On Wed, Apr 12, 2023 at 11:12:17AM +0200, Richard Biener wrote:
> > 108139 fixed this by not evaluating any equivalences if the equivalence
> > was the LHS.
> >
> > What it missed, was it possible we are calculating the range of a_3.
> > b_2 is not defined in a phi node, so it happily used the equivalence.
> > This PR demonstrates that we can't always use that equivlence either
> > without more context. There can be places in the IL where a_3 is used,
> > but b_2 has moved to a new value within a loop.
>
> I think that's only possible when b_2 flows in via a backedge (from BB3).
> So isn't this all about backedges? Indeed creating equivalences across
Apparently backedges and undefined phi arg, without that it doesn't seem
to trigger because then it isn't considered equivalent even if the two have
same range.
> backedges is futile with SSA. I think ranger requires dominators, so
> to have the above scenario - a_3 used after the b_2 definition - requires
> BB3 to be dominated by the a_3 definition which is what you could check.
Would be nice.
Though, I'm afraid it still wouldn't fix the PR101912 testcase, because
it has exactly what happens in this PR, undefined phi arg from the
pre-header and uses of the previous iteration's value (i.e. across
backedge).
Jakub
