On Tue, Mar 21, 2017 at 07:43:51PM -0300, Alexandre Oliva wrote:
> When two VALUEs are recorded in the cselib equivalence table such that
> they are equivalent to each other XORed with the same expression, if
> we started a cselib equivalence test between say the odd one and the
> even one, we'd end up recursing to compare the even one with the odd
> one, and then again, indefinitely.
>
> This patch cuts off this infinite recursion by recognizing the XOR
> special case (it's the only binary commutative operation in which
> applying one of the operands of an operation to the result of that
> operation brings you back to the other operand) and determining
> whether we have an equivalence without recursing down the infinite
> path.
Is XOR the only special case though? E.g. PLUS or MINUS with
the most negative constant act the same, and I don't see why if unlucky
enough with reverse ops etc. you couldn't get something similar through
combination thereof, perhaps indirectly through multiple VALUEs.
So I think it is safer to just put a cap on the rtx_equal_for_cselib_1
recursion depth (should be enough to bump the depth only when walking
locs of a VALUE). I'll test such a patch.
Jakub
