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i8 and u8::to_string() specialisation (far less asm). #82576

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Merged
merged 6 commits into from
May 3, 2021
Merged

i8 and u8::to_string() specialisation (far less asm). #82576

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a69960a
u8::to_string() specialisation (far less asm).
gilescope Feb 26, 2021
d07c43a
Alternative LUT rather than dividing.
gilescope Mar 4, 2021
a678b9a
less uB in i8
gilescope Mar 4, 2021
e83378b
vec![0;4] is a fast path.
gilescope Mar 7, 2021
6a58b6a
Update library/alloc/src/string.rs
gilescope Mar 8, 2021
05330aa
Closer similarities.
gilescope Mar 8, 2021
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Alternative LUT rather than dividing.
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@gilescope
gilescope committed Mar 4, 2021
commit d07c43af3129c8e160bb62d98ebcb8b9cf6f3ccd
35 changes: 34 additions & 1 deletion library/alloc/src/string.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2224,7 +2224,7 @@ impl ToString for char {
}
}

#[stable(feature = "u8_to_string_specialization", since="1.999.0")]
#[stable(feature = "u8_to_string_specialization", since = "1.999.0")]
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@pickfire pickfire May 12, 2021 •
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What is with the since tag? Why 1.999?

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Already fixed on master.

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Ah sorry, wasn't sure what to put for that. Yes 1.54 makes sense, but at the time I didn't want to presume so put something a little further out.

impl ToString for u8 {
#[inline]
fn to_string(&self) -> String {
Expand All @@ -2243,6 +2243,39 @@ impl ToString for u8 {
}
}

// 2 digit decimal look up table
static DEC_DIGITS_LUT: &[u8; 200] = b"0001020304050607080910111213141516171819\
2021222324252627282930313233343536373839\
4041424344454647484950515253545556575859\
6061626364656667686970717273747576777879\
8081828384858687888990919293949596979899";

#[stable(feature = "i8_to_string_specialization", since = "1.999.0")]
impl ToString for i8 {
#[inline]
fn to_string(&self) -> String {
let mut vec: Vec<u8> = if *self < 0 {
let mut v = Vec::with_capacity(4);
v.push(b'-');
v
} else {
Vec::with_capacity(3)
};
let mut n = self.abs();
if n >= 10 {
if n >= 100 {
n -= 100;
vec.push(b'1');
}
let nn = n * 2;
vec.extend_from_slice(&DEC_DIGITS_LUT[nn as usize..=nn as usize + 1]);
} else {
vec.push(b'0' + (n as u8));
}
unsafe { String::from_utf8_unchecked(vec) }
}
}

#[stable(feature = "str_to_string_specialization", since = "1.9.0")]
impl ToString for str {
#[inline]
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