Niels LohmannandClaude Opus 4.8 ac6b505923 Encode big-endian numbers with a byte swap instead of std::reverse ...

write_number() reordered multi-byte numbers for the big-endian formats
(CBOR/MessagePack/UBJSON) with std::reverse over the byte array. GCC
lowered only some sizes to a bswap; clang kept a scalar byte shuffle
(0 bswap instructions in the CBOR number path). Replace the reverse with
size-dispatched __builtin_bswap16/32/64 helpers (portable shift fallback
for other compilers; std::reverse retained for exotic sizes such as a
long double number_float_t).

Codegen: the CBOR number path now emits bswap on both compilers
(gcc 2 -> 16, clang 0 -> 4). Output is byte-for-byte identical to the
previous implementation across the binary differential corpus.

Throughput (isolated vs the std::reverse version, best of 9):
  CBOR int64 array   gcc +7%   clang +10%
  CBOR uint16 array  gcc +27%  clang flat

Modest but consistent on number-dense encodings; negligible on
string/blob-heavy output, as expected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XAYM1qhSA2FDaDcGfPW3fG
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-07-22 10:20:33 +00:00

..

nlohmann

Encode big-endian numbers with a byte swap instead of std::reverse

2026-07-22 10:20:33 +00:00