[llvm-dev] Supporting LLVM_BUILD_LLVM_DYLIB on Windows

[Chris Tetreault via llvm-dev]

Let me start by saying that I think this is important work that you are undertaking. This is a huge task, and it you pull it off you will be a hero. Do you have any notion of how big the distribution would be on Windows with the dll build? 2 GB is ridiculous!

At a previous workplace, we had a similar problem, and we solved it in much the way that you propose: by having a magic export macro that we decorated all external symbols with. The main problem was the human factor: there are those that don’t develop on Windows (we didn’t disable exporting everything by default on Linux and Mac), and it was a constant battle to get these developers to use the export macro. If we disable exporting everything on not-Windows, that will at least reduce the amount of people that fail to use the macro. (as they will get linker failures and presumably the build bots will complain) However, there will always be a contingent of people saying things like “This problem that you are solving doesn’t affect me, and now I a am being asked to do extra work! I don’t like this!” My personal opinion is that:

```
class LLVM_SUPPORT_ABI Foo { …
```

… is not materially harder to write than:

```
class Foo { …
```

… and this sort of stuff is just a fact of life in writing C++ code that is portable between Windows and Linux.

However, if you don’t dynamically link in your day-to-day work, then it’s easy to mess this up. Is it `void LLVM_SUPPORT_ABI bar()` or `LLVM_SUPPORT_ABI void bar()`? Do I have to decorate class members, or just the class? When can I omit it? Do I have to decorate templated code? If you’re statically linking `LLVM_SUPPORT_ABI` preprocesses to nothing, so you can put it literally anywhere and your code will compile. I can’t think of how to do it offhand, and maybe it’s not possible, but if the system could somehow be rigged to fail to compile if you do it wrong, even for statically linked builds that would be nice. If this could be done, it would also help with the transition. At some point you’re going to flip the switch on this and everybody is going to get random breakages for a few days while they hunt down the stragglers.

All I can say is: “good luck, I’ll be rooting for you!”

Thanks,
   Chris Tetreault

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Saleem Abdulrasool via llvm-dev
Sent: Wednesday, September 8, 2021 3:52 PM
To: llvm-dev <llvm-dev at lists.llvm.org>
Cc: Saleem Abdulrasool <abdulras at google.com>; Hans Wennborg <hwennborg at google.com>
Subject: [llvm-dev] Supporting LLVM_BUILD_LLVM_DYLIB on Windows


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Hello llvm-dev,

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  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
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