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

[Hal Finkel]

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