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base_n: speedup push_str #86214

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base_n: speedup push_str #86214

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15 changes: 8 additions & 7 deletions compiler/rustc_data_structures/src/base_n.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -14,24 +14,25 @@ const BASE_64: &[u8; MAX_BASE as usize] =

#[inline]
pub fn push_str(mut n: u128, base: usize, output: &mut String) {
debug_assert!(base >= 2 && base <= MAX_BASE);
assert!(base >= 2 && base <= MAX_BASE);
let mut s = [0u8; 128];
let mut index = 0;
let mut first_index = 0;

let base = base as u128;

loop {
s[index] = BASE_64[(n % base) as usize];
index += 1;
for idx in (0..128).rev() {
// SAFETY: given `base <= MAX_BASE`, so `n % base < MAX_BASE`
s[idx] = unsafe { *BASE_64.get_unchecked((n % base) as usize) };
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This probably doesn't need to be unsafe. LLVM will eliminate the bounds check because the assertion above already allows it to proof the condition given in the comment.

n /= base;

if n == 0 {
first_index = idx;
break;
}
}
s[0..index].reverse();

output.push_str(str::from_utf8(&s[0..index]).unwrap());
// SAFETY: all chars in given range is nonnull ascii
output.push_str(unsafe { str::from_utf8_unchecked(&s[first_index..]) });
}

#[inline]
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