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[wpiutil] Faster nanopb submessage encode (#7374)

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Due to how submessages are encoded (with a length prefix), nanopb currently does the encoding twice. It encodes once to get the length to write, then writes the length, then reencodes the entire message a 2nd time.

This results in a requirement that each encode always encodes the same. Generally, this is fine, but it'd be nice to not make this a requirement.

The double encode also requires going through the entire set of fields again, which has the possibility to be slow.

Instead of doing this, write to a temporary SmallVector. Then we can just encode the length of that buffer, and do a memcpy into primary stream. Theoretically in most cases, this should be much faster.
This commit is contained in:
Thad House
2024-11-16 15:26:10 +00:00
committed by GitHub
parent 1e545c38a8
commit 969664ceaa
2 changed files with 30 additions and 1 deletions
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6
wpiutil/src/main/native/include/wpi/protobuf/Protobuf.h
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@@ -43,6 +43,9 @@ bool WriteFromSmallVector(pb_ostream_t* stream, const pb_byte_t* buf,
bool WriteFromStdVector(pb_ostream_t* stream, const pb_byte_t* buf,
size_t count);
bool WriteSubmessage(pb_ostream_t* stream, const pb_msgdesc_t* desc,
const void* msg);
} // namespace detail
/**
@@ -208,7 +211,8 @@ class ProtoOutputStream {
bool Encode(
const typename Protobuf<std::remove_cvref_t<T>>::MessageStruct& msg) {
if (m_streamMsg) {
return pb_encode_submessage(m_streamMsg, m_msgDesc, &msg);
return detail::WriteSubmessage(m_streamMsg, m_msgDesc, &msg);
// return pb_encode_submessage(m_streamMsg, m_msgDesc, &msg);
}
return pb_encode(&m_streamLocal, m_msgDesc, &msg);
}