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perf(gzip): index BytesView directly in crc32_update #388

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perf(gzip): index BytesView directly in crc32_update #388

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18 changes: 13 additions & 5 deletions src/internal/gzip_internal/crc32.mbt
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Original file line number Diff line number Diff line change
Expand Up @@ -23,10 +23,18 @@ let crc32_table : FixedArray[UInt] = FixedArray::makei(256, i => {

///|
fn crc32_update(current : UInt, chunk : BytesView) -> UInt {
for byte in chunk; crc = current {
let index = ((crc ^ byte.to_uint()) & 0xffU).reinterpret_as_int()
continue crc32_table[index] ^ (crc >> 8)
} nobreak {
crc
// Pull out the backing Bytes + start offset once and index that
// directly. `for byte in chunk` desugars through BytesView::iter +
// Iter::next (15-16% in a gzip_roundtrip profile), and even
// `chunk[i]` (= BytesView::at) is a non-inlined function call
// (6.88% before this change). Indexing the raw Bytes is intrinsic.
let mut crc = current
let bytes = chunk.data()
let start = chunk.start_offset()
let len = chunk.length()
for i in 0..<len {
let index = ((crc ^ bytes[start + i].to_uint()) & 0xffU).reinterpret_as_int()
crc = crc32_table[index] ^ (crc >> 8)
}
crc
}
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