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

Mon Feb 9 09:25:38 PST 2015

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)
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]

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 <mailto: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 <mailto: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 <mailto:george.burgess.iv at gmail.com>>
> > To: "Hal J. Finkel" <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>
> > Cc: "Chandler Carruth" <chandlerc at google.com <mailto:chandlerc at google.com>>, "Jiangning Liu" <Jiangning.Liu at arm.com <mailto:Jiangning.Liu at arm.com>>, "LLVM Developers Mailing
> > List" <llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu>>, "Daniel Berlin" <dberlin at dberlin.org <mailto: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 <mailto:hfinkel at anl.gov> > wrote:
> >
> > ----- Original Message -----
> >
> >
> > From: "George Burgess IV" < george.burgess.iv at gmail.com <mailto:george.burgess.iv at gmail.com> >
> > To: "Hal Finkel" < hfinkel at anl.gov <mailto:hfinkel at anl.gov> >
> > Cc: "Chandler Carruth" < chandlerc at google.com <mailto:chandlerc at google.com> >, "Jiangning Liu" <
> > Jiangning.Liu at arm.com <mailto:Jiangning.Liu at arm.com> >, "LLVM Developers Mailing
> > List" < llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu> >, "Daniel Berlin" < dberlin at dberlin.org <mailto: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 <mailto:hfinkel at anl.gov> >
> > wrote:
> >
> >
> > ----- Original Message -----
> >
> >
> > From: "George Burgess IV" < george.burgess.iv at gmail.com <mailto:george.burgess.iv at gmail.com> >
> > To: "Daniel Berlin" < dberlin at dberlin.org <mailto:dberlin at dberlin.org> >
> > Cc: "Chandler Carruth" < chandlerc at google.com <mailto:chandlerc at google.com> >, "Hal Finkel" <
> > hfinkel at anl.gov <mailto:hfinkel at anl.gov> >, "Jiangning Liu"
> > < Jiangning.Liu at arm.com <mailto:Jiangning.Liu at arm.com> >, "LLVM Developers Mailing List" <
> > llvmdev at cs.uiuc.edu <mailto: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 <mailto:george.burgess.iv at gmail.com> > wrote:
> >
> >
> >
> >
> > Inline
> >
> >
> > George
> >
> >
> >
> >
> > On Jan 26, 2015, at 1:05 PM, Daniel Berlin < dberlin at dberlin.org <mailto: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 <mailto:chandlerc at google.com> > wrote:
> >
> >
> >
> >
> >
> >
> > On Sun, Jan 25, 2015 at 6:37 PM, George Burgess IV <
> > george.burgess.iv at gmail.com <mailto: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
> 
> 

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