Copy the text between escapes while scanning it

A string that contains an escape is decoded by builder::slow_string(). After
each escape it scanned the next run of plain bytes (8 or 16 byte-by-byte
steps, then a vector loop), and then copied that run to the decode arena in a
second pass. The run is now copied 16 bytes at a time while it is scanned
(one vector load, a store of the same 16 bytes, and the mask of the bytes
that end the run), and the write position lives in a local variable, not in
the builder, since a store of a character may alias the builder's members and
made the compiler reload it after every escape. A run that ends at a quote
needs no scan, and non-ASCII text goes through the UTF-8 check as before;
the portable code is unchanged.

Parsing, Apple M1 instructions: jeopardy.json 304.0 M -> 293.8 M (-3.4 %), 3000 strings with
escapes 1.72 M -> 1.54 M (-10 %), twitter -0.6 %.
Linux x86-64 Ir, GCC 13 / Clang 18: escaped strings -7.6 % / -11.4 %, twitter 0.0 % / 0.0 %.
Documents and error messages are identical on 900 k random strings of escapes, UTF-8 and control
characters (also under ASan), and on mutants of the test corpus.

Belongs to the PR "json_view: read-side speedups".

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/simd.hpp

@@ -116,6 +116,34 @@ NLOHMANN_VIEW_ALWAYS_INLINE const unsigned char* vector_plain_run(const unsigned
     }
     return p;
 }
+
+/*!
+@brief copy 16 bytes from p to w, and find the first quote, backslash,
+control character, or non-ASCII byte among them
+
+The bytes are copied whether or not they are all plain (the caller has room
+for 16 bytes and keeps only the plain ones).
+
+@return the offset of that byte, or 16 if there is none
+*/
+NLOHMANN_VIEW_ALWAYS_INLINE unsigned copy_plain_run16(const unsigned char* p, char* w) noexcept
+{
+#if NLOHMANN_VIEW_NEON
+    const uint8x16_t in = vld1q_u8(p);
+    vst1q_u8(reinterpret_cast<std::uint8_t*>(w), in); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+    const uint8x16_t special = vorrq_u8(vorrq_u8(vceqq_u8(in, vdupq_n_u8('"')), vceqq_u8(in, vdupq_n_u8('\\'))),
+                                        vcltq_s8(vreinterpretq_s8_u8(in), vdupq_n_s8(0x20)));
+    const std::uint64_t bits = vget_lane_u64(vreinterpret_u64_u8(vshrn_n_u16(vreinterpretq_u16_u8(special), 4)), 0);
+    return bits != 0 ? static_cast<unsigned>(count_trailing_zeros(bits) >> 2u) : 16u;
+#else
+    const __m128i in = _mm_loadu_si128(static_cast<const __m128i*>(static_cast<const void*>(p))); // NOLINT(bugprone-casting-through-void)
+    _mm_storeu_si128(static_cast<__m128i*>(static_cast<void*>(w)), in); // NOLINT(bugprone-casting-through-void)
+    const __m128i special = _mm_or_si128(_mm_or_si128(_mm_cmpeq_epi8(in, _mm_set1_epi8('"')), _mm_cmpeq_epi8(in, _mm_set1_epi8('\\'))),
+                                         _mm_cmplt_epi8(in, _mm_set1_epi8(0x20)));
+    const auto bits = static_cast<std::uint64_t>(static_cast<unsigned>(_mm_movemask_epi8(special)));
+    return bits != 0 ? static_cast<unsigned>(count_trailing_zeros(bits)) : 16u;
+#endif
+}
 #endif
 
 #if NLOHMANN_VIEW_SSSE3_DISPATCH