[llvm-dev] RFC: Resolving TBAA issues
Hal Finkel via llvm-dev
llvm-dev at lists.llvm.org
Tue Aug 22 17:08:19 PDT 2017
On 08/21/2017 04:43 PM, Xinliang David Li wrote:
>
>
> On Mon, Aug 21, 2017 at 2:19 PM, Hal Finkel <hfinkel at anl.gov
> <mailto:hfinkel at anl.gov>> wrote:
>
> On 08/20/2017 08:47 PM, Xinliang David Li via llvm-dev wrote:
>> Hi Ivan, thanks for writing it up. This is a pretty long thread,
>> and there are many good points brought up in the heated
>> discussions. Here is my take on many of the points that have
>> been mentioned:
>>
>> 1) The type based aliasing IR annotation should be generic enough
>> to represent aliasing constraints for any frontend languages;
>> 2) The offset based aliasing rules and type based aliasing rule
>> have a lot in common. The difference is that the offset based
>> aliasing rule require that the two accesses to have same base
>> pointer value, while type based aliasing rules specifies whether
>> two base pointers can possibly point to the same underlying
>> object, and if yes, what are the legally allowed offsets at which
>> the two base pointers can be aligned;
>> 3) Type specific information is usually not interesting to offset
>> based aliasing rules if offsets can be folded into constant. It
>> is when there is a variable involved in the indexing that makes
>> language guaranteed bounds information also useful.
>>
>> Given the above, the type based aliasing annotation can be
>> represented as list of <base_type, offset> pairs, as well as the
>> original base pointer type in the source. Each pair represent a
>> way to access the memory legally allowed by the language -- this
>> field can be accessed with a base pointer of type 'base_type' at
>> 'offset'.
>>
>> For instance:
>>
>> struct A { struct B { struct C {int m, n; } c1; int i, j; } b1,
>> b2; int k; };
>> struct A*ap;
>> struct B *bp;
>> struct C *cp;
>> int *ip;
>>
>> (1) access ap->b2.c1.n has following annotation:
>>
>> {A*, [ <A, 12>, <B, 4>, <C,4>, <int, 0>] }
>>
>> What it means
>> Access (1) may only be aliased with another access if that
>> access's base pointer is type A and the offset is 12, or type B
>> at offset 4, or type C at offset 4, or type int at offset 0.
>> Also access (1) is originally accessed via path <A, 12>.
>>
>> (2) access ap->k has only
>> {A*, [<A, 16>, <int, 0>}
>>
>> (3) access bp->c1.n has
>> {B*, [<B, 4>, <int, 0>]}
>>
>> (4) Access bp->j has
>>
>> {B*, [<B, 12>, <int, 0>]}
>>
>> (5) access *ip (implicitly) has
>>
>> {int *, <int, 0>}
>>
>> From the above, we can see (5) is aliased with all the above, and
>> (3) is also aliased with (1).
>
> This sounds reasonable. It is almost exactly an encoding of our
> current TBAA scheme where, instead of causing the client to walk
> up the tree, we explicitly provide the path up the tree at each
> access. It should also handle unions naturally because, regardless
> of the "currently live" union member, all potentially-aliasing
> accesses will have an entry in their list like <U, offset>.
>
>
>
>
>>
>> With this representation, the type based alias rules
>> implementation becomes: given two memory accesses, align the
>> pointer of one access to the other. If it can be done, they are
>> not aliased. If yes, the use the same base+offset+access_size
>> aliasing rule to check overlapping.
>
> I'm not entirely sure what you mean by aligning pointers (and why
> aligning them would mean they couldn't alias). Can you please explain?
>
>
> I had a typo there. I meant 'if it can not be done, they are not
> aliased ...".
>
> By aligning pointers, I meant align the base pointer of another access
> with one of the allowed types in the access list.
>
> Given:
>
> Access (1) {A*, [ <A, 12>, <B, 4>, <C,4>, <int, 0>] } and access (2)
> {B*, [<B, 4>, <int, 0>]}, we know that access (2)'s base pointer
> type is 'B*'. Its base pointer typed then needs to be matched up
> against the types in Access (1)'s list. In this case, it finds a
> match, so (2)'s base pointer will be aligned to point to the type B
> subobject of the access (1). After alignment, we then compare offset
> and access size to determine overlapping.
