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Revert "[Docs][Windows Itanium] Add a How-To document for Windows Itanium."
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==========================================
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How to build Windows Itanium applications.
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==========================================
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Introduction
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============
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This document contains information describing how to create a Windows Itanium toolchain.
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Windows Itanium allows you to deploy Itanium C++ ABI applications on top of the MS VS CRT.
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This environment can use the Windows SDK headers directly and does not required additional
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headers or additional runtime machinery (such as is used by mingw).
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Windows Itanium Stack:
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* Uses the Itanium C++ abi.
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* libc++.
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* libc++-abi.
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* libunwind.
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* The MS VS CRT.
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* Is compatible with MS Windows SDK include headers.
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* COFF/PE file format.
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* LLD
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Note: compiler-rt is not used. This functionality is supplied by the MS VCRT.
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Prerequisites
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=============
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* The MS SDK is installed as part of MS Visual Studio.
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* Clang with support for the windows-itanium triple.
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* COFF LLD with support for the -autoimport switch.
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Known issues:
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=============
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SJLJ exceptions, "-fsjlj-exceptions", are the only currently supported model.
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link.exe (the MS linker) is unsuitable as it doesn't support auto-importing which
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is currently required to link correctly. However, if that limitation is removed
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then there are no other known issues with using link.exe.
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Currently, there is a lack of a usable Windows compiler driver for Windows Itanium.
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A reasonable work-around is to build clang with a windows-msvc default target and
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then override the triple with e.g. "-Xclang -triple -Xclang x86_64-unknown-windows-itanium".
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The linker can be specified with: "-fuse-ld=lld".
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In the Itanium C++ ABI the first member of an object is a pointer to the vtable
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for its class. The vtable is often emitted into the object file with the key function
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and must be imported for classes marked dllimport. The pointers must be globally
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unique. Unfortunately, the COFF/PE file format does not provide a mechanism to
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store a runtime address from another DLL into this pointer (although runtime
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addresses are patched into the IAT). Therefore, the compiler must emit some code,
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that runs after IAT patching but before anything that might use the vtable pointers,
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and sets the vtable pointer to the address from the IAT. For the special case of
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the references to vtables for __cxxabiv1::__class_type_info from typeinto objects
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there is no declaration available to the compiler so this can't be done. To allow
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programs to link we currently rely on the -auto-import switch in LLD to auto-import
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references to __cxxabiv1::__class_type_info pointers (see: https://reviews.llvm.org/D43184
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for a related discussion). This allows for linking; but, code that actually uses
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such fields will not work as they these will not be fixed up at runtime. See
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_pei386_runtime_relocator which handles the runtime component of the autoimporting
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scheme used for mingw and comments in https://reviews.llvm.org/D43184 and
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https://reviews.llvm.org/D89518 for more.
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Assembling a Toolchain:
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=======================
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The procedure is:
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# Build an LLVM toolchain with support for Windows Itanium.
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# Use the toolchain from step 1. to build libc++, libc++abi, and libunwind.
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It is also possible to cross-compile from Linux.
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One method of building the libraries in step 2. is to build them "stand-alone".
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A stand-alone build doesn't involve the rest of the LLVM tree. The steps are:
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* ``cd build-dir``
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* ``cmake -DLLVM_PATH=<path to llvm checkout e.g. /llvm-project/> -DCMAKE_INSTALL_PREFIX=<install path> <other options> <path to project e.g. /llvm-project/libcxxabi>``
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* ``<make program e.g. ninja>``
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* ``<make program> install``
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More information on standalone builds can be found in the build documentation for
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the respective libraries. The next section discuss the salient options and modifications
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required for building and installing the libraries using standalone builds. This assumes
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that we are building libunwind and ibc++ as DLLs and statically linking libc++abi into
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libc++. Other build configurations are possible, but they are not discussed here.
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Common CMake configuration options:
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-----------------------------------
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* ``-D_LIBCPP_ABI_FORCE_ITANIUM'``
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Tell the libc++ headers that the Itanium C++ ABI is being used.
