Ror can do byteswaps all right, but it's not very remarkable at that.

#! /usr/bin/perl

use Inline C => Config => CC => 'gcc', OPTIMIZE => '-O3 -mssse3 -funro
+ll-all-loops';
use Inline C => <<'__CUT__', NAME => 'swab';

#include <x86intrin.h>

void swab_ror(SV *v)
{
    STRLEN slen; char *s = SvPV(v, slen);
    uint16_t *w = (uint16_t*) s;
    size_t n = slen >> 1;

    for (; n; n--) {
        asm("rorw $8, %0" : "+r,m" (w[n-1]) : : "cc");
    }
}

void swab_sse(SV *v)
{
    STRLEN slen; char *s = SvPV(v, slen);
    __m128i x, t;
    size_t n = slen & ~(size_t)1;

    for (; (n & 0xe); n -= 2) {
        uint16_t *w = (uint16_t*) &s[n-2];
        *w = __rorw(*w, 8);
    }
    t = _mm_set_epi8(14,15,12,13,10,11,8,9,6,7,4,5,2,3,0,1);
    for (; n; n -= 16) {
        x = _mm_lddqu_si128((__m128i*)&s[n-16]);
        x = _mm_shuffle_epi8(x, t);
        _mm_storeu_si128((__m128i*)&s[n-16], x);
    }
}
__CUT__

our $str = pack "C*", map rand(256), 1..34567;

use Benchmark 'cmpthese';
cmpthese -5, {
    swab_ror => q( swab_ror $str ),
    swab_sse => q( swab_sse $str ),
};
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that is a big performance difference! Thanks for showing that.
Rate swab_ror swab_sse
swab_ror 155077/s -- -82%
swab_sse 885905/s 471% --