Scan strings with SSE2, SSSE3, or NEON

The string fast path found the end of a run of plain characters 8 bytes at
a time (SWAR) and validated non-ASCII text one sequence at a time, calling
validate_one_utf8() out of line for each.

The vector code of json_view's scanner, in the new shared header
detail/input/simd_scan.hpp, now does both 16 bytes at a time: SSE2 finds
the end of a run on x86-64, and the UTF-8 check of Keiser and Lemire runs
with NEON on AArch64 and with SSSE3 on x86-64. SSSE3 is not part of x86-64,
so the check is compiled for it with a function attribute and used when the
CPU has it (cached in a constant-initialized atomic), else the scalar check;
code compiled with -mssse3 skips the CPU check. The first word of a run is
still checked with SWAR (the first four with NEON, where a vector mask
costs more), as most keys and short values end there, and the scalar
helpers are always inlined. On anything else, the scalar code is unchanged.

The new macro JSON_NO_SIMD selects the portable code everywhere, including
the SSE2/NEON digit formatting of the floating-point writer. The lexer tests
now also run with JSON_NO_SIMD and, on x86-64, with -mssse3, and a new test
places every byte that ends a run at each offset across several 16-byte
blocks, after ASCII and after CJK text.

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

include/nlohmann/detail/input/string_scan.hpp

@@ -14,6 +14,7 @@
 #include <cstring> // memcpy
 
 #include <nlohmann/detail/bit_ops.hpp>
+#include <nlohmann/detail/input/simd_scan.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 
 // Optional SIMD backend for bulk UTF-8 validation. This is an opt-in external
@@ -66,7 +67,7 @@ inline std::uint64_t swar_string_special(std::uint64_t v) noexcept
 
 // return the index of the first is_string_special() byte in [data, data+n), or
 // n if every byte is ordinary; scans 8 bytes at a time
-inline std::size_t find_string_special(const unsigned char* data, std::size_t n) noexcept
+JSON_HEDLEY_ALWAYS_INLINE std::size_t find_string_special(const unsigned char* data, std::size_t n) noexcept
 {
     std::size_t i = 0;
     for (; i + 8 <= n; i += 8)
@@ -235,35 +236,136 @@ inline std::size_t valid_utf8_prefix(const unsigned char* data, std::size_t n) n
     return pos;
 }
 