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* ``-DCMAKE_C_FLAGS="-lmsvcrt -llegacy_stdio_definitions -D_NO_CRT_STDIO_INLINE"``
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Supply CRT definitions including stdio definitions that have been removed from the MS VS CRT.
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We don't want the stdio functions decalred inline as they will casuse multiple defintiion
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errors when the same symbols are pulled in from legacy_stdio_definitions.ib.
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* ``-DCMAKE_INSTALL_PREFIX=<install path>``
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Where to install the library and headers.
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Building libunwind:
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-------------------
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* ``-DLIBUNWIND_ENABLE_SHARED=ON``
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* ``-DLIBUNWIND_ENABLE_STATIC=OFF``
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libunwind can be built as a DLL. It is not dependent on other projects.
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* ``-DLIBUNWIND_USE_COMPILER_RT=OFF``
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We use the MS runtime.
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The CMake files will need to be edited to prevent them adding GNU specific libraries to the link line.
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Building libc++abi:
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------------------
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* ``-DLIBCXXABI_ENABLE_SHARED=OFF``
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* ``-DLIBCXXABI_ENABLE_STATIC=ON``
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* ``-DLIBCXX_ENABLE_SHARED=ON'``
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* ``-DLIBCXX_ENABLE_STATIC_ABI_LIBRARY=ON``
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To break the symbol dependency between libc++abi and libc++ we
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build libc++abi as a static library and then statically link it
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into the libc++ DLL. This necessitates setting the CMake file
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to ensure that the visibility macros (which expand to dllexport/import)
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are expanded as they will be needed when creating the final libc++
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DLL later, see: https://reviews.llvm.org/D90021.
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* ``-DLIBCXXABI_LIBCXX_INCLUDES=<path to libcxx>/include``
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Where to find the libc++ headers
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Building libc++:
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----------------
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* ``-DLIBCXX_ENABLE_SHARED=ON``
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* ``-DLIBCXX_ENABLE_STATIC=OFF``
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We build libc++ as a DLL and statically link libc++abi into it.
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* ``-DLIBCXX_INSTALL_HEADERS=ON``
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Install the headers.
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* ``-DLIBCXX_USE_COMPILER_RT=OFF``
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We use the MS runtime.
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* ``-DLIBCXX_HAS_WIN32_THREAD_API=ON``
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Windows Itanium does not offer a POSIX-like layer over WIN32.
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* ``-DLIBCXX_ENABLE_STATIC_ABI_LIBRARY=ON``
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* ``-DLIBCXX_CXX_ABI=libcxxabi``
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* ``-DLIBCXX_CXX_ABI_INCLUDE_PATHS=<libcxxabi src path>/include``
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* ``-DLIBCXX_CXX_ABI_LIBRARY_PATH=<libcxxabi build path>/lib``
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Use the static libc++abi library built earlier.
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* ``-DLIBCXX_NO_VCRUNTIME=ON``
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Remove any dependency on the VC runtime - we need libc++abi to supply the C++ runtime.
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* ``-DCMAKE_C_FLAGS=<path to installed unwind.lib>``
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As we are statically linking against libcxxabi we need to link
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against the unwind import library to resolve unwind references
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from the libcxxabi objects.
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* ``-DCMAKE_C_FLAGS+=' -UCLOCK_REALTIME'``
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Prevent the inclusion of sys/time that MS doesn't provide.
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Notes:
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------
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An example build recipe is available here: https://reviews.llvm.org/D88124
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@ -33,7 +33,6 @@ intermediate LLVM representation.
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GoldPlugin
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HowToBuildOnARM
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HowToBuildWithPGO
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HowToBuildWindowsItaniumPrograms
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HowToCrossCompileBuiltinsOnArm
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HowToCrossCompileLLVM
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HowToUpdateDebugInfo
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@ -199,9 +198,6 @@ Additional Topics
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Gives the steps necessary when adding a new constrained math intrinsic
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to LLVM.
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:doc:`HowToBuildWindowsItaniumPrograms`
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Notes on assembling a Windows Itanium enviroment.
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:doc:`HowToCrossCompileBuiltinsOnArm`
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Notes on cross-building and testing the compiler-rt builtins for Arm.
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