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

[Hal Finkel]

----- 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