-// Scalar (C++11) computation of the bulk run length: the number of leading
-// bytes in [data, data+n) that are ordinary ASCII or complete well-formed UTF-8
+#if JSON_SCAN_VECTOR_UTF8
+/// the end of scan_string_vector from block, where the vector loop stopped
+/// (ill-formed UTF-8, or fewer than 16 bytes left): one byte or sequence at a
+/// time, from the start of a sequence that crosses into the block
+inline const unsigned char* scan_string_finish(const unsigned char* p, const unsigned char* block, const unsigned char* e) noexcept
+{
+    for (int i = 1; i <= 3 && block - i >= p; ++i)
+    {
+        const unsigned char c = block[-i];
+        if (c < 0x80)
+        {
+            break;
+        }
+        if (c >= 0xC0)
+        {
+            const int len = 2 + static_cast<int>(c >= 0xE0) + static_cast<int>(c >= 0xF0);
+            if (len > i)
+            {
+                block -= i;
+            }
+            break;
+        }
+    }
+    for (p = block; p != e;)
+    {
+        if (*p < 0x80)
+        {
+            if (is_string_special(*p))
+            {
+                return p;
+            }
+            ++p;
+            continue;
+        }
+        const std::size_t n = validate_one_utf8(p, static_cast<std::size_t>(e - p));
+        if (n == 0)
+        {
+            return p;
+        }
+        p += n;
+    }
+    return p;
+}
+
+/// the rest of a string from p (a character boundary), with the vector UTF-8
+/// check and the scalar check for the end of the block where it stopped
+inline const unsigned char* scan_string_vector(const unsigned char* p, const unsigned char* e) noexcept
+{
+    const unsigned char* block = p;
+    if (vector_utf8_run(p, e, block))
+    {
+        return block;
+    }
+    return scan_string_finish(p, block, e);
+}
+#endif
+
+#if JSON_SCAN_VECTOR
+/// the end of the run of plain string characters at @a p: the first quote,
+/// backslash, control character, or non-ASCII byte, or @a e. The first words
+/// are checked with SWAR (one; four with NEON, where the vector mask costs
+/// more), as most keys and short values end there; longer runs continue 16
+/// bytes at a time.
+JSON_HEDLEY_ALWAYS_INLINE const unsigned char* plain_run_end(const unsigned char* p, const unsigned char* e) noexcept
+{
+    constexpr int prefix_words = JSON_SCAN_NEON ? 4 : 1;
+    for (int words = prefix_words; words > 0 && e - p >= 8; --words)
+    {
+        const std::uint64_t special = swar_string_special(read_eight_bytes(p));
+        if (special != 0)
+        {
+            return p + (static_cast<std::size_t>(count_trailing_zeros(special)) / 8);
+        }
+        p += 8;
+    }
+    p = vector_plain_run_long(p, e);
+    if (e - p < 16) // else the vector loop stopped at a special byte
+    {
+        p += find_string_special(p, static_cast<std::size_t>(e - p));
+    }
+    return p;
+}
+#endif
+
+// Computation of the bulk run length: the number of leading bytes in
+// [data, data+n) that are ordinary ASCII or complete well-formed UTF-8
 // sequences, stopping before the first byte that needs individual handling (the
 // closing quote, an escape, a control character, or an ill-formed/truncated
-// sequence). ASCII is skipped 8 bytes at a time.
-inline std::size_t scalar_string_bulk_run(const unsigned char* data, std::size_t n) noexcept
+// sequence). ASCII is skipped 16 bytes at a time with SSE2 and 8 bytes at a
+// time otherwise; runs of non-ASCII text use the vector UTF-8 check where
+// available.
+JSON_HEDLEY_ALWAYS_INLINE std::size_t scalar_string_bulk_run(const unsigned char* data, std::size_t n) noexcept
 {
-    std::size_t pos = 0;
-    while (pos < n)
+    const unsigned char* const e = data + n;
+    const unsigned char* p = data;
+    while (p != e)
     {
-        pos += find_string_special(data + pos, n - pos);
-        if (pos >= n || data[pos] < 0x80u)
+#if JSON_SCAN_VECTOR
+        p = plain_run_end(p, e);
+#else
+        p += find_string_special(p, static_cast<std::size_t>(e - p));
+#endif
+        if (p == e || *p < 0x80u)
         {
             break; // end of buffer, or a quote/escape/control byte
         }
+#if JSON_SCAN_VECTOR_UTF8
+#if JSON_SCAN_SSSE3_DISPATCH
+        if (JSON_HEDLEY_LIKELY(cpu_has_ssse3()))
+#endif
+        {
+            return static_cast<std::size_t>(scan_string_vector(p, e) - data);
+        }
+#endif
+#if !JSON_SCAN_VECTOR_UTF8 || JSON_SCAN_SSSE3_DISPATCH
         // a run of multi-byte sequences (e.g. CJK text) is validated sequence
         // by sequence without searching for the next special byte in between
         do
         {
-            const std::size_t seq = validate_one_utf8(data + pos, n - pos);
+            const std::size_t seq = validate_one_utf8(p, static_cast<std::size_t>(e - p));
             if (seq == 0)
             {
-                return pos; // ill-formed or truncated: let the byte path diagnose it
+                return static_cast<std::size_t>(p - data); // ill-formed or truncated: let the byte path diagnose it
             }
-            pos += seq;
+            p += seq;
         }
-        while (pos < n && data[pos] >= 0x80u);
+        while (p != e && *p >= 0x80u);
+#endif
     }
-    return pos;
+    return static_cast<std::size_t>(p - data);
 }
 
 #if defined(JSON_USE_SIMDUTF) && defined(JSON_HAS_CPP_17)
@@ -304,7 +406,7 @@ inline std::size_t find_string_delimiter(const unsigned char* data, std::size_t
 // next delimiter is validated in one shot by simdutf; on the rare failure the
 // scalar helper recomputes the exact valid prefix so the byte path still
 // produces the precise diagnostic. Without it, the pure scalar path is used.
-inline std::size_t string_bulk_run(const unsigned char* data, std::size_t n) noexcept
+JSON_HEDLEY_ALWAYS_INLINE std::size_t string_bulk_run(const unsigned char* data, std::size_t n) noexcept
 {
 #if defined(JSON_USE_SIMDUTF) && defined(JSON_HAS_CPP_17)
     const std::size_t run = find_string_delimiter(data, n);