Merge #438: Use -0.0 as the neutral additive float

-0.0 +  0.0 is  0.0
-0.0 + -0.0 is -0.0

Thus, the float additive-zero is -0.0, not its positive cousin.
This aligns with a recent change to the impl of Sum for floats,
in rust-lang/rust@4908188

Author: workingjubilee

crates/core_simd/src/simd/num/float.rs

@@ -419,7 +419,7 @@ macro_rules! impl_trait {
                     self.as_array().iter().sum()
                 } else {
                     // Safety: `self` is a float vector
-                    unsafe { core::intrinsics::simd::simd_reduce_add_ordered(self, 0.) }
+                    unsafe { core::intrinsics::simd::simd_reduce_add_ordered(self, -0.) }
                 }
             }
 

crates/core_simd/tests/ops_macros.rs

@@ -527,6 +527,9 @@ macro_rules! impl_float_tests {
                 }
 
                 fn is_normal<const LANES: usize>() {
+                    // Arm v7 Neon violates float opsem re: subnormals, see
+                    // https://github.com/rust-lang/portable-simd/issues/439
+                    #[cfg(not(target_arch = "arm"))]
                     test_helpers::test_unary_mask_elementwise(
                         &Vector::<LANES>::is_normal,
                         &Scalar::is_normal,
@@ -535,6 +538,9 @@ macro_rules! impl_float_tests {
                 }
 
                 fn is_subnormal<const LANES: usize>() {
+                    // Arm v7 Neon violates float opsem re: subnormals, see
+                    // https://github.com/rust-lang/portable-simd/issues/439
+                    #[cfg(not(target_arch = "arm"))]
                     test_helpers::test_unary_mask_elementwise(
                         &Vector::<LANES>::is_subnormal,
                         &Scalar::is_subnormal,

rust-toolchain.toml

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "nightly-2024-06-13"
+channel = "nightly-2024-09-11"
 components = ["rustfmt", "clippy", "miri", "rust-src"]