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

Tue Aug 19 19:25:11 PDT 2014

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 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
>> >
>> >
>> >
>> > _______________________________________________
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>> > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
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>> >
>>
>>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory