[llvm-dev] Supporting LLVM_BUILD_LLVM_DYLIB on Windows

Peter Collingbourne via llvm-dev llvm-dev at lists.llvm.org
Wed Sep 8 19:09:28 PDT 2021

Hi Saleem,

I am concerned that your change will increase the maintenance burden for
those of us who would prefer to develop without shared libraries. Since it
is unclear a priori where the macros will be required, developers will need
to build both with and without shared libraries in order to verify that
they aren't breaking the build for shared library users -- in effect
slowing down the development for folks who prefer to develop without shared

I think your goal should be achievable without littering the code with
macros. Perhaps on Windows you can achieve your goal with a variant of
Leonard Chan's "busybox" proposal [1] with some adjustments to account for
a lack of symlink support on Windows. Perhaps something like:

- Create a <tool name>_main() entry point for each tool that does not use
llvm::cl to parse options.
- Create a llvm.dll in the bin directory that links together all the <tool
name>_main() entry points.
- Each tool <tool name>.exe consists of:
int main() {
  <tool name>_main();
- Tools that use llvm::cl will need to be linked with all of their code in
the .exe for now. However, they can be incrementally switched away from
llvm::cl and moved into llvm.dll.


[1] https://lists.llvm.org/pipermail/llvm-dev/2021-June/151321.html

On Wed, Sep 8, 2021 at 3:52 PM Saleem Abdulrasool via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello llvm-dev,
> One of the current limitations on LLVM on Windows is that you cannot use
> 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.
> 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
> 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
> - 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 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.
> --
> Saleem Abdulrasool
> compnerd (at) compnerd (dot) org
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

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