How unsafe are unsafeThawArray# and unsafeFreezeArray#

[Johan Tibell]

Hi,

I just ran across some code that calls unsafeThawArray#, writeArray#,
and unsafeFreezeArray#, in that order. How unsafe is that?

 * Is it unsafe in the sense that if someone has a reference to the
original Array# they will see the value of that pure array change?

 * Is it unsafe in the sense things will crash and burn?

I looked at the implementation of unsafeFreezeArray#, which is

    emitPrimOp [res] UnsafeFreezeArrayOp [arg] _
       = stmtsC [ setInfo arg (CmmLit (CmmLabel mkMAP_FROZEN_infoLabel)),
           CmmAssign (CmmLocal res) arg ]

but I couldn't find any implementation of unsafeThawArray#. Is it a
no-op? It seems to me that if unsafeFreezeArray# changes the head of
the array heap object so should unsafeThawArray#.

Cheers,
Johan