Optimize large array creation in const-eval

This changes repeated memcpy's to a memset for the case that we're
propagating a single byte into a region of memory.

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -1209,21 +1209,25 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         throw_ub_custom!(fluent::const_eval_copy_nonoverlapping_overlapping);
                     }
                 }
+            }
 
-                for i in 0..num_copies {
-                    ptr::copy(
-                        src_bytes,
-                        dest_bytes.add((size * i).bytes_usize()), // `Size` multiplication
-                        size.bytes_usize(),
-                    );
+            let size_in_bytes = size.bytes_usize();
+            // For particularly large arrays (where this is perf-sensitive) it's common that
+            // we're writing a single byte repeatedly. So, optimize that case to a memset.
+            if size_in_bytes == 1 {
+                let value = *src_bytes;
+                dest_bytes.write_bytes(value, (size * num_copies).bytes_usize());
+            } else if src_alloc_id == dest_alloc_id {
+                let mut dest_ptr = dest_bytes;
+                for _ in 0..num_copies {
+                    ptr::copy(src_bytes, dest_ptr, size_in_bytes);
+                    dest_ptr = dest_ptr.add(size_in_bytes);
                 }
             } else {
-                for i in 0..num_copies {
-                    ptr::copy_nonoverlapping(
-                        src_bytes,
-                        dest_bytes.add((size * i).bytes_usize()), // `Size` multiplication
-                        size.bytes_usize(),
-                    );
+                let mut dest_ptr = dest_bytes;
+                for _ in 0..num_copies {
+                    ptr::copy_nonoverlapping(src_bytes, dest_ptr, size_in_bytes);
+                    dest_ptr = dest_ptr.add(size_in_bytes);
                 }
             }
         }