Auto merge of #107007 - TDecking:float_parsing_improvments, r=Mark-Simulacrum

Improve the floating point parser in dec2flt.

Greetings everyone,

I've benn studying the rust floating point parser recently and made the following tweaks:

* Remove all remaining traces of `unsafe`. The parser is now 100% safe Rust.
* The trick in which eight digits are processed in parallel is now in a loop.
* Parsing of inf/NaN values has been reworked.

On my system, the changes result in performance improvements for some input values.

5 files changed, 188 insertions, 288 deletions

diff --git a/library/core/src/num/dec2flt/common.rs b/library/core/src/num/dec2flt/common.rs
index 3e8d75df378..11a62648519 100644
--- a/library/core/src/num/dec2flt/common.rs
+++ b/library/core/src/num/dec2flt/common.rs
@@ -1,165 +1,60 @@
 //! Common utilities, for internal use only.
 
-use crate::ptr;
-
 /// Helper methods to process immutable bytes.
-pub(crate) trait ByteSlice: AsRef<[u8]> {
-    unsafe fn first_unchecked(&self) -> u8 {
-        debug_assert!(!self.is_empty());
-        // SAFETY: safe as long as self is not empty
-        unsafe { *self.as_ref().get_unchecked(0) }
-    }
-
-    /// Get if the slice contains no elements.
-    fn is_empty(&self) -> bool {
-        self.as_ref().is_empty()
-    }
-
-    /// Check if the slice at least `n` length.
-    fn check_len(&self, n: usize) -> bool {
-        n <= self.as_ref().len()
-    }
-
-    /// Check if the first character in the slice is equal to c.
-    fn first_is(&self, c: u8) -> bool {
-        self.as_ref().first() == Some(&c)
-    }
-
-    /// Check if the first character in the slice is equal to c1 or c2.
-    fn first_is2(&self, c1: u8, c2: u8) -> bool {
-        if let Some(&c) = self.as_ref().first() { c == c1 || c == c2 } else { false }
-    }
-
-    /// Bounds-checked test if the first character in the slice is a digit.
-    fn first_isdigit(&self) -> bool {
-        if let Some(&c) = self.as_ref().first() { c.is_ascii_digit() } else { false }
-    }
-
-    /// Check if self starts with u with a case-insensitive comparison.
-    fn starts_with_ignore_case(&self, u: &[u8]) -> bool {
-        debug_assert!(self.as_ref().len() >= u.len());
-        let iter = self.as_ref().iter().zip(u.iter());
-        let d = iter.fold(0, |i, (&x, &y)| i | (x ^ y));
-        d == 0 || d == 32
-    }
-
-    /// Get the remaining slice after the first N elements.
-    fn advance(&self, n: usize) -> &[u8] {
-        &self.as_ref()[n..]
-    }
-
-    /// Get the slice after skipping all leading characters equal c.
-    fn skip_chars(&self, c: u8) -> &[u8] {
-        let mut s = self.as_ref();
-        while s.first_is(c) {
-            s = s.advance(1);
-        }
-        s
-    }
-
-    /// Get the slice after skipping all leading characters equal c1 or c2.
-    fn skip_chars2(&self, c1: u8, c2: u8) -> &[u8] {
-        let mut s = self.as_ref();
-        while s.first_is2(c1, c2) {
-            s = s.advance(1);
-        }
-        s
-    }
-
+pub(crate) trait ByteSlice {
     /// Read 8 bytes as a 64-bit integer in little-endian order.
-    unsafe fn read_u64_unchecked(&self) -> u64 {
-        debug_assert!(self.check_len(8));
-        let src = self.as_ref().as_ptr() as *const u64;
-        // SAFETY: safe as long as self is at least 8 bytes
-        u64::from_le(unsafe { ptr::read_unaligned(src) })
-    }
+    fn read_u64(&self) -> u64;
 
