[llvm] Enable LLVM_LINK_LLVM_DYLIB by default on non-Windows platforms

As discussed in the RFC thread [1], this build configuration is a better
default because it uses less RAM and disk to build LLVM, which makes it
more accessible to beginners. The downside to the libLLVM build is that
it increases tool binary startup time, which can have major impacts on
configuration script times and the LLVM test suite itself. However, it
is better for new users configuring LLVM builds for the first time.

Long-time developers with beefy machines who prefer the finer grained
dependencies of static linking can still use the old model by opting out
with -DLLVM_LINK_LLVM_DYLIB=OFF. The fine-grained shared lib build is
also still supported via -DBUILD_SHARED_LIBS=ON.

MLIR's dylib build is configured to follow the LLVM dylib build, so this
changes the default for MLIR as well.

[1] https://discourse.llvm.org/t/rfc-llvm-link-llvm-dylib-should-default-to-on-on-posix-platforms/85908/1

Author: rnk

llvm/CMakeLists.txt

@@ -904,14 +904,26 @@ if(NOT MSVC OR LLVM_BUILD_LLVM_DYLIB_VIS)
   set(CAN_BUILD_LLVM_DYLIB ON)
 endif()
 
-cmake_dependent_option(LLVM_LINK_LLVM_DYLIB "Link tools against the libllvm dynamic library" OFF
+# Link the tools against the libllvm DSO by default.
+set(LLVM_LINK_LLVM_DYLIB_default ON)
+if (BUILD_SHARED_LIBS OR WIN32)
+  set(LLVM_LINK_LLVM_DYLIB_default OFF)
+endif()
+
+cmake_dependent_option(LLVM_LINK_LLVM_DYLIB "Link tools against the libllvm dynamic library"
+                       "${LLVM_LINK_LLVM_DYLIB_default}"
                        "CAN_BUILD_LLVM_DYLIB" OFF)
 
+message("LLVM_LINK_LLVM_DYLIB_default ${LLVM_LINK_LLVM_DYLIB_default}")
+message("BUILD_SHARED_LIBS ${BUILD_SHARED_LIBS}")
+message("LLVM_LINK_LLVM_DYLIB ${LLVM_LINK_LLVM_DYLIB}")
+
 set(LLVM_BUILD_LLVM_DYLIB_default OFF)
 if(LLVM_LINK_LLVM_DYLIB OR LLVM_BUILD_LLVM_C_DYLIB)
   set(LLVM_BUILD_LLVM_DYLIB_default ON)
 endif()
-cmake_dependent_option(LLVM_BUILD_LLVM_DYLIB "Build libllvm dynamic library" ${LLVM_BUILD_LLVM_DYLIB_default}
+cmake_dependent_option(LLVM_BUILD_LLVM_DYLIB "Build libllvm dynamic library"
+                       "${LLVM_BUILD_LLVM_DYLIB_default}"
                        "CAN_BUILD_LLVM_DYLIB" OFF)
 
 cmake_dependent_option(LLVM_DYLIB_EXPORT_INLINES "Force inline members of classes to be DLL exported when

llvm/docs/ReleaseNotes.md

@@ -77,6 +77,11 @@ Changes to LLVM infrastructure
 Changes to building LLVM
 ------------------------
 
+* On non-Windows platforms, LLVM now builds as a large shared library, libLLVM,
+  by default. To revert to the old behavior of producing and linking static
+  libraries, pass ``-DLLVM_LINK_LLVM_DYLIB=OFF`` to CMake when configuring your
+  build.
+
 Changes to TableGen
 -------------------