different implementation of parse_unsigned

Author: grisumbras

include/boost/json/detail/sse2.hpp

@@ -315,34 +315,7 @@ inline int count_digits( char const* p ) noexcept
 
 inline uint64_t parse_unsigned( uint64_t r, char const * p, std::size_t n ) noexcept
 {
-    while( n >= 4 )
-    {
-        // faster on on clang for x86,
-        // slower on gcc
-#ifdef __clang__
-        r = r * 10 + p[0] - '0';
-        r = r * 10 + p[1] - '0';
-        r = r * 10 + p[2] - '0';
-        r = r * 10 + p[3] - '0';
-#else
-        uint32_t v;
-        std::memcpy( &v, p, 4 );
-        endian::native_to_little_inplace(v);
-
-        v -= 0x30303030;
-
-        unsigned w0 = v & 0xFF;
-        unsigned w1 = (v >> 8) & 0xFF;
-        unsigned w2 = (v >> 16) & 0xFF;
-        unsigned w3 = (v >> 24);
-
-        r = (((r * 10 + w0) * 10 + w1) * 10 + w2) * 10 + w3;
-#endif
-        p += 4;
-        n -= 4;
-    }
-
-    switch( n )
+    switch( n & 3 )
     {
     case 0:
         break;
@@ -359,6 +332,23 @@ inline uint64_t parse_unsigned( uint64_t r, char const * p, std::size_t n ) noex
         r = r * 10 + p[2] - '0';
         break;
     }
+
+    auto const e = p + n;
+    p += n & 3;
+    while( p != e )
+    {
+        uint32_t v;
+        std::memcpy( &v, p, 4 );
+        endian::native_to_little_inplace(v);
+
+        v = (v & 0x0F0F0F0F) * 2561 >> 8;
+        v = (v & 0x00FF00FF) * 6553601 >> 16;
+
+        r = r * 10000 + v;
+
+        p += 4;
+    }
+
     return r;
 }