[LLVMdev] [cfe-dev] For alias analysis, It's gcc too aggressive or LLVM need to improve?

Wed Aug 20 15:48:08 PDT 2014

----- Original Message -----
> From: "Daniel Berlin" <dannyb at google.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Richard Smith" <richard at metafoo.co.uk>, "Reid Kleckner" <rnk at google.com>, "LLVM Developers Mailing List"
> <llvmdev at cs.uiuc.edu>, "cfe-dev Developers" <cfe-dev at cs.uiuc.edu>, "Daniel Berlin" <dberlin at dberlin.org>
> Sent: Tuesday, August 19, 2014 9:25:11 PM
> Subject: Re: [LLVMdev] [cfe-dev] For alias analysis, It's gcc too aggressive or LLVM need to improve?
> 
> The traditional way to communicate this is by filtering some analysis
> with other ones (which is different than chaining them and asking
> them
> each the same question)

For better or worse, for types within LLVM's current infrastructure, I'm not sure there is a distinction: we only have type information at the accesses themselves. But for accesses within structs, we have full "path" information from the base access type, through the parents with offsets included. So we might be able to recover some of the 'recursive filtering' semantics that you describe.

For this case, we have an access to ptr[i]->index and an access to some global int aa. When looking at the access to ptr[i]->index we should have path information that tells us that ptr[i]->index is an access to an int at offset zero from 'struct heap'. We also know from the metadata node describing 'struct heap' that it also has an int at offset 4, and so must be at least 8 bytes in total. We can also look at the address of aa and, because it is a global, easily determine that any object created there must be no larger than 4 bytes (and, thus, no object of type 'struct heap' could ever have existed there). From this, we could conclude NoAlias.

We could conceivably enhance this further by also tagging globals, calls to operator new, etc. with types to provide explicit object types where known. Then we could use these types, instead of just the number of known dereferenceable bytes, to determine what kinds of objects can exist at some pointer address.

Does this make sense?

