PRE (sometimes) confuses ivopts/scev?

[Daniel Berlin]

On Mon, 24 Jan 2005, Richard Guenther wrote:

> Hi!
>
> Are there known bad interaction PRs wrt PRE and loop optimizations?
> I found only PR18848 for PRE problems.  Sometimes a lot better code
> is created with -fno-tree-pre -- SCEV seems to be confused
> and produces (for only extra IVs?) scev_not_known with PRE, but not
> without, i.e. some uses are not detected?

Yes
This is the wraparound variable problem.
I have fixes for PRE to stop it from creating induction variables.
Sebastian has improvements to SCEV that let it handle the type of 
induction variables PRE was creating (wraparound variables).

Both patches are queued for 4.1