-    /// Try to read the next 8 bytes from the slice.
-    fn read_u64(&self) -> Option<u64> {
-        if self.check_len(8) {
-            // SAFETY: self must be at least 8 bytes.
-            Some(unsafe { self.read_u64_unchecked() })
-        } else {
-            None
-        }
-    }
-
-    /// Calculate the offset of slice from another.
-    fn offset_from(&self, other: &Self) -> isize {
-        other.as_ref().len() as isize - self.as_ref().len() as isize
-    }
-}
-
-impl ByteSlice for [u8] {}
-
-/// Helper methods to process mutable bytes.
-pub(crate) trait ByteSliceMut: AsMut<[u8]> {
     /// Write a 64-bit integer as 8 bytes in little-endian order.
-    unsafe fn write_u64_unchecked(&mut self, value: u64) {
-        debug_assert!(self.as_mut().len() >= 8);
-        let dst = self.as_mut().as_mut_ptr() as *mut u64;
-        // NOTE: we must use `write_unaligned`, since dst is not
-        // guaranteed to be properly aligned. Miri will warn us
-        // if we use `write` instead of `write_unaligned`, as expected.
-        // SAFETY: safe as long as self is at least 8 bytes
-        unsafe {
-            ptr::write_unaligned(dst, u64::to_le(value));
-        }
-    }
-}
+    fn write_u64(&mut self, value: u64);
 
-impl ByteSliceMut for [u8] {}
+    /// Calculate the offset of a slice from another.
+    fn offset_from(&self, other: &Self) -> isize;
 
-/// Bytes wrapper with specialized methods for ASCII characters.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub(crate) struct AsciiStr<'a> {
-    slc: &'a [u8],
+    /// Iteratively parse and consume digits from bytes.
+    /// Returns the same bytes with consumed digits being
+    /// elided.
+    fn parse_digits(&self, func: impl FnMut(u8)) -> &Self;
 }
 
-impl<'a> AsciiStr<'a> {
-    pub fn new(slc: &'a [u8]) -> Self {
-        Self { slc }
+impl ByteSlice for [u8] {
+    #[inline(always)] // inlining this is crucial to remove bound checks
+    fn read_u64(&self) -> u64 {
+        let mut tmp = [0; 8];
+        tmp.copy_from_slice(&self[..8]);
+        u64::from_le_bytes(tmp)
     }
 
-    /// Advance the view by n, advancing it in-place to (n..).
-    pub unsafe fn step_by(&mut self, n: usize) -> &mut Self {
-        // SAFETY: safe as long n is less than the buffer length
-        self.slc = unsafe { self.slc.get_unchecked(n..) };
-        self
+    #[inline(always)] // inlining this is crucial to remove bound checks
+    fn write_u64(&mut self, value: u64) {
+        self[..8].copy_from_slice(&value.to_le_bytes())
     }
 
-    /// Advance the view by n, advancing it in-place to (1..).
-    pub unsafe fn step(&mut self) -> &mut Self {
-        // SAFETY: safe as long as self is not empty
-        unsafe { self.step_by(1) }
+    #[inline]
+    fn offset_from(&self, other: &Self) -> isize {
+        other.len() as isize - self.len() as isize
     }
 
-    /// Iteratively parse and consume digits from bytes.
-    pub fn parse_digits(&mut self, mut func: impl FnMut(u8)) {
-        while let Some(&c) = self.as_ref().first() {
+    #[inline]
+    fn parse_digits(&self, mut func: impl FnMut(u8)) -> &Self {
+        let mut s = self;
+
+        // FIXME: Can't use s.split_first() here yet,
+        // see https://github.com/rust-lang/rust/issues/109328
+        while let [c, s_next @ ..] = s {
             let c = c.wrapping_sub(b'0');
             if c < 10 {
                 func(c);
-                // SAFETY: self cannot be empty
-                unsafe {
-                    self.step();
-                }
+                s = s_next;
             } else {
                 break;
             }
         }
-    }
-}
 
-impl<'a> AsRef<[u8]> for AsciiStr<'a> {
-    #[inline]
-    fn as_ref(&self) -> &[u8] {
-        self.slc
+        s
     }
 }
 
