"How does common subexpression elimination work?"

It answered it correctly and gave some basic code examples to demonstrate the concept. Then I followed up with:

"But can it do the elimination if the variables are flipped, but semantically equivalent, like 'y + x' in the example above?"

It again gave what I would consider a correct answer. Note 'x + y' was what was eliminated previously, so I reversed it here.

Then I asked: "Are there cases where the compiler might fail to eliminate common subexpressions?"

And again a good answer.

Now all of this could be found on the web somewhere, but for example the second question didn't show an obvious answer on Google when I searched for it. I'm sure I could find it, but I know this field well. If I was someone new to the field I'd probably spend a lot of time parsing useless articles to find something that answered the question in a way I could understand.

I'm less concerned about it writing code (which is cool). For me, the ability to help me learn an area quickly is far more useful. It doesn't need novel answers, but the ability to understand what I'm asking and answer it. I think it's really close to being able to do this now.