The dominant cost in the reflection serializer was the strict-aliasing
reload pattern: every char* write through `string_builder::buffer.get()`
forced the compiler to assume `string_builder::position` and `::capacity`
may have been clobbered, so it reloaded both members from memory after
each byte. perf annotate showed `ldr [position]` and `ldr [capacity]`
taking 15-20% of CITM serialization time.
This change refactors the reflection atom path to use an internal
`writer` struct (buffer pointer + position + capacity + back-ref to
string_builder). The writer lives as a stack-local in the top-level
append() entry point, threaded through the inlined atom() chain via
`writer&`. After SROA, the three fields are register-resident across
the entire serialization. The `pos` is never round-tripped through
memory between writes — it's a local size_t.
This mirrors the pattern Glaze uses (passing `B&& b, auto&& ix` through
every helper). The simdjson-specific bit is keeping the public
string_builder API intact: only the internal atom() family is
refactored to take `writer&`. The top-level append(string_builder&, T)
constructs a writer on the stack, threads it through atom(), then
syncs the local position back at the end.
For string fields specifically, the escape path is also inlined
through the writer — the existing `write_string_escaped(input, out)`
helper already takes a destination pointer and returns bytes-written,
so it composes cleanly with `w.ptr + w.pos`. Without this, sync/
reload around each string write was a real cost on Twitter (-7%).
Performance
===========
TRUE A/B in single docker invocation, 7 alternating rounds, byte-
identical output, all static_reflection_comprehensive_tests pass.
Build: clang-p2996 21.0.0git, -O3 -DNDEBUG, aarch64 Linux.
CITM Catalog (496 682 bytes):
baseline (master): 4356 MB/s
patched: 6776 MB/s +55.5%
Glaze: 4860 MB/s simdjson now 39.4% AHEAD of Glaze
Twitter (81 927 bytes):
baseline (master): 6437 MB/s
patched: 6452 MB/s +0.2% (break-even)
Twitter is string-heavy; the writer's gain over the existing
member-based path is small here, but it no longer regresses.
Default initial capacity is unchanged at 1024.
Francisco Geiman Thiesen