[LLVMdev] question about enabling cfl-aa and collecting a57 numbers

Tue Feb 10 13:27:15 PST 2015

Sorry for the delay, i'll review these in the next few days.

On Tue Feb 10 2015 at 10:27:29 AM Hal Finkel <hfinkel at anl.gov> wrote:

> [moving to llvm-commits for patch review]
>
> ----- Original Message -----
> > From: "George Burgess IV" <george.burgess.iv at gmail.com>
> > To: "Hal J. Finkel" <hfinkel at anl.gov>
> > Cc: "Chandler Carruth" <chandlerc at google.com>, "Jiangning Liu" <
> Jiangning.Liu at arm.com>, "LLVM Developers Mailing
> > List" <llvmdev at cs.uiuc.edu>, "Daniel Berlin" <dberlin at dberlin.org>,
> "Yogesh Chavan" <cs13m1012 at iith.ac.in>
> > Sent: Monday, February 9, 2015 11:25:38 AM
> > Subject: Re: [LLVMdev] question about enabling cfl-aa and collecting a57
> numbers
> >
> >
> >
> > As promised, attached are two diffs:
> >
> >
> > cflaa-asm-bugfix.diff fixes that we crashed given InlineAsm [1], and
> > has a minor style update (remove trailing whitespace from comments)
>
> Can you please attach a test case?
>
> > cflaa-inttoptr-fix.diff fixes how we treat inttoptr/ptrtoint
> > structures. I ended up implementing this with StratifiedAttrs, and
> > will speak with Danny about his concerns with this approach
> > (hopefully) later this week. It seems to meet the goal of not
> > unifying everything, so I don’t see it being problematic. [2]
>
> Okay. I'd prefer that Danny's thoughts on this end up recorded somewhere,
> so either we can do this on-list, or someone should provide a summary.
>
> Thanks again,
> Hal
>
> >
> >
> > Patches should be applied in the order noted above.
> >
> >
> > [1] - The bug was, more generally, when we’re given two Value*s that
> > weren’t in any way related to a single function (e.g. two globals, a
> > global + InlineAsm, …), we’d crash. Early in implementing CFLAA, I
> > wanted crashes because those are easy to detect/trace, and I had
> > minimal knowledge of what was getting passed in. Now that we have an
> > impl that’s hopefully mostly working, a debug print will suffice in
> > these cases.
> >
> >
> > [2] - This implies that given %A and %B, where %A = inttoptr %Arg (or
> > %Arg = ptrtoint %A), CFLAA *may* report NoAlias iff %B’s set has no
> > StratifiedAttrs.
> >
> >
> > Thanks again to Yogesh for the bug report,
> > George
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > On Feb 5, 2015, at 4:46 PM, George Burgess IV <
> > george.burgess.iv at gmail.com > wrote:
> >
> >
> > +Yogesh -- he found bugs (llvm_unreachable reached -- looking into
> > why now) and said that he'd be interested in helping. Yay!
> >
> >
> > Status update: Toy implementation of properly handling
> > inttoptr/ptrtoint conversions (see: the fix for #2 on Danny's list)
> > passes tests. I need to do a bit of clean up (and want to test it
> > more thoroughly), and will hopefully have that patch out + a fix for
> > the bug noted above by the end of the weekend
> >
> >
> >
> > George
> >
> >
> > On Wed, Feb 4, 2015 at 12:43 AM, George Burgess IV <
> > george.burgess.iv at gmail.com > wrote:
> >
> >
> >
> > Sounds good, I'll reword that comment. Also, the assert you mentioned
> > turned out to be a bad assumption when combined with how I foresee
> > us handling inttoptr/ptrtoint in the future, so I'll just replace it
> > with slightly more robust code. :)
> >
> >
> > Thanks for the feedback,
> > George
> >
> >
> >
> >
> > On Tue, Feb 3, 2015 at 11:30 PM, Hal Finkel < hfinkel at anl.gov >
> > wrote:
> >
> >
> > Hi George,
> >
> > +// Given an Instruction, this will add it to the graph, along with
> > any
> >
> > +// Instructions that are potentially only available from said
> > Instruction
> >
> > I think this comment is somewhat misleading. You can't really have
> > orphaned instructions: instructions that have been inserted into a
> > basic block must appear in its linked list of instructions that
> > you'll visit when you iterate over all of them. You can have
> > constantexprs, and I think that's what you're try to say.
> >
> > + assert(Edge.From == Inst.get() &&
> >
> > + "Expected ConstantExpr edge `From` to evaluate to the
> > ConstantExpr");
> >
> > Indentation is odd here.
