Function SkipWhitespace_SIMD

rapidjson/reader.h:281–313

Skip whitespace with SSE 4.2 pcmpistrm instruction, testing 16 8-byte characters at once.

#ifdef RAPIDJSON_SSE42
//! Skip whitespace with SSE 4.2 pcmpistrm instruction, testing 16 8-byte characters at once.
inline const char *SkipWhitespace_SIMD(const char* p) {
    // Fast return for single non-whitespace
    if (*p == ' ' || *p == '\n' || *p == '\r' || *p == '\t')
        ++p;
    else
        return p;

    // 16-byte align to the next boundary
    const char* nextAligned = reinterpret_cast<const char*>((reinterpret_cast<size_t>(p) + 15) & static_cast<size_t>(~15));
    while (p != nextAligned)
        if (*p == ' ' || *p == '\n' || *p == '\r' || *p == '\t')
            ++p;
        else
            return p;

    // The rest of string using SIMD
    static const char whitespace[16] = " \n\r\t";
    const __m128i w = _mm_loadu_si128(reinterpret_cast<const __m128i *>(&whitespace[0]));

    for (;; p += 16) {
        const __m128i s = _mm_load_si128(reinterpret_cast<const __m128i *>(p));
        const int r = _mm_cvtsi128_si32(_mm_cmpistrm(w, s, _SIDD_UBYTE_OPS | _SIDD_CMP_EQUAL_ANY | _SIDD_BIT_MASK | _SIDD_NEGATIVE_POLARITY));
        if (r != 0) {   // some of characters is non-whitespace
#ifdef _MSC_VER         // Find the index of first non-whitespace
            unsigned long offset;
            _BitScanForward(&offset, r);
            return p + offset;
#else
            return p + __builtin_ffs(r) - 1;
#endif
        }
    }
}

inline const char *SkipWhitespace_SIMD(const char* p, const char* end) {
    // Fast return for single non-whitespace