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

[Hal Finkel]

[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