cmake: Add check for libatomic linking requirement

Some archs require linking against libatomic, others don't. We add some
CMake code that checks for libatomic.so requirement if:
- We are not on MSVC, AND
- Compiling std::atomics code would cause a linker error.

We then check for the existence of libatomic.so, and fail if we can't
find it.

1 files changed, 80 insertions, 0 deletions

diff --git a/host/cmake/Modules/UHDAtomics.cmake b/host/cmake/Modules/UHDAtomics.cmake
new file mode 100644
index 000000000..4d53a2d0b
--- /dev/null
+++ b/host/cmake/Modules/UHDAtomics.cmake
@@ -0,0 +1,80 @@
+#
+# This file is mostly inspired by
+# https://github.com/llvm-mirror/llvm/blob/master/cmake/modules/CheckAtomic.cmake
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+
+include(CheckCXXSourceCompiles)
+include(CheckLibraryExists)
+
+# Sometimes linking against libatomic is required for atomic ops, if
+# the platform doesn't support lock-free atomics.
+
+function(CHECK_WORKING_CXX_ATOMICS varname)
+    CHECK_CXX_SOURCE_COMPILES("
+        #include <atomic>
+        std::atomic<int> x;
+        int main() {
+        return x;
+        }
+        " ${varname})
+endfunction(CHECK_WORKING_CXX_ATOMICS)
+
+function(CHECK_WORKING_CXX_ATOMICS64 varname)
+    CHECK_CXX_SOURCE_COMPILES("
+        #include <atomic>
+        #include <cstdint>
+        std::atomic<uint64_t> x (0);
+        int main() {
+          uint64_t i = x.load(std::memory_order_relaxed);
+          return 0;
+        }
+        " ${varname})
+endfunction(CHECK_WORKING_CXX_ATOMICS64)
+
+macro(CHECK_ATOMICS_LIB_REQUIRED required_var)
+    set(${required_var} FALSE)
+    if(MSVC)
+        set(${required_var} FALSE)
+    else()
+        # First check if atomics work without the library.
+        CHECK_WORKING_CXX_ATOMICS(HAVE_CXX_ATOMICS_WITHOUT_LIB)
+        # If not, check if the library exists, and atomics work with it.
+        if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
+            check_library_exists(atomic __atomic_fetch_add_4 "" HAVE_LIBATOMIC)
+            if(HAVE_LIBATOMIC)
+                set(${required_var} TRUE)
+                set(CMAKE_REQUIRED_LIBRARIES "atomic")
+                CHECK_WORKING_CXX_ATOMICS(HAVE_CXX_ATOMICS_WITH_LIB)
+                if (NOT HAVE_CXX_ATOMICS_WITH_LIB)
+                    message(FATAL_ERROR "Host compiler must support std::atomic!")
+                endif()
+                unset(CMAKE_REQUIRED_LIBRARIES)
+            else()
+                message(
+                    FATAL_ERROR
+                    "Host compiler appears to require libatomic, but cannot find it.")
+            endif()
+        endif()
+        # Same check, but for 64-bit atomics.
+        CHECK_WORKING_CXX_ATOMICS64(HAVE_CXX_ATOMICS64_WITHOUT_LIB)
+        # If not, check if the library exists, and atomics work with it.
+        if(NOT HAVE_CXX_ATOMICS64_WITHOUT_LIB)
+            check_library_exists(atomic __atomic_load_8 "" HAVE_CXX_LIBATOMICS64)
+            if(HAVE_CXX_LIBATOMICS64)
+                set(${required_var} TRUE)
+                set(CMAKE_REQUIRED_LIBRARIES "atomic")
+                CHECK_WORKING_CXX_ATOMICS64(HAVE_CXX_ATOMICS64_WITH_LIB)
+                unset(CMAKE_REQUIRED_LIBRARIES)
+                if (NOT HAVE_CXX_ATOMICS64_WITH_LIB)
+                    message(FATAL_ERROR "Host compiler must support 64-bit std::atomic!")
+                endif()
+            else()
+                message(
+                    FATAL_ERROR
+                    "Host compiler appears to require libatomic for 64-bit operations, but cannot find it.")
+            endif()
+        endif()
+    endif()
+endmacro(CHECK_ATOMICS_LIB_REQUIRED)