[llvm-dev] RFC: Generate plain !tbaa tags in place of !tbaa.struct ones

[Ivan Kosarev via llvm-dev]

Hello,

The motivation behind this proposal is to make it possible to
propagate TBAA information through scalarizing transformations,
such as SROA, that rewrite accesses to aggregates, e.g., memcpy()
calls, into accesses to scalars, that is, load and store
instructions.

Currently, we decorate instructions that initialize and copy
aggregates with !tbaa.struct tags that generally cannot be
transformed to !tbaa tags. For this reason every time we
scalarize an aggregate, we leave resulting loads and stores
undecorated, meaning optimization of such instructions cannot
benefit from TBAA.

Furthermore, our analysis indicates that the only place where
!tbaa.struct tags may potentially impact code generation is
simplification of memcpy() and memmove() calls, see
SimplifyMemTransfer() in InstCombineCalls.cpp. Ironically, what
the code that makes that sole use of such tags is trying to do is
to construct a !tbaa tag from the information encoded in the
given !tbaa.struct tag. Note that it can only do that if the
!tbaa.struct tag describes a structure with a single member of a
scalar type.

Here's how we propose to resolve the issue in terms of specific
steps:

1. Extend the TBAA facilities in clang to support aggregate types
    as final access types. This patch:

    [CodeGen] Propagate may-alias'ness of lvalues with TBAA info
    https://reviews.llvm.org/D39008

    implements this for the needs of fixing issues with
    propagation of TBAA information, which in turn is necessary to
    support TBAA for unions. So once this patch is committed, this
    step is done.

2. Generate !tbaa tags in addition to !tbaa.struct tags for
    aggregate accesses.

3. Fix the TBAA analysis to support aggregate accesses as
    explained in this proposal:

    RFC: Resolving TBAA issues
    http://lists.llvm.org/pipermail/llvm-dev/2017-August/116652.html

4. Switch the code that simplifies memcpy() and memmove() calls
    to !tbaa tags.

5. Remove the support for !tbaa.struct tags.

    I guess we would want to remove all code which only purpose is
    to generate such tags, but I'm not sure what we should do with
    the MD_tbaa_struct enumerator. Some possible options:
    a) Leave it as is, just don't use it.
    b) Rename to something like MD_unused.
    c) Remove, but do not change the values of other MD_*
       enumerators.
    d) Remove and adjust values of other MD_* enumerators
       respectively.
    Or, maybe we want some multi-stage plan here?

Further steps are supposed to include things like fixing SROA to
propagate TBAA information.

Any feedback is highly appreciated.

Thanks,

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