-impl<'a> ByteSlice for AsciiStr<'a> {}
-
 /// Determine if 8 bytes are all decimal digits.
 /// This does not care about the order in which the bytes were loaded.
 pub(crate) fn is_8digits(v: u64) -> bool {
@@ -168,19 +63,6 @@ pub(crate) fn is_8digits(v: u64) -> bool {
     (a | b) & 0x8080_8080_8080_8080 == 0
 }
 
-/// Iteratively parse and consume digits from bytes.
-pub(crate) fn parse_digits(s: &mut &[u8], mut f: impl FnMut(u8)) {
-    while let Some(&c) = s.get(0) {
-        let c = c.wrapping_sub(b'0');
-        if c < 10 {
-            f(c);
-            *s = s.advance(1);
-        } else {
-            break;
-        }
-    }
-}
-
 /// A custom 64-bit floating point type, representing `f * 2^e`.
 /// e is biased, so it be directly shifted into the exponent bits.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Default)]
diff --git a/library/core/src/num/dec2flt/decimal.rs b/library/core/src/num/dec2flt/decimal.rs
index 2019f71e69b..350f64bb4f7 100644
--- a/library/core/src/num/dec2flt/decimal.rs
+++ b/library/core/src/num/dec2flt/decimal.rs
@@ -9,7 +9,7 @@
 //! algorithm can be found in "ParseNumberF64 by Simple Decimal Conversion",
 //! available online: <https://nigeltao.github.io/blog/2020/parse-number-f64-simple.html>.
 
-use crate::num::dec2flt::common::{is_8digits, parse_digits, ByteSlice, ByteSliceMut};
+use crate::num::dec2flt::common::{is_8digits, ByteSlice};
 
 #[derive(Clone)]
 pub struct Decimal {
@@ -205,29 +205,32 @@ impl Decimal {
 pub fn parse_decimal(mut s: &[u8]) -> Decimal {
     let mut d = Decimal::default();
     let start = s;
-    s = s.skip_chars(b'0');
-    parse_digits(&mut s, |digit| d.try_add_digit(digit));
-    if s.first_is(b'.') {
-        s = s.advance(1);
+
+    while let Some((&b'0', s_next)) = s.split_first() {
+        s = s_next;
+    }
+
+    s = s.parse_digits(|digit| d.try_add_digit(digit));
+
+    if let Some((b'.', s_next)) = s.split_first() {
+        s = s_next;
         let first = s;
         // Skip leading zeros.
         if d.num_digits == 0 {
-            s = s.skip_chars(b'0');
+            while let Some((&b'0', s_next)) = s.split_first() {
+                s = s_next;
+            }
         }
         while s.len() >= 8 && d.num_digits + 8 < Decimal::MAX_DIGITS {
-            // SAFETY: s is at least 8 bytes.
-            let v = unsafe { s.read_u64_unchecked() };
+            let v = s.read_u64();
             if !is_8digits(v) {
                 break;
             }
-            // SAFETY: d.num_digits + 8 is less than d.digits.len()
-            unsafe {
-                d.digits[d.num_digits..].write_u64_unchecked(v - 0x3030_3030_3030_3030);
-            }
+            d.digits[d.num_digits..].write_u64(v - 0x3030_3030_3030_3030);
             d.num_digits += 8;
-            s = s.advance(8);
+            s = &s[8..];
         }
-        parse_digits(&mut s, |digit| d.try_add_digit(digit));
+        s = s.parse_digits(|digit| d.try_add_digit(digit));
         d.decimal_point = s.len() as i32 - first.len() as i32;
     }
     if d.num_digits != 0 {
@@ -248,22 +251,26 @@ pub fn parse_decimal(mut s: &[u8]) -> Decimal {
             d.num_digits = Decimal::MAX_DIGITS;
         }
     }
-    if s.first_is2(b'e', b'E') {
-        s = s.advance(1);
-        let mut neg_exp = false;
-        if s.first_is(b'-') {
-            neg_exp = true;
-            s = s.advance(1);
-        } else if s.first_is(b'+') {
-            s = s.advance(1);
-        }
-        let mut exp_num = 0_i32;
-        parse_digits(&mut s, |digit| {
-            if exp_num < 0x10000 {
-                exp_num = 10 * exp_num + digit as i32;
+    if let Some((&ch, s_next)) = s.split_first() {
+        if ch == b'e' || ch == b'E' {
+            s = s_next;
+            let mut neg_exp = false;
+            if let Some((&ch, s_next)) = s.split_first() {
+                neg_exp = ch == b'-';
+                if ch == b'-' || ch == b'+' {
+                    s = s_next;
+                }
             }
-        });
-        d.decimal_point += if neg_exp { -exp_num } else { exp_num };
+            let mut exp_num = 0_i32;
+
+            s.parse_digits(|digit| {
+                if exp_num < 0x10000 {
+                    exp_num = 10 * exp_num + digit as i32;
+                }
+            });
+
+            d.decimal_point += if neg_exp { -exp_num } else { exp_num };
+        }
     }
     for i in d.num_digits..Decimal::MAX_DIGITS_WITHOUT_OVERFLOW {
         d.digits[i] = 0;
diff --git a/library/core/src/num/dec2flt/mod.rs b/library/core/src/num/dec2flt/mod.rs
index 58ffb950ad8..a4bc8b1c9b0 100644
--- a/library/core/src/num/dec2flt/mod.rs
+++ b/library/core/src/num/dec2flt/mod.rs
@@ -79,7 +79,7 @@ use crate::error::Error;
 use crate::fmt;
 use crate::str::FromStr;
 
