[llvm-dev] r250501 adds dependancy to ole32.dll on MSVC

Tue Jan 12 11:55:57 PST 2016

I guess I don’t see much downside in having all of these libs default in. It’s not like you can’t link against these libs, once you take a dependence on them, so not defaulting just passes the obligation down to every client to figure it out.

/NOD usually indicates you are up to something tricky.

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Reid Kleckner via llvm-dev
Sent: Tuesday, January 12, 2016 11:40 AM
To: Aaron Ballman <aaron at aaronballman.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] r250501 adds dependancy to ole32.dll on MSVC

My feeling is that using the autolink pragma is consistent with LLVM being distributed primarily as static libraries, and having no runtime dependencies other than the base system libraries. All these DLLs are "system libraries" similar to libc on Linux or libSystem.dylib on Mac.

Users should be able to take the LLVM libraries and link their own executable, without adding extra linker flags, i.e. 'cl myapp.cpp LLVMSupport.lib' or 'gcc myapp.cpp libLLVMSupport.a'.

I wasn't aware of the distinction between default libs and non-default libs before you brought it up. I just think it's nice for users to not have to screw around with flags. I don't feel strongly, and if you want to standardize on only auto-linking non-default system libs (advapi32 and dbghelp), that sounds good to me.

On Tue, Jan 12, 2016 at 11:25 AM, Aaron Ballman <aaron at aaronballman.com<mailto:aaron at aaronballman.com>> wrote:
I am not comfortable with this commit. I was under the mistaken
impression that we were in agreement on not linking in default
libraries like OLE32.dll, so this commit is a bit of a surprise. I
don't think it needs to be immediately reverted (though if this is
still being discussed by the time we do the 3.8 freeze, I would like
it reverted so we're not committing to supporting it until we're
ready), but I really would like to understand the design guidance. I
don't see why OLE32.dll is special, while user32.dll is not. Right now
we are very inconsistent, we use the pragma for psapi (twice, not a
default lib), dbghelp (not not a default lib), shell32 (default lib),
advapi32 (default lib), and now OLE32 (default lib).

If we don't want default libs, I would like to see shell32, ole32, and
advapi32 pragmas removed. If we do want default libs, I would like to
(1) find a common place to put all of these pragmas for the default
libs (and perhaps the nondefault libs?) so we don't needlessly
duplicate them, and (2) add pragmas for all of the standard default
libraries we rely on, and (3) set /NODEFAULTLIB to be clear that we
are managing dependencies manually. If we don't want any pragmas, we
should remove all of them and update the CMake files appropriately.

