switch to a whitelist based on platform

Author: RustyYato

library/core/src/num/f32.rs

@@ -1017,12 +1017,17 @@ impl f32 {
     #[unstable(feature = "num_midpoint", issue = "110840")]
     pub fn midpoint(self, other: f32) -> f32 {
         cfg_if! {
-            if #[cfg(all(target_arch = "arm", target_pointer_width = "32",
-                         not(target_feature = "vfp2")))] {
-                // some 32-bit ARM architectures don't have native double-precision floats
-                // so fall back to a similar algorithm as in f64, but using f32
-                // This should only differ in the specific NaNs reported.
-
+            if #[cfg(any(
+                    target_arch = "x86_64",
+                    target_arch = "aarch64",
+                    all(any(target_arch="riscv32", target_arch= "riscv64"), target_feature="d"),
+                    all(target_arch = "arm", target_feature="vfp2"),
+                ))] {
+                // whitelist the faster implementation to targets that have known good 64-bit float
+                // implementations. Falling back to the branchy code on targets that don't have
+                // 64-bit hardware floats or buggy implementations.
+                ((f64::from(self) + f64::from(other)) / 2.0) as f32
+            } else {
                 const LO: f32 = f32::MIN_POSITIVE * 2.;
                 const HI: f32 = f32::MAX / 2.;
 
@@ -1043,8 +1048,6 @@ impl f32 {
                     // Not safe to halve a and b
                     (a / 2.) + (b / 2.)
                 }
-            } else {
-                ((f64::from(self) + f64::from(other)) / 2.0) as f32
             }
         }
     }