-use self::common::{BiasedFp, ByteSlice};
+use self::common::BiasedFp;
 use self::float::RawFloat;
 use self::lemire::compute_float;
 use self::parse::{parse_inf_nan, parse_number};
@@ -238,17 +238,18 @@ pub fn dec2flt<F: RawFloat>(s: &str) -> Result<F, ParseFloatError> {
     };
     let negative = c == b'-';
     if c == b'-' || c == b'+' {
-        s = s.advance(1);
+        s = &s[1..];
     }
     if s.is_empty() {
         return Err(pfe_invalid());
     }
 
-    let num = match parse_number(s, negative) {
+    let mut num = match parse_number(s) {
         Some(r) => r,
         None if let Some(value) = parse_inf_nan(s, negative) => return Ok(value),
         None => return Err(pfe_invalid()),
     };
+    num.negative = negative;
     if let Some(value) = num.try_fast_path::<F>() {
         return Ok(value);
     }
diff --git a/library/core/src/num/dec2flt/parse.rs b/library/core/src/num/dec2flt/parse.rs
index 1a90e0d206f..b0a23835c5b 100644
--- a/library/core/src/num/dec2flt/parse.rs
+++ b/library/core/src/num/dec2flt/parse.rs
@@ -1,6 +1,6 @@
 //! Functions to parse floating-point numbers.
 
-use crate::num::dec2flt::common::{is_8digits, AsciiStr, ByteSlice};
+use crate::num::dec2flt::common::{is_8digits, ByteSlice};
 use crate::num::dec2flt::float::RawFloat;
 use crate::num::dec2flt::number::Number;
 
@@ -26,24 +26,39 @@ fn parse_8digits(mut v: u64) -> u64 {
 }
 
 /// Parse digits until a non-digit character is found.
-fn try_parse_digits(s: &mut AsciiStr<'_>, x: &mut u64) {
+fn try_parse_digits(mut s: &[u8], mut x: u64) -> (&[u8], u64) {
     // may cause overflows, to be handled later
-    s.parse_digits(|digit| {
-        *x = x.wrapping_mul(10).wrapping_add(digit as _);
+
+    while s.len() >= 8 {
+        let num = s.read_u64();
+        if is_8digits(num) {
+            x = x.wrapping_mul(1_0000_0000).wrapping_add(parse_8digits(num));
+            s = &s[8..];
+        } else {
+            break;
+        }
+    }
+
+    s = s.parse_digits(|digit| {
+        x = x.wrapping_mul(10).wrapping_add(digit as _);
     });
+
+    (s, x)
 }
 