~Aaron

On Tue, Jan 12, 2016 at 1:37 PM, Reid Kleckner <rnk at google.com<mailto:rnk at google.com>> wrote:
> r257499
>
> On Sun, Jan 10, 2016 at 1:23 PM, Jakob Bornecrantz <wallbraker at gmail.com<mailto:wallbraker at gmail.com>>
> wrote:
>>
>> On Thu, Dec 24, 2015 at 1:28 AM, Jakob Bornecrantz <wallbraker at gmail.com<mailto:wallbraker at gmail.com>>
>> wrote:
>> > On Wed, Dec 23, 2015 at 9:38 PM, Aaron Ballman <aaron at aaronballman.com<mailto:aaron at aaronballman.com>>
>> > wrote:
>> >> On Wed, Dec 23, 2015 at 3:28 PM, Reid Kleckner <rnk at google.com<mailto:rnk at google.com>> wrote:
>> >>> On Wed, Dec 23, 2015 at 11:25 AM, Aaron Ballman via llvm-dev
>> >>> <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
>> >>>>
>> >>>> On Wed, Dec 23, 2015 at 1:55 PM, Jakob Bornecrantz
>> >>>> <wallbraker at gmail.com<mailto:wallbraker at gmail.com>>
>> >>>> wrote:
>> >>>> > On Wed, Dec 23, 2015 at 5:49 PM, Aaron Ballman
>> >>>> > <aaron at aaronballman.com<mailto:aaron at aaronballman.com>>
>> >>>> > wrote:
>> >>>> >> On Wed, Dec 23, 2015 at 11:29 AM, Jakob Bornecrantz via llvm-dev
>> >>>> >> <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
>> >>>> >>> I'm building on Windows x64 using cmake, Ninja and VS 2013
>> >>>> >>> express on
>> >>>> >>> Windows 7.
>> >>>> >>>
>> >>>> >>> So I have been using the LLVMSharp method on getting a usable
>> >>>> >>> loadable
>> >>>> >>> LLVM.dll[1][2].
>> >>>
>> >>>
>> >>> One day, I would like to make it easier to make LLVM.dll...
>> >
>> > We (not I) have rewritten the powershell script into a python
>> > script[1] and where hoping that we could commit that to LLVM and have
>> > it make a DLL during regular MSVC builds. And if possible shipping
>> > that DLL with the Windows package (and Windows snapshot builds[2]).
>> >
>> >>>
>> >>>>
>> >>>> > Further investigation show that for the MinGW build OLE32 is added
>> >>>> > to
>> >>>> > the dependencies for LLVMSupport.lib in the CMakeLists.txt file,
>> >>>> > there
>> >>>> > are a couple of other libraries added as well. This lead me to find
>> >>>> > that those where added for the MSVC build with a pragma(lib, xx),
>> >>>> > so I
>> >>>> > added such a pragma for OLE32 and that fixes my script that is
>> >>>> > generating LLVM.dll, will also fix other peoples problems with
>> >>>> > OLE32
>> >>>> > missing as well.
>> >>>>
>> >>>> Generally speaking, static libraries like LLVMSupport are just an
>> >>>> archive of a bunch of compiled object files, and whatever ultimately
>> >>>> consumes that library is responsible for providing external
>> >>>> definitions. So, for instance, clang.exe consumes LLVMSupport.lib and
>> >>>> so it also links in OLE32. I believe that we usually only use
>> >>>> #pragma(lib) to signal that non-standard libraries need to be linked
>> >>>> in, otherwise every source file in an archive would wind up with an
>> >>>> unwieldy number of pragmas for all its imports, or you would have to
>> >>>> manually maintain that list in some common header file. I don't think
>> >>>> that the pragma(lib) for advapi32.lib should be there either, FWIW,
>> >>>> but I don't know that we've ever had a hard-and-fast rule for this
>> >>>> sort of thing.
>> >>>
>> >>>
>> >>> Sure, but the fact that static libraries can't encode their
>> >>> dependencies has
>> >>> always been an annoying missing feature, not something that we want to
>> >>> follow if we can avoid it.
>> >>
>> >> Agreed.
>> >>
>> >>>>
>> >>>> I'm not opposed to this patch, per se, but it feels like a slippery
>> >>>> slope as to what makes the cut and what does not. I would rather see
>> >>>> *less* non-standard pragma usage instead of more.
>> >>>
>> >>>
>> >>> For these kinds of DLLs that are available on all modern versions of
>> >>> Windows, I think it's perfectly reasonable to use the 'pragma comment
>> >>> lib'
>> >>> auto linking mechanism. Realistically, no consumer of LLVMSupport.lib
>> >>> is
>> >>> going to be surprised if it needs ole32.dll.
>> >>
>> >> I think we're making the same point -- no consumer of LLVMSupport.lib
>> >> should expect to work if it doesn't use the stock set of library
>> >> dependencies. So use #pragma(lib) for everything that's not in that
>> >> set, but expect consumers to at least be using the stock set. Unless
>> >> you're suggesting we stick a #pragma(lib, user32) (et al) somewhere as
>> >> well?
>> >
>> > So ole32 and friends are added by default by cmake[3] which is why you
>> > are seeing them on the executables.
>> >
>> > Looking through the source I find advapi32.lib, psapi.lib, shell32.lib
>> > and dbghelp.lib are added via pragmas. Of these psapi.lib and
>> > dghelp.lib are not added to the default libs by cmake[3]. So just
>> > removing the pragmas would break the build without adding them back
>> > into the linking libs in cmake. Tho it would break all non-cmake based
>> > builds (that are using LLVM components) out there that don't happen to
>> > also link those libs. That is for versions < 3.8 non-cmake builds
>> > worked magically thanks to the pragmas.
>> >
>> > I have no real say in what path you should take here. Pragmas for me
>> > would be the easiest for me, since they just magically works, tho I
>> > appreciate the technical ugliness they present. That said if you
>> > remove the pragmas the list of libraries needed should be documented
>> > somewhere and probably also be returned by llvm-config.exe.
>> >
>> > If we keep the pragmas I guess we could have does link.exe add the
>> > libs by default as a basis for which pragmas we should add in. I can't
>> > experiment right now but user32, kernel32 and friends seems be
>> > included by default by link.exe or they are pragmas in the MSVC
>> > headers that the LLVM build picks.
>> >
>> > Thanks again for your time.
>>
>> Ping?
>>
>> Cheers, Jakob.
>>
>> >
>> > [1] https://github.com/VoltLang/GenLLVMDLLPy/blob/master/GenLLVMDLL.py
>> > [2] http://llvm.org/builds/<https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fllvm.org%2fbuilds%2f&data=01%7c01%7candya%40microsoft.com%7c8ef1260a717644277df108d31b8831be%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=%2buYaaCNdm2VuNMJ8zsV0SUttHdPtvX6PQEgdubeGYwU%3d>
>> > [3]
>> > https://cmake.org/gitweb?p=cmake.git;a=blob;f=Modules/Platform/Windows-MSVC.cmake<https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fcmake.org%2fgitweb%3fp%3dcmake.git%3ba%3dblob%3bf%3dModules%2fPlatform%2fWindows-MSVC.cmake&data=01%7c01%7candya%40microsoft.com%7c8ef1260a717644277df108d31b8831be%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=q9jKg2rbOkktksPWQGiOzhKhQLglYW4WoDDs%2ffuQMFg%3d>
>
>

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