Speed up checked_isqrt and isqrt methods

* Use a lookup table for 8-bit integers and the Karatsuba square root
  algorithm for larger integers.
* Include optimization hints that give the compiler the exact numeric
  range of results.

library/core/src/num/int_macros.rs

@@ -1641,7 +1641,33 @@ macro_rules! int_impl {
             if self < 0 {
                 None
             } else {
-                Some((self as $UnsignedT).isqrt() as Self)
+                // SAFETY: Input is nonnegative in this `else` branch.
+                let result = unsafe {
+                    crate::num::int_sqrt::$ActualT(self as $ActualT) as $SelfT
+                };
+
+                // Inform the optimizer what the range of outputs is. If
+                // testing `core` crashes with no panic message and a
+                // `num::int_sqrt::i*` test failed, it's because your edits
+                // caused these assertions to become false.
+                //
+                // SAFETY: Integer square root is a monotonically nondecreasing
+                // function, which means that increasing the input will never
+                // cause the output to decrease. Thus, since the input for
+                // nonnegative signed integers is bounded by
+                // `[0, <$ActualT>::MAX]`, sqrt(n) will be bounded by
+                // `[sqrt(0), sqrt(<$ActualT>::MAX)]`.
+                unsafe {
+                    // SAFETY: `<$ActualT>::MAX` is nonnegative.
+                    const MAX_RESULT: $SelfT = unsafe {
+                        crate::num::int_sqrt::$ActualT(<$ActualT>::MAX) as $SelfT
+                    };
+
+                    crate::hint::assert_unchecked(result >= 0);
+                    crate::hint::assert_unchecked(result <= MAX_RESULT);
+                }
+
+                Some(result)
             }
         }
 
@@ -2862,15 +2888,11 @@ macro_rules! int_impl {
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline]
+        #[track_caller]
         pub const fn isqrt(self) -> Self {
-            // I would like to implement it as
-            // ```
-            // self.checked_isqrt().expect("argument of integer square root must be non-negative")
-            // ```
-            // but `expect` is not yet stable as a `const fn`.
             match self.checked_isqrt() {
                 Some(sqrt) => sqrt,
-                None => panic!("argument of integer square root must be non-negative"),
+                None => crate::num::int_sqrt::panic_for_negative_argument(),
             }
         }
 

