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[api] Merge varint length + memcpy into single pos scope in encode_string

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Previously encode_string called encode_varint_raw(len) then
encode_raw(data, len) as separate methods, each with their own
__restrict__ pos scope. This caused a redundant store-load pair
of pos_ between the two operations.

Inline the length varint write and memcpy under a single local
pos variable so the compiler can keep pos_ in a register across
both operations. Eliminates one load-store pair per string encode.
This commit is contained in:
J. Nick Koston
2026-03-28 19:51:55 -10:00
parent c6938adb61
commit 3a76f9d5d2
1 changed files with 10 additions and 4 deletions
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esphome/components/api/proto.h
+10 -4
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@@ -288,11 +288,17 @@ class ProtoWriteBuffer {
return;
this->encode_field_raw(field_id, 2); // type 2: Length-delimited string
this->encode_varint_raw(len);
// Direct memcpy into pre-sized buffer — avoids push_back() per-byte capacity checks
// and vector::insert() iterator overhead. ~10-11x faster for 16-32 byte strings.
this->debug_check_bounds_(len);
// Inline the length varint + memcpy under a single __restrict__ pos
// to avoid a store-load pair between encode_varint_raw and encode_raw.
this->debug_check_bounds_(1 + len);
uint8_t *__restrict__ pos = this->pos_;
if (len < 128) [[likely]] {
*pos++ = static_cast<uint8_t>(len);
} else {
// Length >= 128: use slow path for the length varint, then re-hoist pos
this->encode_varint_raw_slow_(len);
pos = this->pos_;
}
std::memcpy(pos, string, len);
this->pos_ = pos + len;
}