Maybe I'm missing something. Isn't the aliasing check just a suffix
check in this scheme? As in:
Might an access with [ <A, 12>, <B, 4>, <C,4>, <int, 0> ], list (1),
alias with an access with [<B, 4>, <C, 4>, <int, 0>], list (2)? Yes,
because the elements in list (2) are a suffix of those in list (1).
I'm assuming here that the offsets are to the start of each field, and
the offsets within the field are separate. Is dealing with the
intra-field-offset issue what you meant by alignment?
To deal with unions, I think that we'd need an additional rule.
Something like: If the suffix check fails, for each union type in the
list, discard the path elements to the right of the union type and try
again. In this way, we can check things like:
[ <A, 12>, <U, 4>, <C,4>, <int, 0> ], list (1), checked against, [ <A,
12>, <U, 4>, <F,4>, <float, 0> ], list (2). The initial suffix check
fails, but we then discard <F, 4> and <float, 0> from list (2), and try
again. At that point, the check succeeds and we conclude that aliasing
is possible.
-Hal
>
> David
>
>
>
>
>
>>
>> For languages that have rules about access bounds of member
>> arrays, the offset information can be replaced with offset +
>> range. By default, the range is the size of the array type.
>
> Yes, and I'd certainly like to include this information going
> forward (along with some flag saying whether it's legal to
> over/under index the field).
>
> Thanks again,
> Hal
>
>
>>
>> David
>>
>>
>> On Wed, Aug 16, 2017 at 11:59 AM, Ivan A. Kosarev via llvm-dev
>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> Hal, Daniel,
>>
>> Thanks for your responses. Here's a quick formal introduction
>> to the proposed approach follows. I also attached a couple
>> files to put some visibility on implementation details. We'd
>> love to hear from you gentlemen and LLVM community in general
>> on how this fits what you know about TBAA.
>>
>>
>> Overview
>> ========
>>
>> With this writing we propose a new approach to the TBAA mechanism
>> designed to overcome current issues such as:
>> - inability to represent accesses to aggregates, unions, union
>> members, bit fields, fields of union and aggregate types,
>> including array members and
>> - lack of a mean to identify user types defined in different
>> translation units.
>>
>> As of today, we have a local patch that implements this approach.
>> This new implementation is known to be at least as functionally
>> complete as the current one. Additionally, with this patch on top
>> we improved SROA to propagate TBAA information, thus making sure
>> the new features work as expected.
>>
>> We should be able to upload that patch for review in a few days.
>>
>>
>> The Approach
>> ============
>>
>> With the proposed approach we represent accesses as sequences
>> that contain all accessed types and fields in order. For example
>> for this access:
>>
>> struct T { union U { struct S { int i1, i2; } s1, s2; } u1,
>> u2; } t;
>> t.u1.s1.i1
>>
>> we generate an access sequence of the form:
>>
>> [T, T::u1, U, U::s1, S, S::i1, int]
>>
>> An array is allowed to overlap with any object of its element
>> type, including other arrays of the same element type, so an
>> access to an array element is represented as an access to an
>> object of the element type:
>>
>> int a[7];
>> a[5]
>>
>> [int]
>>
>> In case of a multi-dimensional array this rule applies
>> recursively:
>>
>> int a[7][9];
>> a[3][5]
>>
>> [int]
>>
>> For member arrays we specify the member field as usual so we can
>> distinct it from other array members:
>>
>> struct S { int a[7], b[9]; } s;
>> s.a[5]
>>
>> [S, S::a, int]
>>
>> Similarly to the scalar and struct-path approaches, we consider
>> every type to be a member of a type group it explicitly refers
>> to. Here's how the tree that describes relations between type
>> groups would look like for the example above:
>>
>> <tbaa_root>
>> |- <may_alias>
>> |- <representation_byte>
>> |-<structure>
>> | |- S
>> |- int
>>
>> The <vtable_pointer> group has a special meaning and is used to
>> describe accesses to virtual table pointers. Similarly, the
>> <union> type group includes all union types and used by the TBAA
>> implementation to distinct union types from other types. The
>> <may_alias> group is technically equivalent to
>> <representation_byte> and supposed to be a group for
>> may_alias-marked types.
>>
>> For two given access sequences we can determine if the accessed
>> objects are allowed to overlap by the rules of the input
>> language. Here's the list of rules complete enough to support C
>> and C++. Ellipsis elements denote sequences of zero or more
>> elements. For other input languages more rules can be supported,
>> if necessary.
