[LLVMdev] GlobalsModRef (and thus LTO) is completely broken

[Chandler Carruth]

On Wed, Jul 15, 2015 at 2:44 AM Chandler Carruth <chandlerc at gmail.com>
wrote:

> I've fixed the obvious bugs I spotted in r242281. These should be pure
> correctness improvements.
>
> I've sent the two patches I'm imagining to address the core issue here:
> http://reviews.llvm.org/D11213
> http://reviews.llvm.org/D11214
>
> Currently, I have the unsafe alias results disabled by default, but with a
> flag that can re-enable them if needed. I don't feel really strongly about
> which way the default is set -- but that may be because I don't have lots
> of users relying on LTO. I'll let others indicate which way they would be
> most comfortable.
>
> Some IRC conversations indicated that early benchmark results with GMR
> completely disabled weren't showing really significant swings, so maybe
> this relatively small reduction in power of GMR won't be too problematic
> for folks. Either way, I'm open to the different approaches. It's D11214
> that I care a lot about. =]
>

Just FYI, I'm landing these patches based on reviews and some initial
benchmarking. There is one report of some trouble on this thread though
that after investigation may need further tweaks though. Just wanted to
close the loop that this is moving forward based on the general sentiment
of the thread. Happy to keep discussing and revising if more concerns come
up!

-Chandler


>
>
> Thanks for all the thoughts here!
> -Chandler
>
> On Tue, Jul 14, 2015 at 11:25 PM Chandler Carruth <chandlerc at gmail.com>
> wrote:
>
>> Replying here, but several of the questions raised boil down to "couldn't
>> you make the usage of GetUnderlyingObject conservatively correct?". I'll
>> try and address that.
>>
>> I think this *is* the right approach, but I think it is very hard to do
>> without effectively disabling this part of GlobalsModRef. That is, the easy
>> ways are likely to fire very frequently IMO.
>>
>> The core idea is to detect a "no information" state coming out of
>> GetUnderlyingObject (likely be providing a custom version just for
>> GlobalsModRef and tailored to its use cases). This is particularly
>> effective at avoiding the problems with the recursion limit. But let's look
>> at what cases we *wouldn't* return that. Here are the cases I see when I
>> thought about this last night with Hal, roughly in descending likelihood I
>> would guess:
>>
>> 1) We detect some global or an alloca. In that case, even BasicAA would
>> be sufficient to provide no-alias. GMR shouldn't be relevant.
>>
>> 2) We detect a phi, select, or inttoptr, and stop there.
>>
>> 3) We detect a load and stop there.
>>
>> 4) We detect a return from a function.
>>
>> 5) We detect an argument to the function.
>>
>> I strongly suspect the vast majority of queries hit #1. That's why
>> BasicAA is *so* effective. Both #4 and #5 I think are actually reasonable
>> places for GMR to potentially say "no-alias" and provide useful definitive
>> information. But I also suspect these are the least common.
>>
>> So let's look at #2 and #3 because I think they're interesting. For
>> these, I think it is extremely hard to return "no-alias". It seems
>> extremely easy for a reasonable and innocuous change to the IR to introduce
>> a phi or a select into one side of the GetUnderlyingObject but not the
>> other. If that ever happens, we can't return "no-alias" for #2, or we need
>> to add really expensive updates. It also seems reasonable (if not as
>> likely) to want adding a store and load to the IR to not trigger a
>> miscompile. If it is possible for a valid optimization pass to do reg2mem
>> on an SSA value, then that could happen to only one side of the paired
>> GetUnderlyingObject and  break GMR with #3. If that seems like an
>> unreasonable thing to do, consider loop re-rolling or other transformations
>> which may need to take things in SSA form at move them out of SSA form.
>> Even if we then try immediately to put it back *into* SSA form, before we
>> do that we create a point where GMR cannot correctly return no-alias.
>>
>> So ultimately, I don't think we want to rely on GMR returning "no-alias"
>> for either #2 or #3 because of the challenge of actually updating it in all
>> of the circumstances that could break them. That means that *only* #4 and
>> #5 are going to return "no-alias" usefully. And even then, function
>> inlining and function outlining both break #4 and #5, so you have to
>> preclude those transforms while GMR is active. And I have serious doubts
>> about these providing enough value to be worth the cost.
>>
>>
>> I think the better way to approach this is the other way around. Rather
>> than doing a backwards analysis to see if one location reaches and global
>> and the other location doesn't reach a global, I think it would be much
>> more effective to re-cast this as a forward analysis that determines all
>> the memory locations in a function that come from outside the function, and
>> use that to drive the no-alias responses.
>>
>>
>> On Tue, Jul 14, 2015 at 12:12 PM Gerolf Hoflehner <ghoflehner at apple.com>
>> wrote:
>>
>>> I wouldn’t be willing to give up performance for hypothetical issues.
>>> Please protect all your changes with options. For some of your concerns it
>>> is probably hard to provide a test case that shows an/the actual issue.
>>>
>>
>> I certainly agree that it will be very hard to provide a test case and
>> extremely rare to see this in the wild for most of these issues. As long as
>> I can remove the problematic update API we currently have (which as its an
>> API change can't really be put behind flags), I'm happy to have flags
>> control whether or not GMR uses the unsound / stale information to try to
>> answer alias queries. Do you have any opinion about what the default value
>> of the flags should be?
>>
>> I'll go ahead and prepare the patches, as it seems like we're all ending
>> up in the same position, and just wondering about the precise tradeoffs we
>> want to settle on.
>>
>>
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