 /// Parse up to 19 digits (the max that can be stored in a 64-bit integer).
-fn try_parse_19digits(s: &mut AsciiStr<'_>, x: &mut u64) {
+fn try_parse_19digits(s_ref: &mut &[u8], x: &mut u64) {
+    let mut s = *s_ref;
+
     while *x < MIN_19DIGIT_INT {
-        if let Some(&c) = s.as_ref().first() {
+        // FIXME: Can't use s.split_first() here yet,
+        // see https://github.com/rust-lang/rust/issues/109328
+        if let [c, s_next @ ..] = s {
             let digit = c.wrapping_sub(b'0');
+
             if digit < 10 {
                 *x = (*x * 10) + digit as u64; // no overflows here
-                // SAFETY: cannot be empty
-                unsafe {
-                    s.step();
-                }
+                s = s_next;
             } else {
                 break;
             }
@@ -51,46 +66,26 @@ fn try_parse_19digits(s: &mut AsciiStr<'_>, x: &mut u64) {
             break;
         }
     }
-}
 
-/// Try to parse 8 digits at a time, using an optimized algorithm.
-fn try_parse_8digits(s: &mut AsciiStr<'_>, x: &mut u64) {
-    // may cause overflows, to be handled later
-    if let Some(v) = s.read_u64() {
-        if is_8digits(v) {
-            *x = x.wrapping_mul(1_0000_0000).wrapping_add(parse_8digits(v));
-            // SAFETY: already ensured the buffer was >= 8 bytes in read_u64.
-            unsafe {
-                s.step_by(8);
-            }
-            if let Some(v) = s.read_u64() {
-                if is_8digits(v) {
-                    *x = x.wrapping_mul(1_0000_0000).wrapping_add(parse_8digits(v));
-                    // SAFETY: already ensured the buffer was >= 8 bytes in try_read_u64.
-                    unsafe {
-                        s.step_by(8);
-                    }
-                }
-            }
-        }
-    }
+    *s_ref = s;
 }
 
 /// Parse the scientific notation component of a float.
-fn parse_scientific(s: &mut AsciiStr<'_>) -> Option<i64> {
-    let mut exponent = 0_i64;
+fn parse_scientific(s_ref: &mut &[u8]) -> Option<i64> {
+    let mut exponent = 0i64;
     let mut negative = false;
-    if let Some(&c) = s.as_ref().get(0) {
+
+    let mut s = *s_ref;
+
+    if let Some((&c, s_next)) = s.split_first() {
         negative = c == b'-';
         if c == b'-' || c == b'+' {
-            // SAFETY: s cannot be empty
-            unsafe {
-                s.step();
-            }
+            s = s_next;
         }
     }
-    if s.first_isdigit() {
-        s.parse_digits(|digit| {
+
+    if matches!(s.first(), Some(&x) if x.is_ascii_digit()) {
+        *s_ref = s.parse_digits(|digit| {
             // no overflows here, saturate well before overflow
             if exponent < 0x10000 {
                 exponent = 10 * exponent + digit as i64;
@@ -98,6 +93,7 @@ fn parse_scientific(s: &mut AsciiStr<'_>) -> Option<i64> {
         });
         if negative { Some(-exponent) } else { Some(exponent) }
     } else {
+        *s_ref = s;
         None
     }
 }
@@ -106,28 +102,29 @@ fn parse_scientific(s: &mut AsciiStr<'_>) -> Option<i64> {
 ///
 /// This creates a representation of the float as the
 /// significant digits and the decimal exponent.
-fn parse_partial_number(s: &[u8], negative: bool) -> Option<(Number, usize)> {
-    let mut s = AsciiStr::new(s);
-    let start = s;
+fn parse_partial_number(mut s: &[u8]) -> Option<(Number, usize)> {
     debug_assert!(!s.is_empty());
 
