_read_lock();    i = index_get(&erts_atom_table, (void*) &a);    res = i < 0 ? 0 : (*ap = make_atom(i), 1);    atom_read_unlock();    return res;}Etermerts_atom_put(const byte *name, int len, ErtsAtomEncoding enc, int trunc){    byte utf8_copy[MAX_ATOM_SZ_FROM_LATIN1];    const byte *text = name;    int tlen = len;    Sint no_latin1_chars;    Atom a;    int aix;#ifdef ERTS_ATOM_PUT_OPS_STAT    erts_smp_atomic_inc_nob(&atom_put_ops);#endif    if (tlen < 0) {    if (trunc)        tlen = 0;    else        return THE_NON_VALUE;    }    switch (enc) {    case ERTS_ATOM_ENC_7BIT_ASCII:    if (tlen > MAX_ATOM_CHARACTERS) {        if (trunc)        tlen = MAX_ATOM_CHARACTERS;        else        return THE_NON_VALUE;    }#ifdef DEBUG    for (aix = 0; aix < len; aix++) {        ASSERT((name[aix] & 0x80) == 0);    }#endif    no_latin1_chars = tlen;    break;    case ERTS_ATOM_ENC_LATIN1:    if (tlen > MAX_ATOM_CHARACTERS) {        if (trunc)        tlen = MAX_ATOM_CHARACTERS;        else        return THE_NON_VALUE;    }    no_latin1_chars = tlen;    latin1_to_utf8(utf8_copy, &text, &tlen);    break;    case ERTS_ATOM_ENC_UTF8:    /* First sanity check; need to verify later */    if (tlen > MAX_ATOM_SZ_LIMIT && !trunc)        return THE_NON_VALUE;    break;    }    a.len = tlen;    a.name = (byte *) text;    atom_read_lock();    aix = index_get(&erts_atom_table, (void*) &a);    atom_read_unlock();    if (aix >= 0) {    /* Already in table no need to verify it */    return make_atom(aix);    }    if (enc == ERTS_ATOM_ENC_UTF8) {    /* Need to verify encoding and length */    byte *err_pos;    Uint no_chars;    switch (erts_analyze_utf8_x((byte *) text,                    (Uint) tlen,                    &err_pos,                    &no_chars, NULL,                    &no_latin1_chars,                    MAX_ATOM_CHARACTERS)) {    case ERTS_UTF8_OK:        ASSERT(no_chars <= MAX_ATOM_CHARACTERS);        break;    case ERTS_UTF8_OK_MAX_CHARS:        /* Truncated... */        if (!trunc)        return THE_NON_VALUE;        ASSERT(no_chars == MAX_ATOM_CHARACTERS);        tlen = err_pos - text;        break;    default:        /* Bad utf8... */        return THE_NON_VALUE;    }    }    ASSERT(tlen <= MAX_ATOM_SZ_LIMIT);    ASSERT(-1 <= no_latin1_chars && no_latin1_chars <= MAX_ATOM_CHARACTERS);    a.len = tlen;    a.latin1_chars = (Sint16) no_latin1_chars;    a.name = (byte *) text;    atom_write_lock();    aix = index_put(&erts_atom_table, (void*) &a);    atom_write_unlock();    return make_atom(aix);}

两个函数都会先进行index_get查找atom表里已经存在的数据，区别仅仅在于list_to_existing_atom不会创建新的atom，和文档中的描述一致。
那么为什么为什么开头的函数要这样写？
俗话说实践是检验整理的唯一标准，所以进行测试
创建1000000个atom，通过list_to_existing_atom和list_to_atom分别进行已有atom的创建，消耗时间几乎没有差别。

所以，此处的list_to_existing_atom没有作用，还多做了一次模式和is_atom判断，没有必要。