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Date: Thu, 30 Jan 2014 02:38:35 +0000
From: Samuel Neves <sneves@....uc.pt>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Combining sliding window and bit-reversal

On 30-01-2014 01:32, Solar Designer wrote:
> I just recalled that AMD XOP's VPPERM, which is an extension of PSHUFB,
> can be directly used to perform bit reversal.  Like PSHUFB, it operates
> on bytes rather than bits, but it has more special modes, including bit
> reversal (within the bytes, which can also be reversed by the same
> instruction).  (It can even do bit reversal and inversion at once, but
> we probably don't need that.)

Excellent point, I had forgotten about this! The only downside here is
that moving between general-purpose register and vector registers is
more expensive on Bulldozer than it is on Intel.

>
>>> it may be an acceptable solution compared to an ad hoc permutation.
>> I agree.
> I've attached a program I wrote, testing bit reversal with VPPERM and
> PSHUFB.  It seems to work fine.  Can the SSSE3 case be optimized further?
>
> $ gcc bitrev.c -o bitrev -s -O2 -mxop; size bitrev; ./bitrev
>    text    data     bss     dec     hex filename
>    1299     504      16    1819     71b bitrev
> a5a55a5a 2480137f 1e6a2c48 ff33ccaa
> $ gcc bitrev.c -o bitrev -s -O2 -mavx; size bitrev; ./bitrev
>    text    data     bss     dec     hex filename
>    1427     504      16    1947     79b bitrev
> a5a55a5a 2480137f 1e6a2c48 ff33ccaa
> $ gcc bitrev.c -o bitrev -s -O2 -mssse3; size bitrev; ./bitrev
>    text    data     bss     dec     hex filename
>    1443     504      16    1963     7ab bitrev
> a5a55a5a 2480137f 1e6a2c48 ff33ccaa
>
> BTW, reusing the value of "sel" that I define for VPPERM also for PSHUFB
> works fine - those extra bits are simply ignored, and the instruction
> appears to be defined to ignore them (so it's probably behavior we can
> rely upon) - but I don't do that in this program just in case (and to
> avoid possibly including a comment on that).  The only savings would be
> in source code, not in compiled code (nor data).

By definition only the most significant bit of each byte is looked at in
PSHUFB, so it's OK to share the same selection mask. I've additionally
used PSHUFB to compute the bit reversal of 4-bit values, which should be
somewhat faster. Here it is:

__m128i bitrev(__m128i x)
{
    const __m128i   m = _mm_set1_epi8(0x40); // Bit reversal mask for VPPERM
    const __m128i sel = _mm_or_si128(m,
                                            _mm_set_epi8(0x0f, 0x0e,
0x0d, 0x0c, 0x0b, 0x0a, 0x09, 0x08,
                                                         0x07, 0x06,
0x05, 0x04, 0x03, 0x02, 0x01, 0x00));
#if   defined(__XOP__)
    return _mm_perm_epi8(x, x, sel);
#elif defined(__SSSE3__)
    const __m128i rev0 = _mm_set_epi8(0x0F, 0x07, 0x0B, 0x03, 0x0D,
0x05, 0x09, 0x01,
                                      0x0E, 0x06, 0x0A, 0x02, 0x0C,
0x04, 0x08, 0x00);
    const __m128i rev1 = _mm_slli_epi32(rev0, 4); // Same table, shifted
4 bits
    const __m128i c0f = _mm_set1_epi8(0x0f);
    __m128i t1, t2, t3;

    t1 = _mm_shuffle_epi8(x, sel); // reverse bytes

    t2 = _mm_and_si128(t1, c0f);   // x & 0xf
    t3 = _mm_srli_epi32(_mm_andnot_si128(c0f, t1), 4); // x >> 4
   
    t2 = _mm_shuffle_epi8(rev1, t2); // rev(x)
    t3 = _mm_shuffle_epi8(rev0, t3); // rev(x)
   
    return _mm_or_si128(t2, t3); // combine 4-bit halves
#else
    #error At least SSSE3 is required
#endif
}


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