     // parse initial digits before dot
     let mut mantissa = 0_u64;
-    let digits_start = s;
-    try_parse_digits(&mut s, &mut mantissa);
-    let mut n_digits = s.offset_from(&digits_start);
+    let start = s;
+    let tmp = try_parse_digits(s, mantissa);
+    s = tmp.0;
+    mantissa = tmp.1;
+    let mut n_digits = s.offset_from(start);
 
     // handle dot with the following digits
     let mut n_after_dot = 0;
     let mut exponent = 0_i64;
     let int_end = s;
-    if s.first_is(b'.') {
-        // SAFETY: s cannot be empty due to first_is
-        unsafe { s.step() };
+
+    if let Some((&b'.', s_next)) = s.split_first() {
+        s = s_next;
         let before = s;
-        try_parse_8digits(&mut s, &mut mantissa);
-        try_parse_digits(&mut s, &mut mantissa);
-        n_after_dot = s.offset_from(&before);
+        let tmp = try_parse_digits(s, mantissa);
+        s = tmp.0;
+        mantissa = tmp.1;
+        n_after_dot = s.offset_from(before);
         exponent = -n_after_dot as i64;
     }
 
@@ -138,65 +135,60 @@ fn parse_partial_number(s: &[u8], negative: bool) -> Option<(Number, usize)> {
 
     // handle scientific format
     let mut exp_number = 0_i64;
-    if s.first_is2(b'e', b'E') {
-        // SAFETY: s cannot be empty
-        unsafe {
-            s.step();
+    if let Some((&c, s_next)) = s.split_first() {
+        if c == b'e' || c == b'E' {
+            s = s_next;
+            // If None, we have no trailing digits after exponent, or an invalid float.
+            exp_number = parse_scientific(&mut s)?;
+            exponent += exp_number;
         }
-        // If None, we have no trailing digits after exponent, or an invalid float.
-        exp_number = parse_scientific(&mut s)?;
-        exponent += exp_number;
     }
 
-    let len = s.offset_from(&start) as _;
+    let len = s.offset_from(start) as _;
 
     // handle uncommon case with many digits
     if n_digits <= 19 {
-        return Some((Number { exponent, mantissa, negative, many_digits: false }, len));
+        return Some((Number { exponent, mantissa, negative: false, many_digits: false }, len));
     }
 
     n_digits -= 19;
     let mut many_digits = false;
-    let mut p = digits_start;
-    while p.first_is2(b'0', b'.') {
-        // SAFETY: p cannot be empty due to first_is2
-        unsafe {
-            // '0' = b'.' + 2
-            n_digits -= p.first_unchecked().saturating_sub(b'0' - 1) as isize;
-            p.step();
+    let mut p = start;
+    while let Some((&c, p_next)) = p.split_first() {
+        if c == b'.' || c == b'0' {
+            n_digits -= c.saturating_sub(b'0' - 1) as isize;
+            p = p_next;
+        } else {
+            break;
         }
     }
     if n_digits > 0 {
         // at this point we have more than 19 significant digits, let's try again
         many_digits = true;
         mantissa = 0;
-        let mut s = digits_start;
+        let mut s = start;
         try_parse_19digits(&mut s, &mut mantissa);
         exponent = if mantissa >= MIN_19DIGIT_INT {
             // big int
-            int_end.offset_from(&s)
+            int_end.offset_from(s)
         } else {
-            // SAFETY: the next byte must be present and be '.'
-            // We know this is true because we had more than 19
-            // digits previously, so we overflowed a 64-bit integer,
-            // but parsing only the integral digits produced less
-            // than 19 digits. That means we must have a decimal
-            // point, and at least 1 fractional digit.
-            unsafe { s.step() };
+            s = &s[1..];
             let before = s;
             try_parse_19digits(&mut s, &mut mantissa);
-            -s.offset_from(&before)
+            -s.offset_from(before)
         } as i64;
         // add back the explicit part
         exponent += exp_number;
     }
 