library/core/src/num/int_sqrt.rs

@@ -0,0 +1,316 @@
+//! These functions use the [Karatsuba square root algorithm][1] to compute the
+//! [integer square root](https://en.wikipedia.org/wiki/Integer_square_root)
+//! for the primitive integer types.
+//!
+//! The signed integer functions can only handle **nonnegative** inputs, so
+//! that must be checked before calling those.
+//!
+//! [1]: <https://web.archive.org/web/20230511212802/https://inria.hal.science/inria-00072854v1/file/RR-3805.pdf>
+//! "Paul Zimmermann. Karatsuba Square Root. \[Research Report\] RR-3805,
+//! INRIA. 1999, pp.8. (inria-00072854)"
+
+/// This array stores the [integer square roots](
+/// https://en.wikipedia.org/wiki/Integer_square_root) and remainders of each
+/// [`u8`](prim@u8) value. For example, `U8_ISQRT_WITH_REMAINDER[17]` will be
+/// `(4, 1)` because the integer square root of 17 is 4 and because 17 is 1
+/// higher than 4 squared.
+const U8_ISQRT_WITH_REMAINDER: [(u8, u8); 256] = {
+    let mut result = [(0, 0); 256];
+
+    let mut n: usize = 0;
+    let mut isqrt_n: usize = 0;
+    while n < result.len() {
+        result[n] = (isqrt_n as u8, (n - isqrt_n.pow(2)) as u8);
+
+        n += 1;
+        if n == (isqrt_n + 1).pow(2) {
+            isqrt_n += 1;
+        }
+    }
+
+    result
+};
+
+/// Returns the [integer square root](
+/// https://en.wikipedia.org/wiki/Integer_square_root) of any [`u8`](prim@u8)
+/// input.
+#[must_use = "this returns the result of the operation, \
+              without modifying the original"]
+#[inline]
+pub const fn u8(n: u8) -> u8 {
+    U8_ISQRT_WITH_REMAINDER[n as usize].0
+}
+
+/// Generates an `i*` function that returns the [integer square root](
+/// https://en.wikipedia.org/wiki/Integer_square_root) of any **nonnegative**
+/// input of a specific signed integer type.
+macro_rules! signed_fn {
+    ($SignedT:ident, $UnsignedT:ident) => {
+        /// Returns the [integer square root](
+        /// https://en.wikipedia.org/wiki/Integer_square_root) of any
+        /// **nonnegative**
+        #[doc = concat!("[`", stringify!($SignedT), "`](prim@", stringify!($SignedT), ")")]
+        /// input.
+        ///
+        /// # Safety
+        ///
+        /// This results in undefined behavior when the input is negative.
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const unsafe fn $SignedT(n: $SignedT) -> $SignedT {
+            debug_assert!(n >= 0, "Negative input inside `isqrt`.");
+            $UnsignedT(n as $UnsignedT) as $SignedT
+        }
+    };
+}
+
+signed_fn!(i8, u8);
+signed_fn!(i16, u16);
+signed_fn!(i32, u32);
+signed_fn!(i64, u64);
+signed_fn!(i128, u128);
+
+/// Generates a `u*` function that returns the [integer square root](
+/// https://en.wikipedia.org/wiki/Integer_square_root) of any input of
+/// a specific unsigned integer type.
+macro_rules! unsigned_fn {
+    ($UnsignedT:ident, $HalfBitsT:ident, $stages:ident) => {
+        /// Returns the [integer square root](
+        /// https://en.wikipedia.org/wiki/Integer_square_root) of any
+        #[doc = concat!("[`", stringify!($UnsignedT), "`](prim@", stringify!($UnsignedT), ")")]
+        /// input.
+        #[must_use = "this returns the result of the operation, \
+                      without modifying the original"]
+        #[inline]
+        pub const fn $UnsignedT(mut n: $UnsignedT) -> $UnsignedT {
+            if n <= <$HalfBitsT>::MAX as $UnsignedT {
+                $HalfBitsT(n as $HalfBitsT) as $UnsignedT
+            } else {
+                // The normalization shift satisfies the Karatsuba square root
+                // algorithm precondition "a₃ ≥ b/4" where a₃ is the most
+                // significant quarter of `n`'s bits and b is the number of
+                // values that can be represented by that quarter of the bits.
+                //
+                // b/4 would then be all 0s except the second most significant
+                // bit (010...0) in binary. Since a₃ must be at least b/4, a₃'s
+                // most significant bit or its neighbor must be a 1. Since a₃'s
