arch/x86/adler32_avx2_tpl.h

/* adler32_avx2_tpl.h -- adler32 avx2 vectorized function templates
 * Copyright (C) 2022 Adam Stylinski
 * For conditions of distribution and use, see copyright notice in zlib.h
 */

#include "../../zbuild.h"
#include <immintrin.h>
#include "../../adler32_fold.h"
#include "../../adler32_p.h"
#include "../../fallback_builtins.h"
#include "adler32_avx2_p.h"

#ifdef X86_SSE42_ADLER32
extern uint32_t adler32_fold_copy_sse42(uint32_t adler, uint8_t *dst, const uint8_t *src, uint64_t len);
extern uint32_t adler32_ssse3(uint32_t adler, const uint8_t *src, uint64_t len);

#define copy_sub32(a, b, c, d) adler32_fold_copy_sse42(a, b, c, d)
#define sub32(a, b, c) adler32_ssse3(a, b, c)
#else
#define copy_sub32(a, b, c, d) adler32_copy_len_16(adler0, c, b, d, adler1)
#define sub32(a, b, c) adler32_len_16(adler0, b, c, adler1)
#endif

#ifdef COPY
Z_INTERNAL uint32_t adler32_fold_copy_avx2(uint32_t adler, uint8_t *dst, const uint8_t *src, uint64_t len) {
#else
Z_INTERNAL uint32_t adler32_avx2(uint32_t adler, const uint8_t *src, uint64_t len) {
#endif
    if (src == NULL) return 1L;
    if (len == 0) return adler;

    uint32_t adler0, adler1;
    adler1 = (adler >> 16) & 0xffff;
    adler0 = adler & 0xffff;

rem_peel:
    if (len < 16) {
#ifdef COPY
        return adler32_copy_len_16(adler0, src, dst, len, adler1);
#else
        return adler32_len_16(adler0, src, len, adler1);
#endif
    } else if (len < 32) {
#ifdef COPY
        return copy_sub32(adler, dst, src, len);
#else
        return sub32(adler, src, len);
#endif
    }

    __m256i vs1, vs2;

    const __m256i dot2v = _mm256_setr_epi8(32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15,
                                           14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1);
    const __m256i dot3v = _mm256_set1_epi16(1);
    const __m256i zero = _mm256_setzero_si256();

    while (len >= 32) {
       vs1 = _mm256_zextsi128_si256(_mm_cvtsi32_si128(adler0));
       vs2 = _mm256_zextsi128_si256(_mm_cvtsi32_si128(adler1));
       __m256i vs1_0 = vs1;
       __m256i vs3 = _mm256_setzero_si256();

       uint64_t k = MIN(len, NMAX);
       k -= k % 32;
       len -= k;

       while (k >= 32) {
           /*
              vs1 = adler + sum(c[i])
              vs2 = sum2 + 32 vs1 + sum( (32-i+1) c[i] )
           */
           __m256i vbuf = _mm256_loadu_si256((__m256i*)src);
           src += 32;
           k -= 32;

           __m256i vs1_sad = _mm256_sad_epu8(vbuf, zero); // Sum of abs diff, resulting in 2 x int32's
                                                          //
#ifdef COPY
            _mm256_storeu_si256((__m256i*)dst, vbuf);
            dst += 32;
#endif
           vs1 = _mm256_add_epi32(vs1, vs1_sad);
           vs3 = _mm256_add_epi32(vs3, vs1_0);
           __m256i v_short_sum2 = _mm256_maddubs_epi16(vbuf, dot2v); // sum 32 uint8s to 16 shorts
           __m256i vsum2 = _mm256_madd_epi16(v_short_sum2, dot3v); // sum 16 shorts to 8 uint32s
           vs2 = _mm256_add_epi32(vsum2, vs2);
           vs1_0 = vs1;
       }

       /* Defer the multiplication with 32 to outside of the loop */
       vs3 = _mm256_slli_epi32(vs3, 5);
       vs2 = _mm256_add_epi32(vs2, vs3);

       /* The compiler is generating the following sequence for this integer modulus
        * when done the scalar way, in GPRs:

        adler = (s1_unpack[0] % BASE) + (s1_unpack[1] % BASE) + (s1_unpack[2] % BASE) + (s1_unpack[3] % BASE) +
                (s1_unpack[4] % BASE) + (s1_unpack[5] % BASE) + (s1_unpack[6] % BASE) + (s1_unpack[7] % BASE);

        mov    $0x80078071,%edi // move magic constant into 32 bit register %edi
        ...
        vmovd  %xmm1,%esi // move vector lane 0 to 32 bit register %esi
        mov    %rsi,%rax  // zero-extend this value to 64 bit precision in %rax
        imul   %rdi,%rsi // do a signed multiplication with magic constant and vector element
        shr    $0x2f,%rsi // shift right by 47
        imul   $0xfff1,%esi,%esi // do a signed multiplication with value truncated to 32 bits with 0xfff1
        sub    %esi,%eax // subtract lower 32 bits of original vector value from modified one above
        ...
        // repeats for each element with vpextract instructions

        This is tricky with AVX2 for a number of reasons:
            1.) There's no 64 bit multiplication instruction, but there is a sequence to get there
            2.) There's ways to extend vectors to 64 bit precision, but no simple way to truncate
                back down to 32 bit precision later (there is in AVX512)
            3.) Full width integer multiplications aren't cheap

        We can, however, and do a relatively cheap sequence for horizontal sums.
        Then, we simply do the integer modulus on the resulting 64 bit GPR, on a scalar value. It was
        previously thought that casting to 64 bit precision was needed prior to the horizontal sum, but
        that is simply not the case, as NMAX is defined as the maximum number of scalar sums that can be
        performed on the maximum possible inputs before overflow
        */


        /* In AVX2-land, this trip through GPRs will probably be unvoidable, as there's no cheap and easy
         * conversion from 64 bit integer to 32 bit (needed for the inexpensive modulus with a constant).
         * This casting to 32 bit is cheap through GPRs (just register aliasing). See above for exactly
         * what the compiler is doing to avoid integer divisions. */
        adler0 = partial_hsum256(vs1) % BASE;
        adler1 = hsum256(vs2) % BASE;
    }

    adler = adler0 | (adler1 << 16);

    if (len) {
        goto rem_peel;
    }

    return adler;
}