From 71830c804be76cf6abe913ac2fe584947b7a91ea Mon Sep 17 00:00:00 2001
From: Samuel Martin <s.martin49@gmail.com>
Date: Tue, 24 May 2016 23:08:40 +0200
Subject: [PATCH] cmake: add check for explicit linking against libatomic

To use atomics functions, some toolchains requires to explicitly add
-latomic to the linker flags (because they are not provided by libc,
but libatomic).

This change adds a helper function trying to build/link a test program
using atomics, then calls it to:
* first check if atomics are directly available in the libc;
* if not and libatomic has been found, then run the same test with
  "-latomic" added to the linker flags.
The pulseview link library list is updated according to the results of
these tests.

This issue was triggered by the Buildroot farms:
  http://autobuild.buildroot.org/results/1e3/1e3101261252d5f30fdf842cc99604e4f4c25eef/build-end.log

Notes:
1- CMAKE_REQUIRED_* variables are only used in check functions. They
   are not automatically forwarded to/handled by the target commands
   (such as target_link_library), because the check functions are
   implemented as macro in CMake code, whereas many target commands
   are native.
2- Because of note #1, CMAKE_REQUIRED_LIBRARIES (or its value) must be
   explicitly passed to the target_link_library command when this is
   needed.
3- In this implementation, LIBATOMIC_LIBRARY is only set when it is
   needed; so, unconditionally appending it to PULSEVIEW_LINK_LIBS
   will produce the expected behavior.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>

---
changes v1->v2:
- use std::atomic_fetch_add_explicit function instead of
  __atomic_fetch_add_4;
- rework code using cmake_*_check_state and find_library helpers;
- quiet-ize checks and clean outputs
- extend the commit log
---
 CMakeLists.txt | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9dac69f..44f810e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -107,6 +107,55 @@ endif()
 # This will set ${CMAKE_THREAD_LIBS_INIT} to the correct, OS-specific value.
 find_package(Threads REQUIRED)
 
+
+# Check for explicit link against libatomic
+#
+# Depending on the toolchain, linking a program using atomic functions may need
+# "-latomic" explicitly passed to the linker
+#
+# This check first tests if atomics are available in the C-library, if not and
+# libatomic exists, then it runs the same test with -latomic added to the
+# linker flags.
+
+# Helper for checking for atomics
+function(check_working_cxx_atomics varname additional_lib)
+  include(CheckCXXSourceCompiles)
+  include(CMakePushCheckState)
+  cmake_push_check_state()
+  set(CMAKE_REQUIRED_FLAGS "-std=c++11")
+  set(CMAKE_REQUIRED_LIBRARIES "${additional_lib}")
+  set(CMAKE_REQUIRED_QUIET 1)
+  CHECK_CXX_SOURCE_COMPILES("
+#include <atomic>
+std::atomic<int> x;
+int main() {
+  return std::atomic_fetch_add_explicit(&x, 1, std::memory_order_seq_cst);
+}
+" ${varname})
+  cmake_pop_check_state()
+endfunction(check_working_cxx_atomics)
+
+# First check if atomics work without the library.
+# If not, check if the library exists, and atomics work with it.
+check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITHOUT_LIB "")
+if(HAVE_CXX_ATOMICS_WITHOUT_LIB)
+  message(STATUS "Atomics provided by the C-library - yes")
+else()
+  message(STATUS "Atomics provided by the C-library - no")
+  find_library(LIBATOMIC_LIBRARY NAMES atomic PATH_SUFFIXES lib)
+  if(LIBATOMIC_LIBRARY)
+    check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITH_LIB "${LIBATOMIC_LIBRARY}")
+    if (HAVE_CXX_ATOMICS_WITH_LIB)
+      message(STATUS "Atomics provided by libatomic - yes")
+    else()
+      message(STATUS "Atomics provided by libatomic - no")
+      message(FATAL_ERROR "Compiler must support std::atomic!")
+    endif()
+  else()
+    message(FATAL_ERROR "Compiler appears to require libatomic, but cannot find it.")
+  endif()
+endif()
+
 #===============================================================================
 #= System Introspection
 #-------------------------------------------------------------------------------
@@ -387,6 +436,7 @@ set(PULSEVIEW_LINK_LIBS
 	${Boost_LIBRARIES}
 	${QT_LIBRARIES}
 	${CMAKE_THREAD_LIBS_INIT}
+	${LIBATOMIC_LIBRARY}
 )
 
 if(STATIC_PKGDEPS_LIBS)
-- 
2.8.3