Add support in CMake to statically link the C++ standard library.

[erichkeane]

Summary

It is sometimes useful to have the C++ standard library linked into the
assembly when compiling clang, particularly when distributing a compiler
onto systems that don't have a copy of stdlibc++ or libc++ installed.

This functionality should work with either GCC or Clang as the host
compiler, though statically linking libc++ (as may be required for
licensing purposes) is only possible if the host compiler is Clang with
a copy of libc++ available.

[erichkeane]

Unfortunately, this ends up adding a warning or two to the build if you enable this, since we have a couple of things built in "C" (flag not used...). In the 'clang' test I did, ./bin/count was the only one. Unfortunately, we don't have separate C vs C++ linking flags, so I think this is just something that someone using this flag needs to deal with.

[erichkeane]

Does anyone have issues with this? Can I commit it?

[MaskRay]

In D65603#1625980, @erichkeane wrote:

Does anyone have issues with this? Can I commit it?

@erichkeane
I think the problem is that LLVM_STATIC_LINK_CXX_STDLIB may be too niche. Do people want to set CMAKE_SHARED_LINKER_FLAGS at the same time? On Linux, unless you set LIBCXX_STATICALLY_LINK_ABI_IN_STATIC_LIBRARY, -static-libstdc++ -stdlib=libc++ does not really work. Considering all these, I do not know whether the configure time variable pulls its weight. Personally I'd avoid such complexity.

llvm/trunk/CMakeLists.txt

 if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
   option(LLVM_ENABLE_MODULE_DEBUGGING "Compile with -gmodules." ON)
   option(LLVM_ENABLE_LOCAL_SUBMODULE_VISIBILITY "Compile with -fmodules-local-submodule-visibility." OFF)
 else()
   option(LLVM_ENABLE_MODULE_DEBUGGING "Compile with -gmodules." OFF)
   option(LLVM_ENABLE_LOCAL_SUBMODULE_VISIBILITY "Compile with -fmodules-local-submodule-visibility." ON)
 endif()
 option(LLVM_ENABLE_LIBCXX "Use libc++ if available." OFF)
+option(LLVM_STATIC_LINK_CXX_STDLIB "Statically link the standard library." OFF)
 option(LLVM_ENABLE_LLD "Use lld as C and C++ linker." OFF)
 option(LLVM_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
 option(LLVM_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 
 option(LLVM_ENABLE_DUMP "Enable dump functions even when assertions are disabled" OFF)
 
 if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
   option(LLVM_ENABLE_ASSERTIONS "Enable assertions" OFF)

llvm/trunk/cmake/modules/HandleLLVMStdlib.cmake

if(LLVM_COMPILER_IS_GCC_COMPATIBLE)
         set(LLVM_LIBCXX_USED 1)
       else()
         message(WARNING "Can't specify libc++ with '-stdlib='")
       endif()
     else()
       message(WARNING "Not sure how to specify libc++ for this compiler")
     endif()
   endif()
+
+  if(LLVM_STATIC_LINK_CXX_STDLIB)
+    if(LLVM_COMPILER_IS_GCC_COMPATIBLE)
+      check_cxx_compiler_flag("-static-libstdc++"
+                              CXX_COMPILER_SUPPORTS_STATIC_STDLIB)
+      check_linker_flag("-static-libstdc++" CXX_LINKER_SUPPORTS_STATIC_STDLIB)
+      if(CXX_COMPILER_SUPPORTS_STATIC_STDLIB AND
+        CXX_LINKER_SUPPORTS_STATIC_STDLIB)
+        append("-static-libstdc++"
+          CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS
+          CMAKE_MODULE_LINKER_FLAGS)
+      else()
+        message(WARNING
+          "Can't specify static linking for the C++ standard library")
+      endif()
+    else()
+      message(WARNING "Not sure how to specify static linking of C++ standard "
+        "library for this compiler")
+    endif()
+  endif()
 endif()

llvm/trunk/docs/CMake.rst

**LLVM_USE_LINKER**:STRING
   search. For example to link LLVM with the Gold linker, cmake can be invoked
   with ``-DLLVM_USE_LINKER=gold``.
 
 **LLVM_ENABLE_LIBCXX**:BOOL
   If the host compiler and linker supports the stdlib flag, -stdlib=libc++ is
   passed to invocations of both so that the project is built using libc++
   instead of stdlibc++. Defaults to OFF.
 
+**LLVM_STATIC_LINK_CXX_STDLIB**:BOOL
+  Statically link to the C++ standard library if possible. This uses the flag
+  "-static-libstdc++", but a Clang host compiler will statically link to libc++
+  if used in conjuction with the **LLVM_ENABLE_LIBCXX** flag. Defaults to OFF.
+
 **LLVM_ENABLE_LLD**:BOOL
   This option is equivalent to `-DLLVM_USE_LINKER=lld`, except during a 2-stage
   build where a dependency is added from the first stage to the second ensuring
   that lld is built before stage2 begins.
 
 **LLVM_PARALLEL_COMPILE_JOBS**:STRING
   Define the maximum number of concurrent compilation jobs.