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

Tue Feb 3 21:43:20 PST 2015

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
>
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