>>
>> [X...] is allowed to overlap with [S1..., X..., S2...]
>> and the most generic access sequence is [X...].
>>
>> [X1..., X2...] is allowed to overlap with [S1..., X1...]
>> with the most generic access sequence to be [X1...].
>>
>> [X1..., U, U::m1, X2...] is allowed to overlap with
>> [S1..., X1..., U, U::m2, S2...]
>> for a union U of an unknown effective type, provided m1 != m2
>> and the most generic access sequence is [X1..., U].
>>
>> If neither of the given sequences contains the leading access
>> type of the other, then they are allowed to overlap if the
>> leading access type of one sequence is a direct or indirect field
>> of the final access type of the other sequence and then the most
>> generic access sequence consists of a single element, which is
>> that final access type.
>>
>> For the purpose of determining whether one type is a direct or
>> indirect member of another type unions are considered to have no
>> members as accesses to members of unions are only allowed to
>> overlap if they have the base union object explicitly specified.
>>
>> Otherwise, given sequences overlap if there is a type group that
>> includes both the leading access types and the most generic
>> access sequence consists of the smallest common type group as its
>> only element.
>>
>> See the attached TypeBasedAliasAnalysis.cpp file and specifically
>> the MatchAccessSequences() function for how these rules can be
>> implemented.
>>
>> TBAA information is encoded as metadata nodes, as usual. Load and
>> store instructions refer to access sequences:
>> store %struct.T* %p, %struct.T** %p.addr, align 8, !tbaa !11
>>
>> A type node is either a terminal type node that names a root type
>> group:
>> !0 = !{ !"<tbaa_root>" }
>>
>> or a non-terminal type node that names a type and refers to a
>> type group it belongs to:
>> !1 = !{ !0, !"int" }
>>
>> Record types also refer to their field descriptors:
>> !3 = !{ !0, !"S", !9 }
>>
>> An access node is either a terminal access node that refers to
>> the corresponding access type:
>> !5 = !{ !1 }
>> !9 = !{ !3 }
>>
>> or a member node that refers to a structure/class or union field
>> descriptor and a subsequent access path node:
>> !7 = !{ !type_group, !field_id, !field_offset, !field_size }
>> !11 = !{ !5, !9, !7 }
>>
>> For a field node the first element refers to its type. The
>> purpose of other elements is to make the field node unique. Their
>> meaning is unspecified. Currently the other members for C and C++
>> are the field name, bit offset and bit size of the member, but
>> this may change in future and front ends for other input
>> languages may act differently, so TBAA implementation in the
>> codegen shall not rely on specific shape or meaning of these
>> elements.
>>
>> For types that are interchangeable for purposes of TBAA it is
>> important to encode them identically so that descriptors of
>> interchangeable types defined in different modules merge into
>> same metadata nodes.
>>
>> Structure/class fields are specified in the order of declaration.
>> For union fields there is a canonical order that guarantee that
>> definitions of the same union type will result in identical
>> descriptors regardless of the order of member declarations.
>> Currently we sort union fields with key (field_id, field_offset,
>> field_size).
>>
>> C++ tagged types with no linkage are to be encoded as "distinct"
>> nodes to guarantee their uniqueness.
>>
>> The support for !tbaa.struct information is to be replaced with
>> plain !tbaa tags representing accesses to the corresponding
>> record types.
>>
>> Another attached file, the tbaa.cpp one, is a test case that can
>> give an idea what encoded TBAA metadata may look like.
>>
>>
>> Space and Performance Analysis
>> ==============================
>>
>> In terms of metadata size, with the new approach we generate
>> about 15% more of metadata nodes. The ratio of the total number
>> of TBAA nodes to the amount of code remains very low, meaning the
>> size of TBAA metadata is unlikely to be a problem any time soon.
>>
>> From the performance perspective, the proposed approach differs
>> from the current one in that:
>> - it does not traverse through record types if one of the access
>> sequences to match contains the leading access type of the
>> other and
>> - it never traverses union types.
>>
>> We thus expect that the new approach is at least as efficient as
>> the current one. Our experiments do not indicate any sensible
>> difference in performance between the implementations.
>>
>> --
>>
>>
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>>
>>
>>
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>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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