No. Simply providing two ways to executate with 256bit or 512bit ALUs is not enough to prevent side channels, you can still time the execution time to read the

No. Simply providing two ways to executate with 256bit or 512bit ALUs is not enough to prevent side channels, you can still time the execution time to read the side channel. The key to closing the side channel is not allowing speculated instructions to trigger the activation of the 512bit ALUs. Hold off for a few cycles until execution of those instructions is confirmed.