Thoughts about semantics of BB_VISITED

[Diego Novillo]

Currently, individual passes are free to assume that the BB_VISITED flag 
will always be clear on every basic block before they start.

I find these semantics rather error prone, as they assume that previous 
passes will have been good citizens and cleared flags on their way out. 
  It's pretty easy to forget and it makes for potentially difficult to 
track bugs.

What's worse is that we may end up doing double work (like in PRE), 
where we first traverse all the blocks asserting that the bit is cleared 
and at the end do another loop clearing the flag.

We don't seem to have that semantics on TREE_VISITED, why do it on 
blocks then?


Thoughts?   Diego.