[Bug tree-optimization/23619] Missed pre opportunity

[dberlin at dberlin dot org]

------- Additional Comments From dberlin at gcc dot gnu dot org  2005-08-29 15:23 -------
Subject: Re:  Missed pre opportunity


> Disabling reassoc, it knows that p + q is antic over the backedge.
> Which means it knows it *could* insert it.

Okay, it determines it's not fully available in block 6, even though
both predecessors of 6 generate it, because neither predecessor strictly
dominates block 6, and nowhere else that generates the expression does.

This is a partial partial case that i never bothered handling because it
just doesn't happen often in practice, AFAICT.

We can do it, and i know how, it just was never worth it.

We'd have to generate a no-cost phi in block 6 to merge the two
available values.

Let me see if i can change compute_avail to do this for us so we don't
have to worry about it later.




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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23619