Ensure userspace allocation is 8-byte aligned

Author: raoulstrackx

library/std/src/sys/sgx/abi/usercalls/alloc.rs

@@ -2,6 +2,7 @@
 
 use crate::arch::asm;
 use crate::cell::UnsafeCell;
+use crate::cmp;
 use crate::convert::TryInto;
 use crate::mem;
 use crate::ops::{CoerceUnsized, Deref, DerefMut, Index, IndexMut};
@@ -212,7 +213,9 @@ where
         unsafe {
             // Mustn't call alloc with size 0.
             let ptr = if size > 0 {
-                rtunwrap!(Ok, super::alloc(size, T::align_of())) as _
+                // `copy_to_userspace` is more efficient when data is 8-byte aligned
+                let alignment = cmp::max(T::align_of(), 8);
+                rtunwrap!(Ok, super::alloc(size, alignment)) as _
             } else {
                 T::align_of() as _ // dangling pointer ok for size 0
             };