SCEV bottom value

I’d like a value, call it Bottom, such that

SE->getAddExpr(Bottom, X) => Bottom
SE->getMulExpr(Bottom, X,) => Bottom
isKnownPredicate(any, Bottom, X) => false

I can write code to make NULL work like I want, but it would be simpler if something was already defined. I’m wondering about SCEV::Unknown. The documentation suggests I could perhaps use it for a “bottom” value.

Think it would work?


The documentation definitely says that SCEVUnknown is the "bottom"
value. But the semantics you have listed here are different from how
SCEVUnknown works. For example, B = A + Unknown is not Unknown, but it
is in fact an add expression. If B is an operand for another add
expression C, then it can be flattened so that A participates in any
folding happening for C. But instead, if B was the expression A +
Bottom, then B would become bottom, and so would C, and no folding will
ever be allowed.

In some sense, Bottom is the anti-thesis to Unknown. Unknown provides
a boundary that encapsulates unknown semantics, while Bottom
invalidates anything it touches!

Also, how would one uniquify Bottom expressions? Should they all be
considered identical?


Hi Preston,

I was wondering ... "Bottom" is a bit overloaded as far as terms go. Would SCEVNaN be a better name for this beast?


Sure. I just want one value that behaves as above. Since Unknown won't
do it for me, I'll just use NULL and code around the problem. Indeed,
I wrote it that way in the first place. This Bottom idea just occurred
to me as a way to simplify my implementation.


In my case, it indicates an unknown value. It's certainly a number of
some sort, but I can't prove anything about it (positive, negative,
size, etc). So I use this special indication to indicate I must make
worst-case assumptions. But, yes, I'd like it to work almost like a
floating-point NaN. The exception is that 0*magic should yield 0.