json_view: check_utf8 skips well-formed runs with the parser's string scan

Strings of 32 bytes or more that are written into an editable document
are validated with scan_string_run(), which skips well-formed UTF-8 16
bytes at a time; the byte-wise check runs only from where the scan found
ill-formed UTF-8, to report the same type_error.316. Shorter strings
keep the byte-wise check (inlined with a constant start, so that this
path is the code it was), where the vector scan costs more than it
saves.

set()/push_back() of strings of 32 bytes or more take 0.29-0.76 of the
time (x86-64 GCC, averaged over code layouts) and 0.25-0.80 (AArch64
clang); shorter strings take 1.5-4 % fewer instructions; the json_view
rows of bench_edit execute at most 0.04 % more instructions.

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

single_include/nlohmann/json_view.hpp

@@ -3727,6 +3727,8 @@ NLOHMANN_JSON_NAMESPACE_END
 
 // #include <nlohmann/detail/view/node.hpp>
 
+// #include <nlohmann/detail/view/scan.hpp>
+
 // #include <nlohmann/detail/view/view_mode.hpp>
 //     __ _____ _____ _____
 //  __|  |   __|     |   | |  JSON for Modern C++
@@ -3775,10 +3777,9 @@ struct first_true<false, Conditions...> : std::integral_constant < int, 1 + firs
 /// (type_error.316 with the same message), so that an editable document
 /// only holds valid UTF-8: the error is at the first byte that no
 /// well-formed sequence can continue with (Unicode, Table 3-7).
-inline void check_utf8(const char* s, std::size_t n)
+NLOHMANN_VIEW_ALWAYS_INLINE void check_utf8_bytewise(const unsigned char* p, std::size_t n, std::size_t i)
 {
-    const auto* const p = reinterpret_cast<const unsigned char*>(s); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
-    for (std::size_t i = 0; i < n;)
+    for (; i < n;)
     {
         const unsigned char c = p[i];
         if (c < 0x80)
@@ -3808,6 +3809,43 @@ inline void check_utf8(const char* s, std::size_t n)
     }
 }
 
+/// The long-string path of check_utf8(): skips well-formed runs with the
+/// parser's string scan (which stops at a quote, a backslash, or a control
+/// character, all legal here, and at ill-formed UTF-8); the byte-wise check
+/// runs only from where the scan found ill-formed UTF-8, to report it.
+NLOHMANN_VIEW_NOINLINE inline void check_utf8_scan(const unsigned char* p, std::size_t n)
+{
+    const unsigned char* const e = p + n;
+    const unsigned char* q = p;
+    for (;;)
+    {
+        q = scan_string_run<true>(q, e);
+        if (q == e)
+        {
+            return;
+        }
+        if (*q >= 0x80)
+        {
+            check_utf8_bytewise(p, n, static_cast<std::size_t>(q - p));
+            return; // not reached: the scan stopped at ill-formed UTF-8
+        }
+        ++q; // a quote, backslash, or control character
+    }
+}
+
+inline void check_utf8(const char* s, std::size_t n)
+{
+    const auto* const p = reinterpret_cast<const unsigned char*>(s); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+    if (NLOHMANN_VIEW_UNLIKELY(n >= 32))
+    {
+        check_utf8_scan(p, n);
+        return;
+    }
+    // short strings: the vector scan costs more than it saves; (the loop is
+    // inlined with a constant start, so this path is the plain byte-wise check)
+    check_utf8_bytewise(p, n, 0);
+}
+
 /*!
 @brief the edits of an editable basic_json_document