Fix linking of static archives with GCC with LTO

CMakeLists.txt

@@ -337,12 +337,25 @@ if(TEST_SUITE_COLLECT_STATS)
   list(APPEND CFLAGS -save-stats=obj)
   list(APPEND CXXFLAGS -save-stats=obj)
   # Collect stats for LTO step too.
-  if (${CMAKE_C_FLAGS} MATCHES ".*-flto.*" AND
-      ${CMAKE_CXX_FLAGS} MATCHES ".*-flto.*")
+  if ((${CMAKE_C_FLAGS} MATCHES ".*-flto.*" AND
+	${CMAKE_CXX_FLAGS} MATCHES ".*-flto.*") OR
+      CMAKE_INTERPROCEDURAL_OPTIMIZATION)
     list(APPEND LDFLAGS -save-stats=obj)
   endif()
 endif()
 
+# When CMAKE_INTERPROCEDURAL_OPTIMIZATION is set, CMake implicitly uses
+# -flto=thin on Clang, but -flto=auto on GCC. Append -flto=auto on Clang so they
+# are equivalent. This needs to be done in CFLAGS/CXXFLAGS/LDFLAGS so it appears
+# after -flto=thin and overrides it.
+# https://gitlab.kitware.com/cmake/cmake/-/blob/b65b7687cc541265f894567174a4c0ee65ea03d6/Modules/Compiler/Clang.cmake#L78-82
+if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND CMAKE_INTERPROCEDURAL_OPTIMIZATION
+    AND NOT TEST_SUITE_THIN_LTO)
+  list(APPEND CFLAGS -flto=auto)
+  list(APPEND CXXFLAGS -flto=auto)
+  list(APPEND LDFLAGS -flto=auto)
+endif()
+
 # Detect and include subdirectories
 # This allows to: Place additional test-suites into the toplevel test-suite
 # directory where they will be picked up automatically. Alternatively you may

cmake/caches/OfastLTO.cmake

@@ -1,4 +1,5 @@
-set(OPTFLAGS "${OPTFLAGS} -Ofast -flto")
+set(OPTFLAGS "${OPTFLAGS} -Ofast")
 set(CMAKE_C_FLAGS_RELEASE "${OPTFLAGS}" CACHE STRING "")
 set(CMAKE_CXX_FLAGS_RELEASE "${OPTFLAGS}" CACHE STRING "")
 set(CMAKE_BUILD_TYPE "Release" CACHE STRING "")
+set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON CACHE BOOL "")

cmake/caches/OsLTO.cmake

@@ -1,5 +1,6 @@
-set(OPTFLAGS "${OPTFLAGS} -Os -flto")
+set(OPTFLAGS "${OPTFLAGS} -Os")
 
 set(CMAKE_C_FLAGS_RELEASE "${OPTFLAGS}" CACHE STRING "")
 set(CMAKE_CXX_FLAGS_RELEASE "${OPTFLAGS}" CACHE STRING "")
 set(CMAKE_BUILD_TYPE "Release" CACHE STRING "")
+set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON CACHE BOOL "")

cmake/caches/ReleaseLTO.cmake

@@ -1,9 +1,10 @@
 # Aggressive settings to produce fast code.
-set(OPTFLAGS "${OPTFLAGS} -O3 -fomit-frame-pointer -flto -DNDEBUG")
+set(OPTFLAGS "${OPTFLAGS} -O3 -fomit-frame-pointer -DNDEBUG")
 if(APPLE)
   set(OPTFLAGS "${OPTFLAGS} -mdynamic-no-pic")
 endif()
 
 set(CMAKE_C_FLAGS_RELEASE "${OPTFLAGS}" CACHE STRING "")
 set(CMAKE_CXX_FLAGS_RELEASE "${OPTFLAGS}" CACHE STRING "")
 set(CMAKE_BUILD_TYPE "Release" CACHE STRING "")
+set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON CACHE BOOL "")

cmake/caches/ReleaseThinLTO.cmake

@@ -1,8 +1,10 @@
-set(OPTFLAGS "${OPTFLAGS} -O3 -fomit-frame-pointer -flto=thin -DNDEBUG")
+set(OPTFLAGS "${OPTFLAGS} -O3 -fomit-frame-pointer -DNDEBUG")
 if(APPLE)
   set(OPTFLAGS "${OPTFLAGS} -mdynamic-no-pic")
 endif()
 
 set(CMAKE_C_FLAGS_RELEASE "${OPTFLAGS}" CACHE STRING "")
 set(CMAKE_CXX_FLAGS_RELEASE "${OPTFLAGS}" CACHE STRING "")
 set(CMAKE_BUILD_TYPE "Release" CACHE STRING "")
+set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON CACHE BOOL "")
+set(TEST_SUITE_THIN_LTO ON CACHE BOOL "")