[llvm-dev] Supporting LLVM_BUILD_LLVM_DYLIB on Windows

Tom Stellard via llvm-dev
Wed, 08 Sep 2021 23:52:53 -0000

On 9/8/21 3:52 PM, Saleem Abdulrasool via llvm-dev
wrote:
> Hello llvm-dev,
>

In general, I am in favor of this change and think it would be a good
improvement.  A few comments below:

> One of the current limitations on LLVM on Windows is that you cannot use
LLVM_BUILD_LLVM_DYLIB:
https://github.com/llvm/llvm-project/blob/main/llvm/tools/llvm-shlib/CMakeLists.txt#L14-L16
<https://github.com/llvm/llvm-project/blob/main/llvm/tools/llvm-shlib/CMakeLists.txt#L14-L16>
 I am interested in trying to see if we can lift this limitation.  There are
others in the community that also seem to be interested in seeing LLVM being
possible to use as a DLL on Windows and the topic does come up on the mailing
lists every so often.
>
> When you build a distribution of a LLVM based toolchain currently, the
result on Windows is ~2GiB for a trimmed down toolset.  This is largely due to
the static linking used for all the tools.  I would like to be able to use the
shared LLVM build for building a toolset on Windows.
>
> Unlike Unix platforms, the default on Windows is that all symbols are
treated as `dso_local` (that is `-fvisibility-default=hidden`).  Symbols which
are meant to participate in dynamic linking are to be attributed as
`__declspec(dllexport)` in the module and `__declspec(dllimport)` external to
the module.  This is similar to Unix platforms where
`__attribute__((__visibility__(...)))` controls the same type of behaviour with
`-fvisibility-default=hidden`.
>
> For the case of distributions, it would remain valuable to minimize the
number of shared objects to reduce the files that require to be shipped but also
to minimize the number of cross-module calls which are not entirely free (i.e.
PLT+GOT or IAT costs).  At the same time, the number of possible labels which
can be exposed from a single module on Windows is limited to 64K.  Experience
from MSys2 indicates that LLVM with all the backends is likely to exceed this
count (with a subset of targets, the number already is close to 60K).  This
means that it may be that we would need two libraries on Windows.
>

The backends only need to export a handful of symbols, so I don't think
enabling more targets will have that big of impact on the number
of exported symbols.

> With the LLVM community being diverse, people often build on different
platforms with different configurations, and I am concerned that adding more
differences in how we build libraries complicates how maintainable LLVM is.  I
would suggest that we actually change the behavior of the Unix builds to match
that of Windows by building with `-fvisibility-default=hidden`.  Although this
is a change, it is not without value.  By explicitly marking the interfaces
which are vended by a library and making everything else internal, it does
enable some potential optimization options for the compiler and linker (to be
clear, I am not suggesting that this will have a guaranteed benefit, just that
it can potentially enable additional opportunities for optimizations and size
reductions).  This should incidentally help static linking.
>
> In order to achieve this, we would need to have a module specific
annotation to indicate what symbols are meant to be used outside of the module
when built in a shared configuration.  The same annotation would apply to all
targets and is expected to be applied uniformly.  This of course has a cost
associated with it: the public interfaces would need to be decorated
appropriately.  However, by having the same behaviour on all the platforms,
developers would not be impacted by the platform differences in their day-to-day
development.  The only time that developers would need to be aware of this is
when they are working on the module boundary, that is, changes which do not
change the API surface of LLVM would not need to consider the annotations.
>
> Concretely, what I believe is required to enable building with
LLVM_BUILD_LLVM_DYLIB on Windows is:
> - introduce module specific decoration (e.g. LLVM_SUPPORT_ABI, ...) to mark
public interfaces of shared library modules
> - decorate all the public interfaces of the shared library modules with the
new decoration
> - switching the builds to use `-fvisibility-default=hidden` by default
>

My recommendation would be to start with a generic decoration (e.g.
LLVM_EXPORTED_API)
and decorate all the currently exported symbols (which currently is all of them
except
for the lib/Target symbols on Linux).  This way you could switch to
-fvisibility-default=hidden
and it would have no impact on library users.  I know it's tedious work, but
you can split it up
and recruit others (like me) to help make the changes.

Once you have that working we can start trimming down the ABI and determine
whether or
not we actually need to have two libraries or not.  If the full build with a few
targets
enabled is already under the limit, I think there is a good chance we can get
a single library to work.

The advantage of this approach is that you can make a lot of progress in a very
non-invasive way.

> I believe that these can be done mostly independently and staged in the
order specified.  Until the last phase, it would have no actual impact on the
builds.  However, by staging it, we could allow others to experiment with the
option while it is under development, and allows for an easier path for
switching the builds over.
>
> Although this would enable LLVM_BUILD_LLVM_DYLIB on Windows, give us better
uniformity between Windows and non-Windows platforms, potentially enable
additional optimization benefits, improve binary sizes for a distribution of the
toolchain (though less on Linux where distributors are already using the build
configuration ignoring the official suggestions in the LLVM guides), and help
with runtime costs of the toolchain (by making the core of the tools a shared
library, the backing pages can now be shared across multiple instances), it is
not entirely without downsides.  The primary downsides that I see are:
> - it becomes less enticing to support both LLVM_BUILD_LLVM_DYLIB and
BUILD_SHARED_LIBS: while technically possible, interfaces will need to be
decorated for both forms of the build

I don't think we should make any changes to the BUILD_SHARED_LIBS build.
This is intended for developer convenience only and not recommend for
end users or distros[1].

Supporting BUILD_SHARED_LIBS will require annotating more of the API
and create more work both initially and also in ongoing maintenance,
and I don't think it is worth it.

-Tom

> - LLVM_DYLIB_COMPONENTS becomes less tractable: in theory it is possible to
apply enough CPP magic to determine where a symbol is homed, but allowing a
symbol to be homed in a shared or static library is significantly more complex
> - BUILD_SHARED_LIBS becomes more expensive to maintain: the decoration is
per-module, which requires that we would need to decorate the symbols of each
module with module specific annotations as well
>
> One argument that people make for BUILD_SHARED_LIBS is that it reduces the
overall time build-test cycle.  With the combination of lld, DWARF Fission, and
LLVM_BUILD_LLVM_DYLIB, I believe that most of the benefits still can be had.
The cost of linking all the tools is amortized across the link of a single
library, which while not as small as the a singular library, is offset by the
following:
> - The LLVM_BUILD_LLVM_DYLIB would not require the re-linking of all the
libraries for each tool.
> - DWARF Fission would avoid the need to relink all of the DWARF
information.
> - lld is faster than the gold and bfd linkers
>
> Header changes would still ripple through the system as before, requiring
rebuilding the transitive closure.  Source file changes do not have the same
impact of course.
>
> For those would like a more concrete example of what a change like this may
shape up into: https://reviews.llvm.org/D109192
<https://reviews.llvm.org/D109192> contains `LLVMSupportExports.h` which
has the expected structure for declaring the decoration macros with the rest of
the change primarily being focused on applying the decoration.  Please ignore
the CMake changes as they are there to ensure that the CI validates this without
changing the configuration and not intended to be part of the final version of
the change.
>

[1] https://llvm.org/docs/CMake.html#llvm-related-variables


> --
> Saleem Abdulrasool
> compnerd (at) compnerd (dot) org
>
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