WordPress save_post hook not firing when checking if _GET[‘post’] is set

How is $_GET['post'] set ? And why are you checking it?

It could be that you’re adding the save_post call too late.

Try:

function foo() {
    die('Saving post');
}

function foo2() {
    if( isset($_GET['post']) ) 
        foo();
}
add_action( 'save_post', 'foo2' );