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perf(sourcemap): shorten main loop encoding VLQ (#4586)

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Reduce number of operations in main loop in source map VLQ encoding.

#4583 made pushing a byte to output only 2 instructions, so that makes it workable to repeat `push_byte_unchecked` inside and outside the loop.

On a local benchmark of just VLQ encoding shows this increases performance by 16% (on top of the 11% from #4583).

Probably main gain is it makes a fast path for encoding `0`, which is common.
This commit is contained in:
overlookmotel 2024-08-01 12:21:26 +00:00
parent e02bac357a
commit 590d79530e
1 changed files with 21 additions and 13 deletions
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34
crates/oxc_sourcemap/src/encode.rs
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@ -217,25 +217,33 @@ static B64_CHARS: Aligned64 = Aligned64([
unsafe fn encode_vlq(out: &mut String, num: i64) {
let mut num = if num < 0 { ((-num) << 1) + 1 } else { num << 1 };
// Breaking out of loop early when have reached last char (rather than conditionally adding
// 32 for last char within the loop) removes 3 instructions from the loop.
// https://godbolt.org/z/Es4Pavh9j
// This translates to a 16% speed-up for VLQ encoding.
let mut digit;
loop {
let mut digit = num & 0b11111;
digit = num & 0b11111;
num >>= 5;
if num > 0 {
digit |= 1 << 5;
}
let b = B64_CHARS.0[digit as usize];
// SAFETY:
// * This loop can execute a maximum of 7 times, caller promises there are at least
// 7 bytes spare capacity in `out` at start, and we only push 1 byte on each turn,
// so guaranteed there is at least 1 byte capacity in `out` here.
// * All values in `B64_CHARS` lookup table are ASCII bytes.
push_byte_unchecked(out, b);
if num == 0 {
break;
}
let b = B64_CHARS.0[digit as usize + 32];
// SAFETY:
// * This loop can execute a maximum of 7 times, and on last turn will exit before getting here.
// Caller promises there are at least 7 bytes spare capacity in `out` at start. We only
// push 1 byte on each turn, so guaranteed there is at least 1 byte capacity in `out` here.
// * All values in `B64_CHARS` lookup table are ASCII bytes.
push_byte_unchecked(out, b);
}
let b = B64_CHARS.0[digit as usize];
// SAFETY:
// * The loop above pushes max 6 bytes. Caller promises there are at least 7 bytes spare capacity
// in `out` at start. So guaranteed there is at least 1 byte capacity in `out` here.
// * All values in `B64_CHARS` lookup table are ASCII bytes.
push_byte_unchecked(out, b);
}
/// Push a byte to `out` without bounds checking.