[LLVM][CMake] Add ffi_static target for the FFI static library

[jhuber6]

Summary:
This patch is an attempt to make the find_package(FFI) support in LLVM
prefer to provide the static library version if present. This is currently
an optional library for building libffi, and its presence implies that
it should likely be used. This patch is an attempt to fix some problems
observed with testing programs linked against libffi on many different
systems that could have conflicting paths. Linking it statically
prevents this.

This patch adds the ffi_static target for this library.

[doru1004]

LGTM

[petrhosek]

With other dependencies, we use dedicated flags, see for example:

llvm-project/llvm/CMakeLists.txt

Line 541 in 30aa9fb

set(LLVM_USE_STATIC_ZSTD FALSE CACHE BOOL "Use static version of zstd. Can be TRUE, FALSE")

Inside the FindFFI module, we would ideally expose the static library as a separate target, see for example:

llvm-project/llvm/cmake/modules/Findzstd.cmake

Lines 21 to 23 in 30aa9fb

	find_library(zstd_STATIC_LIBRARY NAMES
	zstd_static
	"${CMAKE_STATIC_LIBRARY_PREFIX}zstd${CMAKE_STATIC_LIBRARY_SUFFIX}")

[MaskRay]

+1 for a new CMake variable. Many distributions prefer dynamic linking. Linking in libffi.a could cause a conflict (usually benign) if a program links against both libLLVM.so and libffi.so.

For libLLVM*.a users, it's possible that libffi.a works better.

[jhuber6]

With other dependencies, we use dedicated flags, see for example:

llvm-project/llvm/CMakeLists.txt

Line 541 in 30aa9fb

set(LLVM_USE_STATIC_ZSTD FALSE CACHE BOOL "Use static version of zstd. Can be TRUE, FALSE")

Inside the FindFFI module, we would ideally expose the static library as a separate target, see for example:

llvm-project/llvm/cmake/modules/Findzstd.cmake

Lines 21 to 23 in 30aa9fb

	find_library(zstd_STATIC_LIBRARY NAMES
	zstd_static
	"${CMAKE_STATIC_LIBRARY_PREFIX}zstd${CMAKE_STATIC_LIBRARY_SUFFIX}")

Thanks for the pointer, made the change and some example usage.

[ronlieb]

applied locally and tested , LGTM

[MaskRay]

I am not familiar with CMake, but should there be a LLVM_USE_STATIC_FFI to select libffi.a when both libffi.so and libffi.a are present?

[jhuber6]

I am not familiar with CMake, but should there be a LLVM_USE_STATIC_FFI to select libffi.a when both libffi.so and libffi.a are present?

The user can decide which one to use based off of whether or not FFI_STATIC_LIBRARIES was set, as in the changed code in libomptarget.