diff --git a/src/libcore/ptr.rs b/src/libcore/ptr.rs
index 92470299366ce..4f118f58441c4 100644
--- a/src/libcore/ptr.rs
+++ b/src/libcore/ptr.rs
@@ -160,7 +160,10 @@ unsafe fn swap_nonoverlapping_bytes(x: *mut u8, y: *mut u8, len: usize) {
     // #[repr(simd)], even if we don't actually use this struct directly.
     //
     // FIXME repr(simd) broken on emscripten and redox
-    #[cfg_attr(not(any(target_os = "emscripten", target_os = "redox")), repr(simd))]
+    // It's also broken on big-endian powerpc64 and s390x.  #42778
+    #[cfg_attr(not(any(target_os = "emscripten", target_os = "redox",
+                       target_endian = "big")),
+               repr(simd))]
     struct Block(u64, u64, u64, u64);
     struct UnalignedBlock(u64, u64, u64, u64);