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Use clz32 for counting trailing zeroes. #9340

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Nov 2, 2023
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23 changes: 9 additions & 14 deletions packages/core/graph/src/BitSet.js
Original file line number Diff line number Diff line change
@@ -1,18 +1,13 @@
// @flow strict-local

// Small wasm program that exposes the `ctz` instruction.
// https://developer.mozilla.org/en-US/docs/WebAssembly/Reference/Numeric/Count_trailing_zeros
const wasmBuf = new Uint8Array([
0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00, 0x01, 0x06, 0x01, 0x60, 0x01,
0x7f, 0x01, 0x7f, 0x03, 0x02, 0x01, 0x00, 0x07, 0x0d, 0x01, 0x09, 0x74, 0x72,
0x61, 0x69, 0x6c, 0x69, 0x6e, 0x67, 0x30, 0x00, 0x00, 0x0a, 0x07, 0x01, 0x05,
0x00, 0x20, 0x00, 0x68, 0x0b, 0x00, 0x0f, 0x04, 0x6e, 0x61, 0x6d, 0x65, 0x02,
0x08, 0x01, 0x00, 0x01, 0x00, 0x03, 0x6e, 0x75, 0x6d,
]);

// eslint-disable-next-line
const {trailing0} = new WebAssembly.Instance(new WebAssembly.Module(wasmBuf))
.exports;
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/clz32#implementing_count_leading_ones_and_beyond
function ctz32(n: number): number {
if (n === 0) {
return 32;
}
let reversed = n & -n;
return 31 - Math.clz32(reversed);
}

export class BitSet {
bits: Uint32Array;
Expand Down Expand Up @@ -95,7 +90,7 @@ export class BitSet {
while (v !== 0) {
let t = (v & -v) >>> 0;
// $FlowFixMe
fn((k << 5) + trailing0(v));
fn((k << 5) + ctz32(v));
v ^= t;
}
}
Expand Down
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