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

Evgeny Astigeevich
2015-07-17T13:32:44Z

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<mailto: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> [mailto: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> [mailto: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<mailto: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<mailto: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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