 -Hal

> 
> For example, GCC post-filters points-to sets with TBAA (or you could
> do it during analysis prior, but it's significantly more expensive to
> do repeated filtering, even though you may gain precision).
> 
> Here, points-to would come up and say "array points to aa and
> element"
> post-filtering the set with TBAA (or whatever you like)  and asking
> "can array legally point to aa by the TBAA rules" would come up with
> "no" (even in LLVM, given the types, you should see that they are in
> different TBAA subtrees).  It will then say "array only points to
> element". The aliases query then answers it right. The optimizer is
> happy.
> 
> (It's a bit more complex for C on the analysis side because you can
> only do this when the pointer is dereferenced, since otherwise they
> can *store* whatever they like in it, if it's never used).
> 
> I'm not aware of a more general mechanism for doing this than the
> above.  The generalization is usually the "type filtering".
> 
> 
> 
> 
> On Tue, Aug 19, 2014 at 4:57 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> > ----- Original Message -----
> >> From: "Richard Smith" <richard at metafoo.co.uk>
> >> To: "Daniel Berlin" <dberlin at dberlin.org>
> >> Cc: "Reid Kleckner" <rnk at google.com>, "Hal Finkel"
> >> <hfinkel at anl.gov>, "Daniel Berlin" <dannyb at google.com>, "LLVM
> >> Developers Mailing List" <llvmdev at cs.uiuc.edu>, "cfe-dev
> >> Developers" <cfe-dev at cs.uiuc.edu>
> >> Sent: Monday, August 11, 2014 9:31:09 PM
> >> Subject: Re: [LLVMdev] [cfe-dev] For alias analysis, It's gcc too
> >> aggressive or LLVM need to improve?
> >>
> >>
> >>
> >>
> >> I'll take this from the C++ angle; the C rules are not the same,
> >> and
> >> I'm not confident they give the same answer.
> >>
> >>
> >> On Mon, Aug 11, 2014 at 2:09 PM, Daniel Berlin <
> >> dberlin at dberlin.org
> >> > wrote:
> >>
> >>
> >> The access path matters (in some sense), but this is, AFIAK, valid
> >> no
> >> matter how you look at it.
> >>
> >> Let's take a look line by line
> >>
> >>
> >> #include <stdio.h>
> >> struct heap {int index; int b;};
> >> struct heap **ptr;
> >> int aa;
> >>
> >> int main() {
> >> struct heap element;
> >> struct heap *array[2];
> >> array[0] = (struct heap *)&aa; <- Okay so far.
> >> array[1] = &element; <- Clearly okay
> >> ptr = array; <- still okay so far
> >> aa = 1; <- not pointer related.
> >> int i; <- not pointer related
> >> for (i =0; i< 2; i++) { <- not pointer related
> >> printf("i is %d, aa is %d\n", i, aa); <- not pointer related
> >> ptr[i]->index = 0; <- Here is where it gets wonky.
> >>
> >> <rest of codeis irrelevan>
> >>
> >> First, ptr[i] is an lvalue, of type struct heap *, and ptr[i]-> is
> >> an
> >> lvalue of type struct heap (in C++03, this is 5.2.5 paragraph 3,
> >> check
> >> footnote 59).
> >>
> >>
> >>
> >> This is where we get the undefined behavior.
> >>
> >>
> >>
> >> 3.8/6: "[If you have a glvalue referring to storage but where
> >> there
> >> is no corresponding object, the] program has undefined behavior
> >> if:
> >>
> >> [...] the glvalue is used to access a non-static data member".
> >>
> >>
> >> There is no object of type 'heap' denoted by *ptr[0] (by 1.8/1, we
> >> can only create objects through definitions, new-expressions, and
> >> by
> >> creating temporary objects). So the behavior is undefined when we
> >> evaluate ptr[0]->index.
> >
> > Any thoughts on how we might communicate this information to the
> > optimizer? In this case we might be able to infer something from
> > the struct-path aware TBAA (because reaching the int though the
> > structure has a different path length than reaching the top-level
> > int), but I'm not sure that addresses the general case.
> >
> > Thanks again,
> > Hal
> >
> >>
> >> (I'm too lazy to parse the rules for whether E1.E2 is an lvalue,
> >> because it doesn't end up making a difference)
> >>
> >> Let's assume, for the sake of argument, the actual access to aa
> >> occurs through an lvalue of type "struct heap" rather than "int"
> >>
> >> In C++03 and C++11, it says:
> >>
> >> An object shall have its stored value accessed only by an lvalue
> >> expression that has one of the following types:
> >>
> >>
> >> a type compatible with the effective type of the object,
> >> a qualified version of a type compatible with the effective type
> >> of
> >> the object,
> >> a type that is the signed or unsigned type corresponding to the
> >>
> >> effective type of the object,
> >> a type that is the signed or unsigned type corresponding to a
> >> qualified version of the effective type of the object,
> >>
> >> an aggregate or union type that includes one of the aforementioned
> >> types among its members (including, recursively, a member of a
> >> subaggregate or contained union), or
> >> a character type.
> >> (C++11 adds something about dynamic type here)
