Attempt to workaround MIPS bug

Author: calebzulawski

crates/core_simd/src/masks/full_masks.rs

@@ -112,7 +112,18 @@ macro_rules! define_mask {
                     // TODO remove the transmute when rustc is more flexible
                     assert_eq!(core::mem::size_of::<U::IntBitMask>(), core::mem::size_of::<U::BitMask>());
                     let mask: U::IntBitMask = crate::intrinsics::simd_bitmask(self.0);
-                    core::mem::transmute_copy(&mask)
+                    let mut bitmask: U::BitMask = core::mem::transmute_copy(&mask);
+
+                    // There is a bug where LLVM appears to implement this operation with the wrong
+                    // bit order.
+                    // TODO fix this in a better way
+                    if cfg!(any(target_arch = "mips", target_arch = "mips64")) {
+                        for x in bitmask.as_mut() {
+                            *x = x.reverse_bits();
+                        }
+                    }
+
+                    bitmask
                 }
             }
         }

crates/core_simd/tests/masks.rs

@@ -70,10 +70,10 @@ macro_rules! test_mask_api {
             fn to_bitmask() {
                 let values = [
                     true, false, false, true, false, false, true, false,
-                    false, false, false, false, false, false, false, false,
+                    true, true, false, false, false, false, false, true,
                 ];
                 let mask = core_simd::$name::<16>::from_array(values);
-                assert_eq!(mask.to_bitmask(), [0b01001001, 0]);
+                assert_eq!(mask.to_bitmask(), [0b01001001, 0b10000011]);
             }
         }
     }