+                // most significant bits are `n`'s most significant bits, the
+                // same applies to `n`.
+                //
+                // The reason to shift by an even number of bits is because an
+                // even number of bits produces the square root shifted to the
+                // left by half of the normalization shift:
+                //
+                // sqrt(n << (2 * p))
+                // sqrt(2.pow(2 * p) * n)
+                // sqrt(2.pow(2 * p)) * sqrt(n)
+                // 2.pow(p) * sqrt(n)
+                // sqrt(n) << p
+                //
+                // Shifting by an odd number of bits leaves an ugly sqrt(2)
+                // multiplied in:
+                //
+                // sqrt(n << (2 * p + 1))
+                // sqrt(2.pow(2 * p + 1) * n)
+                // sqrt(2 * 2.pow(2 * p) * n)
+                // sqrt(2) * sqrt(2.pow(2 * p)) * sqrt(n)
+                // sqrt(2) * 2.pow(p) * sqrt(n)
+                // sqrt(2) * (sqrt(n) << p)
+                const EVEN_MAKING_BITMASK: u32 = !1;
+                let normalization_shift = n.leading_zeros() & EVEN_MAKING_BITMASK;
+                n <<= normalization_shift;
+
+                let s = $stages(n);
+
+                let denormalization_shift = normalization_shift >> 1;
+                s >> denormalization_shift
+            }
+        }
+    };
+}
+
+/// Generates the first stage of the computation after normalization.
+///
+/// # Safety
+///
+/// `$n` must be nonzero.
+macro_rules! first_stage {
+    ($original_bits:literal, $n:ident) => {{
+        debug_assert!($n != 0, "`$n` is  zero in `first_stage!`.");
+
+        const N_SHIFT: u32 = $original_bits - 8;
+        let n = $n >> N_SHIFT;
+
+        let (s, r) = U8_ISQRT_WITH_REMAINDER[n as usize];
+
+        // Inform the optimizer that `s` is nonzero. This will allow it to
+        // avoid generating code to handle division-by-zero panics in the next
+        // stage.
+        //
+        // SAFETY: If the original `$n` is zero, the top of the `unsigned_fn`
+        // macro recurses instead of continuing to this point, so the original
+        // `$n` wasn't a 0 if we've reached here.
+        //
+        // Then the `unsigned_fn` macro normalizes `$n` so that at least one of
+        // its two most-significant bits is a 1.
+        //
+        // Then this stage puts the eight most-significant bits of `$n` into
+        // `n`. This means that `n` here has at least one 1 bit in its two
+        // most-significant bits, making `n` nonzero.
+        //
+        // `U8_ISQRT_WITH_REMAINDER[n as usize]` will give a nonzero `s` when
+        // given a nonzero `n`.
+        unsafe { crate::hint::assert_unchecked(s != 0) };
+        (s, r)
+    }};
+}
+
+/// Generates a middle stage of the computation.
+///
+/// # Safety
+///
+/// `$s` must be nonzero.
+macro_rules! middle_stage {
+    ($original_bits:literal, $ty:ty, $n:ident, $s:ident, $r:ident) => {{
+        debug_assert!($s != 0, "`$s` is  zero in `middle_stage!`.");
+
+        const N_SHIFT: u32 = $original_bits - <$ty>::BITS;
+        let n = ($n >> N_SHIFT) as $ty;
+
+        const HALF_BITS: u32 = <$ty>::BITS >> 1;
+        const QUARTER_BITS: u32 = <$ty>::BITS >> 2;
+        const LOWER_HALF_1_BITS: $ty = (1 << HALF_BITS) - 1;
+        const LOWEST_QUARTER_1_BITS: $ty = (1 << QUARTER_BITS) - 1;
+
+        let lo = n & LOWER_HALF_1_BITS;
+        let numerator = (($r as $ty) << QUARTER_BITS) | (lo >> QUARTER_BITS);
+        let denominator = ($s as $ty) << 1;
+        let q = numerator / denominator;
+        let u = numerator % denominator;
+
+        let mut s = ($s << QUARTER_BITS) as $ty + q;
+        let (mut r, overflow) =
+            ((u << QUARTER_BITS) | (lo & LOWEST_QUARTER_1_BITS)).overflowing_sub(q * q);
+        if overflow {
+            r = r.wrapping_add(2 * s - 1);
+            s -= 1;
+        }
+
+        // Inform the optimizer that `s` is nonzero. This will allow it to
+        // avoid generating code to handle division-by-zero panics in the next