> >>
> >> struct heap is "an aggregate or union type that includes one of
> >> the
> >> aforementioned types among it's members".
> >>
> >> Thus, this is legal to access this int through an lvalue
> >> expression
> >> that has a type of struct heap.
> >> Whether the actual store is legal for other reasons, i don't know.
> >> There are all kinds of rules about object alignment and value
> >> representation that aren't my baliwick. I leave it to another
> >> language lawyer to say whether it's okay to do a store to
> >> something
> >> that is essentially, a partial object.
> >>
> >> Note that GCC actually knows this is legal to alias, at least at
> >> the
> >> tree level. I debugged it there, and it definitely isn't
> >> eliminating
> >> it at a high level. It also completely understands the call to
> >> ->index
> >> = 0 affects "aa", and has a reload for aa before the printf call.
> >>
> >> I don't know what is eliminating this at the RTL level, but i
> >> can't
> >> see why it's illegal from *aliasing rules*. Maybe this is invalid
> >> for
> >> some other reason.
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> }
> >> return 0;
> >>
> >> }
> >>
> >> ptr[i]->index = 0;
> >>
> >>
> >>
> >> On Mon, Aug 11, 2014 at 1:13 PM, Reid Kleckner < rnk at google.com >
> >> wrote:
> >> > +aliasing people
> >> >
> >> > I *think* this is valid, because the rules have always been
> >> > described to me
> >> > in terms of underlying storage type, and not access path. These
> >> > are
> >> > all
> >> > ints, so all loads and stores can alias.
> >> >
> >> >
> >> > On Sat, Aug 9, 2014 at 3:07 PM, Hal Finkel < hfinkel at anl.gov >
> >> > wrote:
> >> >>
> >> >> ----- Original Message -----
> >> >> > From: "Tim Northover" < t.p.northover at gmail.com >
> >> >> > To: "Jonas Wagner" < jonas.wagner at epfl.ch >
> >> >> > Cc: "cfe-dev Developers" < cfe-dev at cs.uiuc.edu >, "LLVM
> >> >> > Developers Mailing
> >> >> > List" < llvmdev at cs.uiuc.edu >
> >> >> > Sent: Friday, August 8, 2014 6:54:50 AM
> >> >> > Subject: Re: [cfe-dev] [LLVMdev] For alias analysis, It's gcc
> >> >> > too
> >> >> > aggressive or LLVM need to improve?
> >> >> >
> >> >> > > your C program invokes undefined behavior when it
> >> >> > > dereferences
> >> >> > > pointers that
> >> >> > > have been converted to other types. See for example
> >> >> > >
> >> >> > > http://stackoverflow.com/questions/4810417/c-when-is-casting-between-pointer-types-not-undefined-behavior
> >> >> >
> >> >> > I don't think it's quite that simple.The type-based aliasing
> >> >> > rules
> >> >> > come from 6.5p7 of C11, I think. That says:
> >> >> >
> >> >> > "An object shall have its stored value accessed only by an
> >> >> > lvalue
> >> >> > expression that has one of
> >> >> > the following types:
> >> >> > + a type compatible with the effective type of the object,
> >> >> > [...]
> >> >> > + an aggregate or union type that includes one of the
> >> >> > aforementioned
> >> >> > types among its members [...]"
> >> >> >
> >> >> > That would seem to allow this usage: aa (effective type
> >> >> > "int")
> >> >> > is
> >> >> > being accessed via an lvalue "ptr[i]->index" of type "int".
> >> >> >
> >> >> > The second point would even seem to allow something like
> >> >> > "ptr[i]
> >> >> > =
> >> >> > ..." if aa was declared "int aa[2];", though that seems to be
> >> >> > going
> >> >> > too far. It also seems to be very difficult to pin down a
> >> >> > meaning
> >> >> > (from the standard) for "a->b" if a is not a pointer to an
> >> >> > object
> >> >> > with
> >> >> > the correct effective type. So the entire area is probably
> >> >> > one
> >> >> > that's
> >> >> > open to interpretation.
> >> >> >
> >> >> > I've added cfe-dev to the list; they're the *professional*
> >> >> > language
> >> >> > lawyers.
> >> >>
> >> >> Coincidentally, this also seems to be PR20585 (adding Jiangning
> >> >> Liu, the
> >> >> reporter of that bug, to this thread too).
> >> >>
> >> >> -Hal
> >> >>
> >> >> >
> >> >> > Cheers.
> >> >> >
> >> >> > Tim.
> >> >> > _______________________________________________
> >> >> > cfe-dev mailing list
> >> >> > cfe-dev at cs.uiuc.edu
> >> >> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
> >> >> >
> >> >>
> >> >> --
> >> >> Hal Finkel
> >> >> Assistant Computational Scientist
> >> >> Leadership Computing Facility
> >> >> Argonne National Laboratory
> >> >> _______________________________________________
> >> >> LLVM Developers mailing list
> >> >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> >> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >> >
> >> >
> >> >
> >> > _______________________________________________
> >> > LLVM Developers mailing list
> >> > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >> >
> >>
> >>
> >
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory