Update documentation and fix i586 inaccuracy

Author: calebzulawski

crates/core_simd/src/reduction.rs

@@ -58,22 +58,38 @@ macro_rules! impl_float_reductions {
             /// Produces the sum of the lanes of the vector.
             #[inline]
             pub fn sum(self) -> $scalar {
-                unsafe { crate::intrinsics::simd_reduce_add_ordered(self, 0.) }
+                // f32 SIMD sum is inaccurate on i586
+                if cfg!(target_arch = "i586") && core::mem::size_of::<$scalar>() == 4 {
+                    self.as_slice().iter().sum()
+                } else {
+                    unsafe { crate::intrinsics::simd_reduce_add_ordered(self, 0.) }
+                }
             }
 
             /// Produces the sum of the lanes of the vector.
             #[inline]
             pub fn product(self) -> $scalar {
-                unsafe { crate::intrinsics::simd_reduce_mul_ordered(self, 1.) }
+                // f32 SIMD product is inaccurate on i586
+                if cfg!(target_arch = "i586") && core::mem::size_of::<$scalar>() == 4 {
+                    self.as_slice().iter().product()
+                } else {
+                    unsafe { crate::intrinsics::simd_reduce_mul_ordered(self, 1.) }
+                }
             }
 
             /// Returns the maximum lane in the vector.
+            ///
+            /// Returns values based on equality, so a vector containing both `0.` and `-0.` may
+            /// return either.  This function will not return `NaN` unless all lanes are `NaN`.
             #[inline]
             pub fn max_lane(self) -> $scalar {
                 unsafe { crate::intrinsics::simd_reduce_max(self) }
             }
 
             /// Returns the minimum lane in the vector.
+            ///
+            /// Returns values based on equality, so a vector containing both `0.` and `-0.` may
+            /// return either.  This function will not return `NaN` unless all lanes are `NaN`.
             #[inline]
             pub fn min_lane(self) -> $scalar {
                 unsafe { crate::intrinsics::simd_reduce_min(self) }