The problem case looks something like x = a[0] a[1] = x x = a[2] a[3] = x for (i = 0; i < N; i++) {   a[i] = ...   ... a[i] ... } It shuffles data in an array a[] using the scalar x, then loops over array a[]. This particular arrangement, with the right optimisations, will do the wrong thing with the shuffle code. The wrong thing is part of optimising the loop, so both parts are required for there to be a problem. A workaround is to use a separate scalar variable for each assignment in the shuffle, ie, "x1 = a[0]; a[1] = x1" and "x2 = a[2]; a[3] = x2".