[cfe-dev] Big Clang DLL, plug-in mechanism revision

Mon May 9 13:00:45 PDT 2011

On May 9, 2011, at 12:52 PM, Nico Weber wrote:

> On Mon, May 9, 2011 at 12:45 PM, Argyrios Kyrtzidis <kyrtzidis at apple.com> wrote:
> I agree with Anton and personally am not too fond of dllimport/dllexport.
> Could we live without the big Clang DLL and just get rid of the current static plug-in registration, replacing it with explicit plugin classes with virtual functions (whose objects get created in the plugin) ?
> 
> Also stepping back a bit, there's a "tooling" effort going on that makes it easier to build custom tools without needing plugins, isn't this a better way to go ?
> 
> We're using a plugin that runs in addition to normal codegen on the chrome buildbots (the -add-plugin flag). It outputs additional warnings ( http://www.chromium.org/developers/coding-style/chromium-style-checker-errors has a list if you're curious). Running with the plugin slows down our build by a few seconds (a full build is 40 minutes, so it's a very small percentage). Doing this with the tooling stuff would be both slower and more annoying. TL;DR: I think the plugin stuff is probably useful even when the tooling stuff is there.

Ok, I'm only questioning the need for the big Clang DLL. We can get rid of the static registration using a "standardized" exported function from the plugin and a couple of virtual abstract classes.
Can we be more specific about what is the use case that the above will not suffice and we need a big DLL with all classes exported ?

