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perf: mark the writer noalias in the writing helpers - #51
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lalinsky
added this pull request to stack #52
October 13, 2026 08:40
The writer's buffer is a []u8, and a byte store may alias anything,
including the writer struct itself. So after every byte written into the
buffer, the compiler has to reload end, buffer.ptr and buffer.len before
the next write, and a run of small writes becomes a chain of dependent
loads and stores through memory.
noalias on writeByte, writeAll, packHeaderAndInt and packStringLiteral
tells it that the stores into the buffer leave the writer alone. These
are leaves that never hand the writer to other code, so the promise is
easy to keep. The functions that do pass the writer on, such as packAny,
which can call a user's msgpackWrite, are left as they are.
Cycles per encode, ReleaseFast, perf stat:
case before after
order 343 257 -25% struct with 8 fields, 4 nested items
ints 8864 8939 flat array of 1000 mixed-size u32
The instruction count barely moves (804 -> 786 for order); the saving is
in the dependency chain. The int array is one call per element, so there
is no run of writes for it to help.
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Stacked on #50.
The writer's buffer is a
[]u8, and a byte store may alias anything, including theWriterstruct itself. So after every write into the buffer, the compiler has to reloadend,buffer.ptrandbuffer.lenbefore the next write. A run of small writes becomes a chain of dependent loads and stores through memory.noaliasonwriteByte,writeAll,packHeaderAndIntandpackStringLiteraltells it that the buffer stores leave the writer alone. These are leaves that never hand the writer to other code, so the promise is easy to keep. Functions that pass the writer on, such aspackAny, which can call a user'smsgpackWrite, are left alone.Cycles per encode, same setup as #50:
u32)The instruction count barely moves (804 → 786 for order); the saving is the dependency chain. The int array is one call per element, so there's no run of writes for it to help.
Tests pass on 0.16 and 0.17, and in ReleaseFast.