+        // stage.
+        //
+        // SAFETY: If the original `$n` is zero, the top of the `unsigned_fn`
+        // macro recurses instead of continuing to this point, so the original
+        // `$n` wasn't a 0 if we've reached here.
+        //
+        // Then the `unsigned_fn` macro normalizes `$n` so that at least one of
+        // its two most-significant bits is a 1.
+        //
+        // Then these stages take as many of the most-significant bits of `$n`
+        // as will fit in this stage's type. For example, the stage that
+        // handles `u32` deals with the 32 most-significant bits of `$n`. This
+        // means that each stage has at least one 1 bit in `n`'s two
+        // most-significant bits, making `n` nonzero.
+        //
+        // Then this stage will produce the correct integer square root for
+        // that `n` value. Since `n` is nonzero, `s` will also be nonzero.
+        unsafe { crate::hint::assert_unchecked(s != 0) };
+        (s, r)
+    }};
+}
+
+/// Generates the last stage of the computation before denormalization.
+///
+/// # Safety
+///
+/// `$s` must be nonzero.
+macro_rules! last_stage {
+    ($ty:ty, $n:ident, $s:ident, $r:ident) => {{
+        debug_assert!($s != 0, "`$s` is  zero in `last_stage!`.");
+
+        const HALF_BITS: u32 = <$ty>::BITS >> 1;
+        const QUARTER_BITS: u32 = <$ty>::BITS >> 2;
+        const LOWER_HALF_1_BITS: $ty = (1 << HALF_BITS) - 1;
+
+        let lo = $n & LOWER_HALF_1_BITS;
+        let numerator = (($r as $ty) << QUARTER_BITS) | (lo >> QUARTER_BITS);
+        let denominator = ($s as $ty) << 1;
+
+        let q = numerator / denominator;
+        let mut s = ($s << QUARTER_BITS) as $ty + q;
+        let (s_squared, overflow) = s.overflowing_mul(s);
+        if overflow || s_squared > $n {
+            s -= 1;
+        }
+        s
+    }};
+}
+
+/// Takes the normalized [`u16`](prim@u16) input and gets its normalized
+/// [integer square root](https://en.wikipedia.org/wiki/Integer_square_root).
+///
+/// # Safety
+///
+/// `n` must be nonzero.
+#[inline]
+const fn u16_stages(n: u16) -> u16 {
+    let (s, r) = first_stage!(16, n);
+    last_stage!(u16, n, s, r)
+}
+
+/// Takes the normalized [`u32`](prim@u32) input and gets its normalized
+/// [integer square root](https://en.wikipedia.org/wiki/Integer_square_root).
+///
+/// # Safety
+///
+/// `n` must be nonzero.
+#[inline]
+const fn u32_stages(n: u32) -> u32 {
+    let (s, r) = first_stage!(32, n);
+    let (s, r) = middle_stage!(32, u16, n, s, r);
+    last_stage!(u32, n, s, r)
+}
+
+/// Takes the normalized [`u64`](prim@u64) input and gets its normalized
+/// [integer square root](https://en.wikipedia.org/wiki/Integer_square_root).
+///
+/// # Safety
+///
+/// `n` must be nonzero.
+#[inline]
+const fn u64_stages(n: u64) -> u64 {
+    let (s, r) = first_stage!(64, n);
+    let (s, r) = middle_stage!(64, u16, n, s, r);
+    let (s, r) = middle_stage!(64, u32, n, s, r);
+    last_stage!(u64, n, s, r)
+}
+
+/// Takes the normalized [`u128`](prim@u128) input and gets its normalized
+/// [integer square root](https://en.wikipedia.org/wiki/Integer_square_root).
+///
+/// # Safety
+///
+/// `n` must be nonzero.
+#[inline]
+const fn u128_stages(n: u128) -> u128 {
+    let (s, r) = first_stage!(128, n);
+    let (s, r) = middle_stage!(128, u16, n, s, r);
+    let (s, r) = middle_stage!(128, u32, n, s, r);
+    let (s, r) = middle_stage!(128, u64, n, s, r);
+    last_stage!(u128, n, s, r)
+}
+
+unsigned_fn!(u16, u8, u16_stages);
+unsigned_fn!(u32, u16, u32_stages);
+unsigned_fn!(u64, u32, u64_stages);
+unsigned_fn!(u128, u64, u128_stages);
+
+/// Instantiate this panic logic once, rather than for all the isqrt methods
+/// on every single primitive type.
+#[cold]
+#[track_caller]
+pub const fn panic_for_negative_argument() -> ! {
+    panic!("argument of integer square root cannot be negative")
+}

library/core/src/num/mod.rs

@@ -41,6 +41,7 @@ mod uint_macros; // import uint_impl!
 
 mod error;
 mod int_log10;
+mod int_sqrt;
 mod nonzero;
 mod overflow_panic;
 mod saturating;