>  
> Keep in mind that dynamically loading plugins is of reduced usefulness since we don't have a stable API.
> 
> (I'm fine with breaking changes to the plugin api.)
>  
> 
> -Argyrios
> 
> On May 9, 2011, at 11:57 AM, John Thompson wrote:
> 
>> My collegue suggested that a clearer statement of the problems to be solved might help, since overall it was kind of spread out over a few threads over a couple of months.
>>  
>> 1. Plug-in registration.  The current plug-in mechanism doesn't work on Windows.  Shared libraries work differently on Windows, such that building a plug-in DLL on windows doesn't create a connection to the plug-in registry in the libraries used in the Clang executable.  The result is that in running a plug-in like the PrintFunctionNames plug-in, there is no output becase the plug-in is adding an entry to a registry that exists only in it's own data space.
>>  
>> My solution for this was to dumb-down the registry a bit, and to have the plug-in export a function to be called that creates and returns the plug-in action object.
>>  
>> Even if the big-DLL change in #2 is not acceptable, I strongly need the plug-in mechanism to work, as I wrote Lua-bindings plug-in, and I need to have it work on Windows.  So it's very important to me to either have this fix accepted, or get feedback on a more acceptable solution, or have someone else fix it.
>>  
>> 2. Big Clang DLL. If you build a plug-in or other shared library on Windows, the DLL is much like a position-independent executable in that it has it's own code and data sections, with absolute addresses fixed up at runtime.  When the DLL runs, it typically will only call versions of code and static data that live in the DLL itself.  An exception is if you pass in an object with virtual function that is created in a separate executable.  By means of the vtable, the virtual functions called will be those from the executable where the constructor lives.
>>  
>> If it is expected that the DLL will read or make changes to static data not explicitly or indirectly passed to it, this will not happen, as the DLL is only operating on it's own data space, or the data pointers explicitly passed to it.
>>  
>> Fortunately, because both the Clang executable and the plug-ins are created using the DLL version of the VC++ runtime, plug-ins that don't rely on static data from the main executable can still work (as my Lua bindings plug-in does).  This means that values returned from calls like malloc can be freed from either the main executable or the DLL.  But I fear that we might encounter other plug-ins that won't work properly, without true sharing of the common code and data.
>>  
>> Again, I need at least #1, and hope it can go in soon (leaving in the old registry for a while until all plug-ins use the new mechanism).  I don't personally need #2, but I think it should be done, and I'm willing to do the work.
>>  
>> Thanks.
>>  
>> -John
>> 
>> On Wed, May 4, 2011 at 6:06 PM, John Thompson <john.thompson.jtsoftware at gmail.com> wrote:
>> Hi,
>>  
>> May I solicit some feedback on the work I've done for these two issues?
>>  
>> Basically I'm attacking the problem from two separate angles, doing the following changes in two separate trees.
>>  
>> In one, I revised the plug-in mechanism, dumbing it down a bit, to avoid the problems encountered on Windows where the plug-in registry referenced in the plug-in DLL doesn't connect with the plug-in registry in the clang executable.  I say dumbing it down a bit because it uses a more basic vector of structures for storing the plugin name and a callback function for creating the action node.  The original patch was kind of old, so I updated it, using one new file from the other tree for the __cdeclspec(dllexport) wrapper.  Note that I left in the original plug-in registry stuff side-by-side.  I tested it the earlier version on both Windows and Linux.  It still has the potential problem of the plug-in not having access to everything in the clang executable, which can only be resolved by using a big Clang DLL.  Here's the patches for this change:
>>  
>> http://www.jtlanguage.com/xfer/plugin_llvm_patch.txt
>> http://www.jtlanguage.com/xfer/plugin_clang_patch.txt
>>  
>> In the other, I did the ground work for creating a big Clang DLL from all the LLVM and Clang sources used in the original Clang executable.  It currently only exports a small subset of the functions, mostly only those needed to get the Clang and other executables to link correctly.  I'll recap the scheme later in this email, revising and pasting info from my original messages, with some updates.  The clang.exe executable links and runs, though there is some dependency problem I don't understand yet when doing the "ALL_BUILD".  In running the tests, some of the Index tests fail for unknown reasons.  Otherwise, without the cmake "LLVM_BUILD_DLL" option, the normal build and test results are unchanged.  Yesterday I tested the patch on Linux and fixed a couple of compile problems.  Basically, this shows that the basic big DLL mechanism works, and that all that is needed is to add the rest of the "*_LINKAGE" tags and fix the dependency and Index test problems.  Here's the patches for this change:
>>  
>> http://www.jtlanguage.com/xfer/dll_llvm_patch.txt
>> http://www.jtlanguage.com/xfer/dll_clang_patch.txt
>>  
>> Note that the big Clang DLL does not automatically fix the original problem of the plug-in registry, as the needed internal registry symbols are not exported.  So the full solution of fixing the plug-in registry mechanism probably requires both of these changes, the first for the registering mechanism, and the second for allowing the plug-in to full access of the Clang code and data.
>>  
>> Basically I'm looking for feedback on these two schemes, or a go-ahead with checking in what I have so far, for a staged adoption.  Because the DLL scheme still needs a lot of work in adding the "*_LINKAGE" tags to so many declarations, I want to find out if the owners of the respective systems approve of these changes, rather than continuing with all the work and finding that it will not be used.  I know it's a pretty big change, and Windows-specific, but I think it's important enough to try.
>>  
>> I'm thinking I could check in what I have at present (with any changes required), which has the build changes and the new module-common headers, and the subset of "*_LINKAGE" tag additions that let the link succeed, and then check in the remaining changes in smaller batches, as I add the remaining "*_LINKAGE" tags.
>>  
>> I'm not sure how the big Clang DLL fits into the long-term scheme of things, whether it remains a special build option on Windows, or becomes the mainstream version.  For now just leave it as a special build option.  I'm also not sure if any other LLVM modules should be in the DLL, and whether there should also be a separate LLVM DLL.  I started doing a little work on this, but didn't include it here.
