[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")

[Hal Finkel via llvm-dev]

----- Original Message -----

> From: "Hal Finkel via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "Xinliang David Li" <xinliangli at gmail.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Friday, February 26, 2016 11:07:35 PM
> Subject: Re: [llvm-dev] Possible soundness issue with
> available_externally (split from "RFC: Add guard intrinsics")

> ----- Original Message -----

> > From: "Xinliang David Li" <xinliangli at gmail.com>
> 
> > To: "Hal Finkel" <hfinkel at anl.gov>
> 
> > Cc: "llvm-dev" <llvm-dev at lists.llvm.org>, "Philip Reames"
> > <listmail at philipreames.com>, "Duncan P. N. Exon Smith"
> > <dexonsmith at apple.com>, "Sanjoy Das"
> > <sanjoy at playingwithpointers.com>, "Chandler Carruth"
> > <chandlerc at google.com>
> 
> > Sent: Friday, February 26, 2016 10:59:40 PM
> 
> > Subject: Re: [llvm-dev] Possible soundness issue with
> > available_externally (split from "RFC: Add guard intrinsics")
> 

> > On Fri, Feb 26, 2016 at 8:57 PM, Hal Finkel < hfinkel at anl.gov >
> > wrote:
> 

> > > > From: "Xinliang David Li" < xinliangli at gmail.com >
> > > 
> > 
> 
> > > > To: "Chandler Carruth" < chandlerc at google.com >
> > > 
> > 
> 
> > > > Cc: "Hal Finkel" < hfinkel at anl.gov >, "llvm-dev" <
> > > > llvm-dev at lists.llvm.org >, "Philip Reames" <
> > > > listmail at philipreames.com >, "Duncan P. N. Exon Smith" <
> > > > dexonsmith at apple.com >, "Sanjoy Das" <
> > > > sanjoy at playingwithpointers.com >
> > > 
> > 
> 
> > > > Sent: Friday, February 26, 2016 10:48:13 PM
> > > 
> > 
> 
> > > > Subject: Re: [llvm-dev] Possible soundness issue with
> > > > available_externally (split from "RFC: Add guard intrinsics")
> > > 
> > 
> 

> > > > On Fri, Feb 26, 2016 at 8:43 PM, Chandler Carruth <
> > > > chandlerc at google.com > wrote:
> > > 
> > 
> 

> > > > > On Fri, Feb 26, 2016 at 8:40 PM Xinliang David Li <
> > > > > xinliangli at gmail.com > wrote:
> > > > 
> > > 
> > 
> 

> > > > > > On Fri, Feb 26, 2016 at 8:30 PM, Chandler Carruth <
> > > > > > chandlerc at google.com > wrote:
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > On Fri, Feb 26, 2016 at 8:15 PM Hal Finkel <
> > > > > > > hfinkel at anl.gov
> > > > > > > >
> > > > > > > wrote:
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > Can you explain your concerns about reliability? My
> > > > > > > > idea
> > > > > > > > is
> > > > > > > > that
> > > > > > > > we
> > > > > > > > have a kind of delayed-name comdat section attribute
> > > > > > > > that
> > > > > > > > says
> > > > > > > > that
> > > > > > > > the name of the comdat section into which the function
> > > > > > > > will
> > > > > > > > be
> > > > > > > > placed will be based on the hash of the contents (which
> > > > > > > > should
> > > > > > > > probably be the pre-isel IR-level contexts plus perhaps
> > > > > > > > the
> > > > > > > > other
> > > > > > > > state that might affect backend optimization). Maybe
> > > > > > > > just
> > > > > > > > using
> > > > > > > > the
> > > > > > > > MI-level function contents will work too. It seems like
> > > > > > > > this
> > > > > > > > should
> > > > > > > > be "perfect". I don't see the circumstances why
> > > > > > > > function
> > > > > > > > definitions
> > > > > > > > optimized under identical optimization settings should
> > > > > > > > differ
> > > > > > > > in
> > > > > > > > this sense.
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > Ok, let's consider Sanjoy's latest post.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > I think we could end up with this routine in N different
> > > > > > > modules
> > > > > > > with
> > > > > > > slightly different amounts of information in each. Each
> > > > > > > of
> > > > > > > these
> > > > > > > in
> > > > > > > turn could cause us to deduce a slightly different set of
> > > > > > > attributes, and thus compute a different hash.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > Now, certainly, there are a small number of variants
> > > > > > > here.
> > > > > > > Two,
> > > > > > > maybe
> > > > > > > three. But this could in the worst case cause us to have
> > > > > > > two
> > > > > > > copies
> > > > > > > of a *huge* number of functions.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > My concern here is not that this is a *theoretically*
> > > > > > > hard
> > > > > > > problem
> > > > > > > to
> > > > > > > solve. I think there are good techniques to handle this.
> > > > > > > My
> > > > > > > concern
> > > > > > > is that it will be a substantial amount of work and
> > > > > > > require
> > > > > > > a
> > > > > > > reasonable amount of testing and experimentation.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > Such data won't be too hard to find out. It can be
> > > > > > conservatively
> > > > > > estimated by looking at the final binary code for each
> > > > > > COMDAT
> > > > > > to
> > > > > > see
> > > > > > how many 'clones's are needed due to different code gen --
> > > > > > the
> > > > > > result is the upper bound as different code gen does not
> > > > > > mean
> > > > > > different deduced function attributes.
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > With PGO, the decision process will also become much
> > > > > > simpler.
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > Sure. And if you want to work on privatization in LLVM's IPO,
> > > > > that
> > > > > would be awesome.
> > > > 
> > > 
> > 
> 

> > > > > But I think it is completely unreasonable to gate checking in
> > > > > code
> > > > > to
> > > > > test for the invalid transformation until this new
> > > > > optimization
> > > > > technique is available. We should be fixing the bug now, and
> > > > > then
> > > > > figuring out how to do privatization.
> > > > 
> > > 
> > 
> 
> > > > Isn't privatization one way to get correctness? Other
> > > > techniques
> > > > such
> > > > as hash based de-dup or use PGO etc for tuning are orthognal
> > > > methods
> > > > to reduce the overall cost of the former.
> > > 
> > 
> 
> > > It is, and I agree. However, the code-size growth seems like it
> > > could
> > > easily be unacceptable even for speed-optimized builds without
> > > the
> > > kind of mitigation options mentioned.
> > 
> 

> > Except that there might be icache/itlb impact so performance is not
> > totally immune to size increase -- otherwise we may as well just
> > force inlining those :)
> 
> This is why I am hoping we can aggressively apply that kind of
> privatization. However, it might turn out not to be the case.

But to be clear, my point is that (at least when optimizing for speed), we should try this when fixing the bug. It might turn out we can't afford the size increase, but we need to make a quantitative decision on that. If we can deal with the increase, then it seems like we can then work on recovering the code-size losses using the mechanisms discussed. These might require tuning to be acceptable, or the tuning might itself be an extra nicety to further recover code size regressions. 

I'm assuming here that privatization can be accomplished simply by switching the linkage on the current function (although do we need to do something special for function pointers?), and so the implementation cost is not high. 

-Hal 

> -Hal

> > David
> 

> > > -Hal
> > 
> 

> > > > David
> > > 
> > 
> 
> > > > > -Chandler
> > > > 
> > > 
> > 
> 
> > > --
> > 
> 

> > > Hal Finkel
> > 
> 
> > > Assistant Computational Scientist
> > 
> 
> > > Leadership Computing Facility
> > 
> 
> > > Argonne National Laboratory
> > 
> 

> --

> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory

> _______________________________________________
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> llvm-dev at lists.llvm.org
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-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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