> >
> > For algorithmic considerations, I think that Danny is certainly the
> > best person to review these.
> >
> > -Hal
> >
> > ----- Original Message -----
> > > From: "George Burgess IV" < george.burgess.iv at gmail.com >
> > > To: "Hal J. Finkel" < hfinkel at anl.gov >
> > > Cc: "Chandler Carruth" < chandlerc at google.com >, "Jiangning Liu" <
> > > Jiangning.Liu at arm.com >, "LLVM Developers Mailing
> > > List" < llvmdev at cs.uiuc.edu >, "Daniel Berlin" <
> > > dberlin at dberlin.org >
> > > Sent: Friday, January 30, 2015 10:34:43 PM
> > > Subject: Re: [LLVMdev] question about enabling cfl-aa and
> > > collecting a57 numbers
> > >
> > >
> >
> >
> > > So, I split it up into three patches:
> > >
> > >
> > > - cflaa-danny-fixes.diff are (some of?) the fixes that Danny gave
> > > us
> > > earlier for tests + the minimal modifications you’d need to make in
> > > CFLAA to make them pass tests.
> > > - cflaa-minor-bugfixes.diff consists primarily of a bug fix for
> > > Argument handling — we’d always report NoAlias when one of the
> > > given
> > > variables was an entirely unused argument
> > > (We never added the appropriate Argument StratifiedAttr)
> > > - cflaa-constexpr-fix.diff - The fix for the constexpr behavior
> > > we’ve
> > > been seeing
> > >
> > >
> > > Patches are meant to be applied in the order listed.
> > >
> > >
> > > Also, I just wanted to thank everyone again for your help so far —
> > > it’s greatly appreciated. :)
> > >
> > >
> > > George
> > >
> > >
> > > On Jan 30, 2015, at 11:56 AM, Hal Finkel < hfinkel at anl.gov > wrote:
> > >
> > > ----- Original Message -----
> > >
> > >
> > > From: "George Burgess IV" < george.burgess.iv at gmail.com >
> > > To: "Hal Finkel" < hfinkel at anl.gov >
> > > Cc: "Chandler Carruth" < chandlerc at google.com >, "Jiangning Liu" <
> > > Jiangning.Liu at arm.com >, "LLVM Developers Mailing
> > > List" < llvmdev at cs.uiuc.edu >, "Daniel Berlin" <
> > > dberlin at dberlin.org
> > > >
> > > Sent: Friday, January 30, 2015 10:29:07 AM
> > > Subject: Re: [LLVMdev] question about enabling cfl-aa and
> > > collecting
> > > a57 numbers
> > >
> > > I had thought that the case that Danny had looked at had a constant
> > > GEP, and so this constant might alias with other global pointers.
> > > How is that handled now?
> > > That issue had to do with that we assumed that for all arguments of
> > > a
> > > given Instruction, each argument was either an Argument,
> > > GlobalValue, or Inst in `for (auto& Bb : Inst.getBasicBlockList())
> > > for (auto& Inst : Bb.getInstList())`. ConstantExprs didn't fit into
> > > this instruction, because they aren't reached by said nested loop.
> > >
> > >
> > > With this fix, if we detect that there's a relevant ConstantExpr,
> > > we'll look into it as if it were a regular Instruction inside of
> > > Bb.getInstList(), which causes us to correctly detect the globals,
> > > etc.
> > >
> > > Sounds good.
> > >
> > > Thanks!
> > >
> > > -Hal
> > >
> > >
> > >
> > >
> > >
> > > (I included a test case specifically for this -- it's ugly, but we
> > > have ~3 nested GEPs with a global at the innermost GEP. It produces
> > > the appropriate output)
> > >
> > >
> > > George
> > >
> > >
> > > On Fri, Jan 30, 2015 at 10:56 AM, Hal Finkel < hfinkel at anl.gov >
> > > wrote:
> > >
> > >
> > > ----- Original Message -----
> > >
> > >
> > > From: "George Burgess IV" < george.burgess.iv at gmail.com >
> > > To: "Daniel Berlin" < dberlin at dberlin.org >
> > > Cc: "Chandler Carruth" < chandlerc at google.com >, "Hal Finkel" <
> > > hfinkel at anl.gov >, "Jiangning Liu"
> > > < Jiangning.Liu at arm.com >, "LLVM Developers Mailing List" <
> > > llvmdev at cs.uiuc.edu >
> > > Sent: Friday, January 30, 2015 8:15:55 AM
> > > Subject: Re: [LLVMdev] question about enabling cfl-aa and
> > > collecting a57 numbers
> > >
> > >
> > >
> > > I'm not exactly thrilled about the size of this diff -- I'll
> > > happily
> > > break it up into more manageable bits later today, because some of
> > > it is test fixes, another bit is a minor bug fix, etc.
