Rollup merge of rust-lang#146225 - Jules-Bertholet:simplify-float-midpoint, r=tgross35

Simplify `{f16, f32, f64, f128}::midpoint()`

`(float_ty::MAX / 2) - (float_ty::MIN_POSITIVE * 2)` equals `(float_ty::MAX / 2) + (float_ty::MIN_POSITIVE * 2)` equals `(float_ty::MAX / 2)`. So these branches are pointless.

CC `@Urgau` who wrote the original implementation in rust-lang#92048; does this seem right?

`@rustbot` label A-floating-point

Author: fmease

core/src/num/f128.rs

@@ -832,7 +832,6 @@ impl f128 {
     #[unstable(feature = "f128", issue = "116909")]
     #[rustc_const_unstable(feature = "f128", issue = "116909")]
     pub const fn midpoint(self, other: f128) -> f128 {
-        const LO: f128 = f128::MIN_POSITIVE * 2.;
         const HI: f128 = f128::MAX / 2.;
 
         let (a, b) = (self, other);
@@ -842,14 +841,7 @@ impl f128 {
         if abs_a <= HI && abs_b <= HI {
             // Overflow is impossible
             (a + b) / 2.
-        } else if abs_a < LO {
-            // Not safe to halve `a` (would underflow)
-            a + (b / 2.)
-        } else if abs_b < LO {
-            // Not safe to halve `b` (would underflow)
-            (a / 2.) + b
         } else {
-            // Safe to halve `a` and `b`
             (a / 2.) + (b / 2.)
         }
     }

core/src/num/f16.rs

@@ -820,7 +820,6 @@ impl f16 {
     #[unstable(feature = "f16", issue = "116909")]
     #[rustc_const_unstable(feature = "f16", issue = "116909")]
     pub const fn midpoint(self, other: f16) -> f16 {
-        const LO: f16 = f16::MIN_POSITIVE * 2.;
         const HI: f16 = f16::MAX / 2.;
 
         let (a, b) = (self, other);
@@ -830,14 +829,7 @@ impl f16 {
         if abs_a <= HI && abs_b <= HI {
             // Overflow is impossible
             (a + b) / 2.
-        } else if abs_a < LO {
-            // Not safe to halve `a` (would underflow)
-            a + (b / 2.)
-        } else if abs_b < LO {
-            // Not safe to halve `b` (would underflow)
-            (a / 2.) + b
         } else {
-            // Safe to halve `a` and `b`
             (a / 2.) + (b / 2.)
         }
     }

core/src/num/f32.rs

@@ -1025,7 +1025,6 @@ impl f32 {
                 ((self as f64 + other as f64) / 2.0) as f32
             }
             _ => {
-                const LO: f32 = f32::MIN_POSITIVE * 2.;
                 const HI: f32 = f32::MAX / 2.;
 
                 let (a, b) = (self, other);
@@ -1035,14 +1034,7 @@ impl f32 {
                 if abs_a <= HI && abs_b <= HI {
                     // Overflow is impossible
                     (a + b) / 2.
-                } else if abs_a < LO {
-                    // Not safe to halve `a` (would underflow)
-                    a + (b / 2.)
-                } else if abs_b < LO {
-                    // Not safe to halve `b` (would underflow)
-                    (a / 2.) + b
                 } else {
-                    // Safe to halve `a` and `b`
                     (a / 2.) + (b / 2.)
                 }
             }

core/src/num/f64.rs

@@ -1026,7 +1026,6 @@ impl f64 {
     #[stable(feature = "num_midpoint", since = "1.85.0")]
     #[rustc_const_stable(feature = "num_midpoint", since = "1.85.0")]
     pub const fn midpoint(self, other: f64) -> f64 {
-        const LO: f64 = f64::MIN_POSITIVE * 2.;
         const HI: f64 = f64::MAX / 2.;
 
         let (a, b) = (self, other);
@@ -1036,14 +1035,7 @@ impl f64 {
         if abs_a <= HI && abs_b <= HI {
             // Overflow is impossible
             (a + b) / 2.
-        } else if abs_a < LO {
-            // Not safe to halve `a` (would underflow)
-            a + (b / 2.)
-        } else if abs_b < LO {
-            // Not safe to halve `b` (would underflow)
-            (a / 2.) + b
         } else {
-            // Safe to halve `a` and `b`
             (a / 2.) + (b / 2.)
         }
     }