[llvm-dev] RFC: Resolving TBAA issues
Hal Finkel via llvm-dev
llvm-dev at lists.llvm.org
Thu Aug 17 17:11:57 PDT 2017
On 08/16/2017 01:59 PM, Ivan A. Kosarev via llvm-dev 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
What exactly do you do, or plan to do, for bit fields? A single
load/store can access more than one field. This is true for other kinds
of things too, but, in particularly common for bit fields. (see also my
comment below about !tbaa.struct).
> , 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.
Do you have a Clang patch as well?
>
>
> 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.
As in the current scheme, this group is a peer to <may_alias> above?
> 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.
Of all of these groups above, only <union> has special semantics, correct?
>
> 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.
Which is presumed to match in both accesses, correct?
> 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.
I'm not following the above paragraph. Can you please explain from where
this comes?
>
> 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
I suspect that we'll want to define a meaning to the size/offset fields.
One thing to think about is that, today, we also have !tbaa.struct
metadata. This is put on memcpy, etc. to indicate what fields are
defined (the rest is padding which the memcpy does not need to actually
copy). This is somewhat akin to the bitfield problem (i.e. a single load
can access multiple fields in the bitfield). If we can represent that,
maybe we can use the same thing to replace !tbaa.struct as well. We
could specify size/offset in bits to handle bitfields.
> 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.
Sounds good. Can you be more specific about your experiments. What kinds
of code did you try?
Thanks again,
Hal
>
>
>
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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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