> > >
> > > Yes, please break it into independent parts.
> > >
> > >
> > >
> > >
> > >
> > > Important bit (WRT ConstantExpr): moved the loop body from
> > > buildGraphFrom into a new function. The body has a few tweaks to
> > > call constexprToEdges on all ConstantExprs that we encounter.
> > > constexprToEdges, naturally, interprets a ConstantExpr (and all
> > > nested ConstantExprs) and places the results into a
> > > SmallVector<Edge>.
> > >
> > >
> > > I'm assuming this method of handling ConstantExprs isn't 100%
> > > correct
> > > because I was told that handling them correctly would be more
> > > difficult than I think it is. I can't quite figure out why, so
> > > examples of cases that break my code would be greatly appreciated.
> > > :)
> > >
> > > I had thought that the case that Danny had looked at had a constant
> > > GEP, and so this constant might alias with other global pointers.
> > > How is that handled now?
> > >
> > > Thanks again,
> > > Hal
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > George
> > >
> > >
> > > On Mon, Jan 26, 2015 at 2:43 PM, George Burgess IV <
> > > george.burgess.iv at gmail.com > wrote:
> > >
> > >
> > >
> > >
> > > Inline
> > >
> > >
> > > George
> > >
> > >
> > >
> > >
> > > On Jan 26, 2015, at 1:05 PM, Daniel Berlin < dberlin at dberlin.org >
> > > wrote:
> > >
> > >
> > > George, given that, can you just build constexpr handling (it's not
> > > as easy as you think) as a separate funciton and have it use it in
> > > the right places?
> > > Will do. :)
> > >
> > >
> > >
> > >
> > >
> > > FWIW, my current list of CFLAA issues is:
> > >
> > > 1. Unknown values (results from ptrtoint, incoming pointers, etc)
> > > are
> > > not treated as unknown. These should be done through graph edge (so
> > > that they can be one way, otherwise, you will unify everything :P)
> > >
> > >
> > >
> > >
> > > 2. Constexpr handling
> > >
> > >
> > >
> > >
> > > ^^^ These are correctness issues. I'm pretty sure there are a few
> > > more but i haven't finished auditing
> > > 3. In a number of places we treat non-pointers as memory-locations
> > > and unify them with pointers. This introduces a lot of spurious
> > > aliasing.
> > > 4. More generally, we induce a lot of spurious aliasing through
> > > things at different dereference levels. In these cases, one may to
> > > the other, but, for example, if we have a foo***, and a foo* (and
> > > neither pointers to unknown things or escapes), the only way for
> > > foo
> > > *** to alias foo* is if there is a graph path with two dereferences
> > > between them.
> > > We seem to get this wrong sometimes. Agreed on all four. Though
> > > naturally it should be fixed, I’d like to see how much of an issue
> > > #4 ends up being when we properly deal with #3.
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > On Sun Jan 25 2015 at 6:44:07 PM Chandler Carruth <
> > > chandlerc at google.com > wrote:
> > >
> > >
> > >
> > >
> > >
> > >
> > > On Sun, Jan 25, 2015 at 6:37 PM, George Burgess IV <
> > > george.burgess.iv at gmail.com > wrote:
> > >
> > >
> > >
> > >
> > > Fixing that still gives a wrong result, i haven't started to
> > > track
> > > down what *else* is going on here.
> > >
> > >
> > > Running with the attached diff + a modified buildGraphFrom to
> > > handle
> > > the constexpr GEPs, we seem to flag everything in test2.ll
> > > (conservatively) correctly.
> > >
> > >
> > > Is `store` the only place we can expect to see these constexpr
> > > analogs, or is just about anywhere fair game?
> > >
> > >
> > > Any Value can be a ConstantExpr, so all operands to instructions
> > > are
> > > fair game.
> > >
> > >
> > >
> > >
> > >
> > > --
> > > Hal Finkel
> > > Assistant Computational Scientist
> > > Leadership Computing Facility
> > > Argonne National Laboratory
> > >
> > >
> > >
> > > --
> > > Hal Finkel
> > > Assistant Computational Scientist
> > > Leadership Computing Facility
> > > Argonne National Laboratory
> > >
> >
> > --
> > 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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