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

Fri Jul 17 12:18:11 PDT 2015

On Fri, Jul 17, 2015 at 9:13 AM Evgeny Astigeevich <
evgeny.astigeevich at arm.com> wrote:

> It’s Dhrystone.
>
Dhrystone has historically not been a good indicator of real-world
performance fluctuations, especially at this small of a shift.

I'd like to see if we see any fluctuation on larger and more realistic
application benchmarks. One advantage of the flag being set is that we
should get runs from folks who have automatic builds and runs periodically
from trunk. Those should help give an accurate picture.

>
>
> *From:* Chandler Carruth [mailto:chandlerc at gmail.com]
> *Sent:* 17 July 2015 16:10
>
>
> *To:* Evgeny Astigeevich; Chandler Carruth
> *Cc:* LLVM Developers Mailing List
>
> *Subject:* Re: [LLVMdev] GlobalsModRef (and thus LTO) is completely broken
>
>
>
> Can you say what Benchmark or give a test case so we understand the nature
> of the regression? As Gerolf said, that will be important to understand
> what is best to do.
>
>
>
> On Fri, Jul 17, 2015, 06:43 Evgeny Astigeevich <Evgeny.Astigeevich at arm.com>
> wrote:
>
> Yes, the regression is stable. I double checked this. A full benchmark run
> consists of at least 10 sub-runs to validate the score.
>
> I also checked if there were regressions of this benchmark across
> different ARM hardware versions. I found all regressions of this benchmark
> were in range 1.6%-2%.
>
>
>
> Kind regards,
>
> Evgeny Astigeevich
>
>
>
> *From:* Chandler Carruth [mailto:chandlerc at gmail.com]
> *Sent:* 17 July 2015 07:52
> *To:* Evgeny Astigeevich; Chandler Carruth
> *Cc:* LLVM Developers Mailing List; Michael Zolotukhin
>
>
> *Subject:* Re: [LLVMdev] GlobalsModRef (and thus LTO) is completely broken
>
>
>
> Hey, thanks for benchmarking.
>
>
>
> How stable is the 2% regression?
>
>
>
> Michael ran some benchmarks with GlobalsModRef completely disabled and the
> only differences were in the noise. This was a complete spec2k6 run along
> with some others. Based on the number of benchmarks run there, I'm going to
> go ahead and submit these patches, but if you can clarify the impact here,
> we can look at potentially some other tradeoff. I'm not particularly set on
> one set of defaults, etc, I just don't want to keep patches held up based
> on that. We can flip the default back and forth as new data arrives.
>
>
>
> On Thu, Jul 16, 2015 at 12:23 PM Evgeny Astigeevich <
> evgeny.astigeevich at arm.com> wrote:
>
> Hi Chandler,
>
>
>
> I ran couple benchmarks with LTO turned on and your patches on ARM
> hardware.
>
> There were no performance degradation of one benchmark and 2% slowdown of
> another benchmark.
>
>
>
> Kind regards,
>
> Evgeny Astigeevich
>
>
>
>
>
> *From:* llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] *On
> Behalf Of *Evgeny Astigeevich
> *Sent:* 15 July 2015 15:12
>
>
> *To:* 'Chandler Carruth'; Gerolf Hoflehner
> *Cc:* LLVM Developers Mailing List
> *Subject:* Re: [LLVMdev] GlobalsModRef (and thus LTO) is completely broken
>
>
>
> Hi Chandler,
>
>
>
> I would like to run some benchmarks on ARM hardware and to look at impact
> of your patches on LTO.
>
>
>
> Kind regards,
>
> Evgeny Astigeevich
>
>
>
> *From:* llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu
> <llvmdev-bounces at cs.uiuc.edu>] *On Behalf Of *Chandler Carruth
>
>
> *Sent:* 15 July 2015 10:45
> *To:* Chandler Carruth; Gerolf Hoflehner
> *Cc:* LLVM Developers Mailing List
> *Subject:* Re: [LLVMdev] GlobalsModRef (and thus LTO) is completely broken
>
>
>
> 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
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__reviews.llvm.org_D11213&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=TDTMPyN-BRKio_S9jhFxP6vHW7gAN3F73DTvS3M46go&s=TjLIFmohippEitr9aFYFcADeLJeQ-z2E_LH0fpsL38Q&e=>
>
> http://reviews.llvm.org/D11214
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__reviews.llvm.org_D11214&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=TDTMPyN-BRKio_S9jhFxP6vHW7gAN3F73DTvS3M46go&s=z8Wdu7ZruKrV5dqBOmaBxWe1aaiDWtf4it9ZI1dVweQ&e=>
>
>
>
> 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. =]
>
>
>
>
>
> 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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