llvm dev - Feb 2015 - [LLVMdev] Reassociate and Canonicalization of Expressions

>> 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.
>>
>> 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.

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

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.
>>
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>

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.
>>> 
>>> 
>>> 
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> 
>> 
> 
>

> On Feb 4, 2015, at 11:30 AM, Mehdi Amini <mehdi.amini at apple.com>
wrote:
> 
>> 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.
FWIW, I believe the term you’re looking for is idempotent.

—Owen
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> 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.
IMO, I think these are reasonable expectations and if you can enforce
these expectations for the common cases, great.  However, if it doubles
your effort to get the remaining 1% exposed by fuzzing, then I don't think
it would be worth your time.

Personally, I would like to invest more time in adding support for vector
operations (I had a patch up on Phabricator at one point).  I'm sure there
are also lots of other types of simplifications that could be added to
this pass.

Just my opinion though.. :)
>>
>> 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.
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>>
>>
>
>