turn off let floating

[Bernard James POPE]

Hi all,

In the documentation for System.IO.Unsafe
it says:

   Make sure that the either you switch off let-floating, 
   or that the call to unsafePerformIO cannot float outside a lambda.  

My question is how can you turn off let floating? I can't seem to
find a flag that suggests this behaviour.

Cheers,
Bernie.