Rollup merge of #139186 - TDecking:float, r=workingjubilee

Refactor `diy_float`

The refactor replaces bespoke algorithms with functions already inside the standard library, improving both codegen and readability.

Author: GuillaumeGomez

library/core/src/num/diy_float.rs

@@ -21,61 +21,29 @@ pub struct Fp {
 
 impl Fp {
     /// Returns a correctly rounded product of itself and `other`.
-    pub fn mul(&self, other: &Fp) -> Fp {
-        const MASK: u64 = 0xffffffff;
-        let a = self.f >> 32;
-        let b = self.f & MASK;
-        let c = other.f >> 32;
-        let d = other.f & MASK;
-        let ac = a * c;
-        let bc = b * c;
-        let ad = a * d;
-        let bd = b * d;
-        let tmp = (bd >> 32) + (ad & MASK) + (bc & MASK) + (1 << 31) /* round */;
-        let f = ac + (ad >> 32) + (bc >> 32) + (tmp >> 32);
+    pub fn mul(self, other: Self) -> Self {
+        let (lo, hi) = self.f.widening_mul(other.f);
+        let f = hi + (lo >> 63) /* round */;
         let e = self.e + other.e + 64;
-        Fp { f, e }
+        Self { f, e }
     }
 
     /// Normalizes itself so that the resulting mantissa is at least `2^63`.
-    pub fn normalize(&self) -> Fp {
-        let mut f = self.f;
-        let mut e = self.e;
-        if f >> (64 - 32) == 0 {
-            f <<= 32;
-            e -= 32;
-        }
-        if f >> (64 - 16) == 0 {
-            f <<= 16;
-            e -= 16;
-        }
-        if f >> (64 - 8) == 0 {
-            f <<= 8;
-            e -= 8;
-        }
-        if f >> (64 - 4) == 0 {
-            f <<= 4;
-            e -= 4;
-        }
-        if f >> (64 - 2) == 0 {
-            f <<= 2;
-            e -= 2;
-        }
-        if f >> (64 - 1) == 0 {
-            f <<= 1;
-            e -= 1;
-        }
+    pub fn normalize(self) -> Self {
+        let lz = self.f.leading_zeros();
+        let f = self.f << lz;
+        let e = self.e - lz as i16;
         debug_assert!(f >= (1 << 63));
-        Fp { f, e }
+        Self { f, e }
     }
 
     /// Normalizes itself to have the shared exponent.
     /// It can only decrease the exponent (and thus increase the mantissa).
-    pub fn normalize_to(&self, e: i16) -> Fp {
+    pub fn normalize_to(self, e: i16) -> Self {
         let edelta = self.e - e;
         assert!(edelta >= 0);
         let edelta = edelta as usize;
         assert_eq!(self.f << edelta >> edelta, self.f);
-        Fp { f: self.f << edelta, e }
+        Self { f: self.f << edelta, e }
     }
 }

library/core/src/num/flt2dec/strategy/grisu.rs

@@ -196,9 +196,9 @@ pub fn format_shortest_opt<'a>(
     let (minusk, cached) = cached_power(ALPHA - plus.e - 64, GAMMA - plus.e - 64);
 
     // scale fps. this gives the maximal error of 1 ulp (proved from Theorem 5.1).
-    let plus = plus.mul(&cached);
-    let minus = minus.mul(&cached);
-    let v = v.mul(&cached);
+    let plus = plus.mul(cached);
+    let minus = minus.mul(cached);
+    let v = v.mul(cached);
     debug_assert_eq!(plus.e, minus.e);
     debug_assert_eq!(plus.e, v.e);
 
@@ -480,7 +480,7 @@ pub fn format_exact_opt<'a>(
     // normalize and scale `v`.
     let v = Fp { f: d.mant, e: d.exp }.normalize();
     let (minusk, cached) = cached_power(ALPHA - v.e - 64, GAMMA - v.e - 64);
-    let v = v.mul(&cached);
+    let v = v.mul(cached);
 
     // divide `v` into integral and fractional parts.
     let e = -v.e as usize;