>>  
>> So here's stuff from the original email for the DLL stuff revisions, as I missed a description of something I did on the LLVM side in the cmake files, and I hadn't gotten the unit test mechanism to work at all.  Sorry for the verbosity; I'm just trying to be complete:
>>  
>> The scheme is meant to satisfy the following design goals/constraints:
>> 1. Create a single Windows DLL including the main Clang and LLVM modules needed for a compiler and other tools, such that it's the only module you need to link against (from the Clang/LLVM project).
>> 2. Preserve the current module organization and build, leaving the static-library and other platform's shared library versions as they are. (This leaves open the option of creating per-module Windows DLLS in the future.)
>> 3. Enable Clang plug-ins to share the code and data space with the Clang or other tool executables.
>> 4. Make the "big" DLL version optional by means of cmake arguments given upon running cmake. (I'm not sure of the criteria for deciding whether to make it the default for Windows or not.)
>> These are the build changes, with respect to the cmake build tool:
>> 1. A "BUILD_LLVM_DLL" configuration flag is added to the cmake build, such that specifying "-DBUILD_LLVM_DLL=ON" in the cmake command line will enable the big-DLL build. By default it is off.
>> 2. Factor out the common portions of the "add_clang_library" into "add_clang_library_common".
>>  
>> 3. Modify the cmake macro "add_clang_library" to collect the source file full path strings in a new target property.
>>  
>> 4. Add a new "add_clang_dll" cmake macro for creating the big DLL. This macro operates like "add_clang_library", but always builds a DLL target, and doesn't collect the source paths. It also passes "LLVM_USE_DLL" and "LLVM_EXPORTS" defines to the compiler, to enable the symbol exports to be described later.
>> 5. Create a new "ClangDLL" project under tools/clang/lib for creating the big DLL, which uses the new "add_clang_dll" macro.
>> 6. Add new "set_clang_components" and "set_clang_llvm_components" cmake macros for use in conjunction with "add_clang_executable" to collect the Clang and LLVM components, as opposed to setting "LLVM_USED_LIBS" and "LLVM_LINK_COMPONENTS" directory. (These macros will substitute the big DLL for the component libraries.)
>> 7. Modify "add_clang_executable" to pass an "LLVM_USE_DLL" define to the compiler if the big-DLL version is enabled. This define enables the symbol imports/exports to be described later.
>>  
>> 8. (new) Do similar changes to the LLVM-side cmake macros, such as chaning add_llvm_library to collect the sources.
>> 9. Update the clang executable, interpreter, and PrintFunctionNames projects to use the new and revised cmake macros.
>> The source changes mainly consist of adding "(moduleNameUppercase)_LINKAGE" macro instances to the declarations of the classes, functions, and other symbols to be exported/imported, and new header files that define these macros. These changes can be summarized as follows:
>> 1. A new "(moduleName)Common.h" file is added to each LLVM and Clang module include directory. This file defines a "(moduleNameUppercase)_LINKAGE" macro for that component module. This macro expands to nothing for the non-big-DLL version ("LLVM_USE_DLL" is not defined to the compiler), or to the LLVM_EXPORT macro when building the DLL ("Clang_EXPORTS" or "LLVM_EXPORTS" or "(moduleName)_EXPORTS" is defined), or to the LLVM_IMPORT macro when building against the DLL ("LLVM_USE_DLL" is defined). This include file will be included by any headers in the component needing the "(moduleNameUppercase)_LINKAGE" macro. 
>> 2. A new "LLVMCommon.h" file is added to the include/llvm directory under the root to define "LLVM_EXPORT" and "LLVM_IMPORT" macros that wrap the Visual C++ "__declspec(dllexport)" and "__declspec(dllimport)" directives, as well as to define the "LLVMCORE_LINKAGE" macro. This file will be included by all the "(moduleName)Common.h" files for the LLVM components.
>> 3. A new "ClangCommon.h" file is added to the tools/clang/include/clang directory. It just includes the "LLVMCommon.h" file, and will be included by the "(moduleName)Common.h" files for the Clang components.
>> In my experimental version, I added most of the "(moduleName)Common.h" files, but I only added the "(moduleNameUppercase)_LINKAGE" macro instance to some of the symbols, at least the ones needed by the Clang driver and interpreter to link.
>> Caveats:
>> 1. The ALL_BUILD project build breaks on some missing .inc files from the X86 target project. I think this has to do with an inherent dependency problem in the cmake build. Even though the "(target)CodeGenTable_gen" project dependency is added to the DLL, the .inc files are not created before the DLL link. I will look into this. A rebuild of the "clang" project does work, as that was the focus of my experiment. The interpreter might build too, as it did earlier.
>> 2. Several of the "Index" tests don't pass.  I haven't figured out why yet.
>> 3. As I mentioned, I only exported some of the symbols, those needed to get the clang executables to build. A big edit (manual or automated) will be needed to add the "(moduleNameUppercase)_LINKAGE" macros to the rest of the symbols, and any needed "(moduleName)Common.h" headers not yet created.
>> 4. I only enabled the X86 target in LLVM (the default for the Windows build in general), for the purpose of this experiment. The other targets will need to be done as well.
>> 5. I tried running the PrintFunctionNames plugin, but there's still a disconnect between the registry referenced by the plugin and the one in the big DLL. I didn't look into it deep enough yet, but I'm guessing it has something to do with the template mechanism used not being exported. I'm not even sure how to export it. But I think the better solution is to switch to the simplified plugin mechanism I provided a patch for in a previous post (and which I'm still waiting for a review fornudge-nudge).
>> 6. In building on Windows with the big DLL mechanism enabled, there are a lot of "class 'someTemplateClass<_Ty>' needs to have dll-interface to be used by clients of class 'someClass'" warnings. These just need to have some extra template specialization declarations with the "*_LINKAGE" tag.
>>  
>> You can search on "CMakeLists.txt" and ".cmake" to see the cmake file changes, or "Common.h" to see the new headers. Most of the rest of the changes are just the adding of the "(moduleNameUppercase)_LINKAGE" macros to the class and function definitions.
>>  
>> Thanks!
>>  
>> -John
>> 
>> -- 
>> John Thompson
>> John.Thompson.JTSoftware at gmail.com
>> 
>> 
>> 
>> 
>> -- 
>> John Thompson
>> John.Thompson.JTSoftware at gmail.com
>> 
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