[LLVMdev] Reassociate and Canonicalization of Expressions

[Mehdi Amini]

Hi Chad,

Thanks for you answer.

> On Feb 4, 2015, at 11:22 AM, Chad Rosier <mcrosier at codeaurora.org> wrote:
> 
>>> On Feb 2, 2015, at 11:12 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:
>>> 
>>> Hi,
>>> 
>>> I encountered some bugs in Reassociate [1] where we are hitting some
>>> assertions:
>>> 
>>>   assert(!Duplicates.count(Factor) &&
>>>              "Shouldn't have two constant factors, missed a
>>> canonicalize");
>>>   assert(NumAddedValues > 1 && "Each occurrence should contribute a
>>> value”);
>>> 
>>> My understanding is that these assertions enforce that when processing
>>> an expression tree, we expect that the nodes have already been
>>> canonicalized by Reassociate.
>>> 
>>> I infer that there should be *one* canonicalization for a function and
>>> it should be deterministic, i.e. if I run Reassociate two times I expect
>>> that the second one does not make any change.
> 
> I don't think deterministic is the right word (as the pass in
> deterministic, AFAICT), but I get what you're saying.  You infer we should
> always arrive at one final solution once the pass has run.

Yes.

> 
>>> 
>>> However right now we are far from that. I have multiple patches in
>>> flight to improve the situation, but the situation is still not perfect.
>>> Before going further I’d like some clarification on the expectation of
>>> Reassociate:
>>> 
>>> - Is there one expected canonicalization in all cases?
>>> - Do we expect that running multiple times Reassociate in a row does not
>>> change the result after the first run?
>>> 
>>> If the answer is no, then I think the two assertions should be relaxed.
> 
> IMHO, I think these asserts are a good idea and we should work towards
> enforcing these assumptions.  One alternative would be to add a slew of
> test cases to ensure functionality doesn't regress and relax the asserts,
> but I'd still prefer the assertions to be in place.

I’m all for keeping the assertion, but do we agree that the answer is “yes” to my two questions above?
Enforcing a “final” result after one run can be quite involved, and I’m asking for clarification on the expectation here before spending too much time on that.

> 
> How you would propose relax the asserts?  Are the asserts hitting code in
> the wild or is it just your fuzz tests?

Only the fuzzer, so I don’t have a high priority on fixing this. But since I started, I rather finish the job.

By relaxing the assertions, I meant being tolerant to expression for which nodes have not been fully canonicalized. 
Again, I’m not suggesting we actually relax these assertions. Just that we clarify the expectation either way.

Thanks,

Mehdi

> 
> I do appreciate your work on cleaning up the pass and I certainly don't
> want to block your efforts, but again I do think these assertions are good
> in general.
> 
> Chad
> 
>>> 
>>> Bonus question: how does InstCombine behave wrt to the two questions
>>> above?
>>> 
>>> Thanks,
>>> 
>>> —
>>> Mehdi
>>> 
>>> [1] : I am stressing it with a fuzzer for a specific language based on
>>> LLVM and Reassociate is used later in the pipeline.
>>> 
>>> 
>>> 
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>> 
>> 
> 
>