-    Some((Number { exponent, mantissa, negative, many_digits }, len))
+    Some((Number { exponent, mantissa, negative: false, many_digits }, len))
 }
 
-/// Try to parse a non-special floating point number.
-pub fn parse_number(s: &[u8], negative: bool) -> Option<Number> {
-    if let Some((float, rest)) = parse_partial_number(s, negative) {
+/// Try to parse a non-special floating point number,
+/// as well as two slices with integer and fractional parts
+/// and the parsed exponent.
+pub fn parse_number(s: &[u8]) -> Option<Number> {
+    if let Some((float, rest)) = parse_partial_number(s) {
         if rest == s.len() {
             return Some(float);
         }
@@ -204,30 +196,48 @@ pub fn parse_number(s: &[u8], negative: bool) -> Option<Number> {
     None
 }
 
-/// Parse a partial representation of a special, non-finite float.
-fn parse_partial_inf_nan<F: RawFloat>(s: &[u8]) -> Option<(F, usize)> {
-    fn parse_inf_rest(s: &[u8]) -> usize {
-        if s.len() >= 8 && s[3..].as_ref().starts_with_ignore_case(b"inity") { 8 } else { 3 }
-    }
-    if s.len() >= 3 {
-        if s.starts_with_ignore_case(b"nan") {
-            return Some((F::NAN, 3));
-        } else if s.starts_with_ignore_case(b"inf") {
-            return Some((F::INFINITY, parse_inf_rest(s)));
-        }
-    }
-    None
-}
-
 /// Try to parse a special, non-finite float.
-pub fn parse_inf_nan<F: RawFloat>(s: &[u8], negative: bool) -> Option<F> {
-    if let Some((mut float, rest)) = parse_partial_inf_nan::<F>(s) {
-        if rest == s.len() {
-            if negative {
-                float = -float;
-            }
-            return Some(float);
-        }
+pub(crate) fn parse_inf_nan<F: RawFloat>(s: &[u8], negative: bool) -> Option<F> {
+    // Since a valid string has at most the length 8, we can load
+    // all relevant characters into a u64 and work from there.
+    // This also generates much better code.
+
+    let mut register;
+    let len: usize;
+
+    // All valid strings are either of length 8 or 3.
+    if s.len() == 8 {
+        register = s.read_u64();
+        len = 8;
+    } else if s.len() == 3 {
+        let a = s[0] as u64;
+        let b = s[1] as u64;
+        let c = s[2] as u64;
+        register = (c << 16) | (b << 8) | a;
+        len = 3;
+    } else {
+        return None;
     }
-    None
+
+    // Clear out the bits which turn ASCII uppercase characters into
+    // lowercase characters. The resulting string is all uppercase.
+    // What happens to other characters is irrelevant.
+    register &= 0xDFDFDFDFDFDFDFDF;
+
+    // u64 values corresponding to relevant cases
+    const INF_3: u64 = 0x464E49; // "INF"
+    const INF_8: u64 = 0x5954494E49464E49; // "INFINITY"
+    const NAN: u64 = 0x4E414E; // "NAN"
+
+    // Match register value to constant to parse string.
+    // Also match on the string length to catch edge cases
+    // like "inf\0\0\0\0\0".
+    let float = match (register, len) {
+        (INF_3, 3) => F::INFINITY,
+        (INF_8, 8) => F::INFINITY,
+        (NAN, 3) => F::NAN,
+        _ => return None,
+    };
+
+    if negative { Some(-float) } else { Some(float) }
 }
diff --git a/library/core/tests/num/dec2flt/parse.rs b/library/core/tests/num/dec2flt/parse.rs
index edc77377d58..4a5d24ba7d5 100644
--- a/library/core/tests/num/dec2flt/parse.rs
+++ b/library/core/tests/num/dec2flt/parse.rs
@@ -32,7 +32,7 @@ fn invalid_chars() {
 }
 
 fn parse_positive(s: &[u8]) -> Option<Number> {
-    parse_number(s, false)
+    parse_number(s)
 }
 
 #[test]