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Re: Thoughts about semantics of BB_VISITED
- From: Daniel Berlin <dberlin at dberlin dot org>
- To: Diego Novillo <dnovillo at redhat dot com>
- Cc: gcc at gcc dot gnu dot org
- Date: Thu, 6 Jan 2005 14:50:41 -0500 (EST)
- Subject: Re: Thoughts about semantics of BB_VISITED
- References: <41DD95EA.firstname.lastname@example.org>
On Thu, 6 Jan 2005, Diego Novillo wrote:
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
I didn't quite understand why either, but Zdenek told me that this is the
way it was (he's the one that added that code in PRE).
Maybe he can tell us the reason we do this?