perf(portable_binary): skip byte-swap for DataSize==1

[type-name-T]

Problem

When serializing a std::vector of a single-byte arithmetic type (e.g. std::vector<std::uint8_t>, std::vector<char>, std::vector<unsigned char>) through PortableBinaryArchive with endianness conversion enabled, the data still enters the byte-swapping slow path even though swapping a single byte is a no-op.

In vector.hpp, arithmetic vectors use the binary_data fast path:

ar( binary_data( vector.data(), vector.size() * sizeof(T) ) );

This dispatches to saveBinary<sizeof(T)> / loadBinary<sizeof(T)>. For sizeof(T) == 1, DataSize is 1, so:

• swap_bytes<1> iterates 0 times — a no-op the compiler can eliminate
• but the surrounding control flow still pays the cost:
  • saveBinary: O(n) single-byte sputn(..., 1) calls instead of one bulk sputn
  • loadBinary: O(n) empty swap-loop iterations

Fix

Gate the byte-swap path behind DataSize > 1 in both functions:

// saveBinary (include/cereal/archives/portable_binary.hpp)
if( itsConvertEndianness && DataSize > 1 )   // was: if( itsConvertEndianness )
{ /* per-byte reversed write */ }
else
  writtenSize = itsStream.rdbuf()->sputn( ... , size );   // bulk write

// loadBinary
if( itsConvertEndianness && DataSize > 1 )   // was: if( itsConvertEndianness )
{ /* swap loop */ }

Why this is safe

• Correctness: Swapping a single byte is a no-op (swap_bytes<1> iterates 0 times), so skipping it produces byte-identical output for DataSize == 1.
• Zero added runtime cost: DataSize is a non-type template parameter, so DataSize > 1 is a compile-time constant. Compilers eliminate the dead branch, leaving the DataSize > 1 path identical to before.
• No behavior change for multi-byte types (uint16/32/64, float, double, etc.).

Performance impact

For std::vector<std::uint8_t> (or any 1-byte vector) serialized with endianness conversion enabled:

• Save: collapses O(n) single-byte sputn calls into one bulk sputn of the whole buffer.
• Load: removes O(n) empty swap-loop iterations.

Files changed

include/cereal/archives/portable_binary.hpp — 2 lines modified (both if conditions).

[redchairman]

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