CMake: Check for func attributes without -Werror.

This matches the modified m4/ax_gcc_func_attribute.m4.
Only constructor and ifunc attribute checks were implemented.

CMake-based build doesn't have the trouble of Autoconf confdefs.h
but writing warning-free tests would still some extra care.
If looking for attribute-related warnings works well enough then
this method should be nicer than the -Werror method in CMake too.

CMakeLists.txt

@@ -53,6 +53,7 @@ include(CheckSymbolExists)
 include(CheckStructHasMember)
 include(CheckCSourceCompiles)
 include(cmake/tuklib_integer.cmake)
+include(cmake/tuklib_gnu_c_attributes.cmake)
 include(cmake/tuklib_cpucores.cmake)
 include(cmake/tuklib_physmem.cmake)
 include(cmake/tuklib_progname.cmake)
@@ -750,17 +751,7 @@ if(NOT TUKLIB_CPUCORES_FOUND OR NOT TUKLIB_PHYSMEM_FOUND)
 endif()
 
 # Check for __attribute__((__constructor__)) support.
-# This needs -Werror because some compilers just warn
-# about this being unsupported.
-cmake_push_check_state()
-set(CMAKE_REQUIRED_FLAGS "-Werror")
-check_c_source_compiles("
-        __attribute__((__constructor__))
-        static void my_constructor_func(void) { return; }
-        int main(void) { return 0; }
-    "
-    HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR)
-cmake_pop_check_state()
+tuklib_check_func_attribute_constructor(HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR)
 tuklib_add_definition_if(liblzma HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR)
 
 # The Win95 threading lacks a thread-safe one-time initialization function.
@@ -782,25 +773,7 @@ option(ALLOW_ATTR_IFUNC "Allow use of __attribute__((__ifunc__())) if \
 supported by the system" ON)
 
 if(ALLOW_ATTR_IFUNC)
-    cmake_push_check_state()
-    set(CMAKE_REQUIRED_FLAGS "-Werror")
-    check_c_source_compiles("
-            static void func(void) { return; }
-            static void (*resolve_func(void)) (void) { return func; }
-            void func_ifunc(void)
-                    __attribute__((__ifunc__(\"resolve_func\")));
-            int main(void) { return 0; }
-            /*
-             * 'clang -Wall' incorrectly warns that resolve_func is
-             * unused (-Wunused-function). Correct assembly output is
-             * still produced. This problem exists at least in Clang
-             * versions 4 to 17. The following silences the bogus warning:
-             */
-            void make_clang_quiet(void);
-            void make_clang_quiet(void) { resolve_func()(); }
-        "
-        HAVE_FUNC_ATTRIBUTE_IFUNC)
-    cmake_pop_check_state()
+    tuklib_check_func_attribute_ifunc(HAVE_FUNC_ATTRIBUTE_IFUNC)
     tuklib_add_definition_if(liblzma HAVE_FUNC_ATTRIBUTE_IFUNC)
 endif()
 

cmake/tuklib_gnu_c_attributes.cmake

@@ -0,0 +1,56 @@
+#
+# tuklib_gnu_c_attributes.cmake - Checks for GNU C attributes
+#
+# Author: Lasse Collin
+#
+# This file has been put into the public domain.
+# You can do whatever you want with this file.
+#
+
+include("${CMAKE_CURRENT_LIST_DIR}/tuklib_common.cmake")
+include(CheckCSourceCompiles)
+
+# This is a helper function for the actual attribute tests.
+function(tuklib_check_attribute CODE RESULT)
+    # GCC, Clang, ICC, Solaris Studio, and XLC don't exit with
+    # a non-zero exit status if an unknown attribute is provided.
+    # They only print a warning. Accept the attribute only if no
+    # attribute-related warnings were issued.
+    #
+    #   - XLC doesn't use the lower-case string "warning".
+    #     It might use "WARNING" on z/OS or "(W)" on AIX.
+    #     So "warning" isn't checked.
+    #
+    #   - A few XLC messages use "Attribute" instead of "attribute".
+    #
+    #   - ICC doesn't use the word "ignored".
+    #
+    # The problem with this method is that the regex might become
+    # outdated with newer compiler versions. Using -Werror would be
+    # an alternative but it fails if warnings occur for reasons other
+    # than unsupported attribute.
+    #
+    # The first pattern is for GCC, Solaris Studio, and XLC.
+    # The second one is for ICC. Both patterns work with Clang.
+    # Matching the leading space or colon reduces chances of
+    # matching any unrelated messages.
+    check_c_source_compiles("${CODE} int main(void) { return 0; }" "${RESULT}"
+                            FAIL_REGEX " [Aa]ttribute .* ignored"
+                                       ": unknown attribute")
+endfunction()
+
+function(tuklib_check_func_attribute_constructor RESULT)
+    tuklib_check_attribute("
+            __attribute__((__constructor__))
+            void my_constructor_func(void) { return; }
+        " "${RESULT}")
+endfunction()
+
+function(tuklib_check_func_attribute_ifunc RESULT)
+    tuklib_check_attribute("
+            static void func(void) { return; }
+            static void (*resolve_func(void))(void) { return &func; }
+            void func_ifunc(void)
+                    __attribute__((__ifunc__(\"resolve_func\")));
+        " "${RESULT}")
+endfunction()