[PATCH v2] configure.ac: add detection of symver gcc attribute

[Giulio Benetti]

From: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

On non-ELF platforms, such as microblaze, builds will fail when trying
to add symver information because  __attribute__((symver ..)) is not
supported even though __has_attribute(__symver__) returns true.

Support for symver needs to be detected via a compile test since
__has_attribute can report false positives [0].

Add a configure compile check for __attribute__((symver ..)) to ensure
it is supported and define a variable to advertise support.

[0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101766#c1

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[Giulio: rework #if nesting]
Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
---
V1->V2:
* rework syscalls #if nesting as suggested by Andreas Grünbacher
---
 configure.ac       | 15 +++++++++++++++
 libattr/syscalls.c |  8 ++++----
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/configure.ac b/configure.ac
index 7e362e9..98477b5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -57,6 +57,21 @@ AS_CASE([$host_os],
        [linux*], [os_linux=yes])
 AM_CONDITIONAL([OS_LINUX], [test "x$os_linux" = "xyes"])
 
+AC_CACHE_CHECK(whether __attribute__((__symver__())) is supported,
+       gcc_cv_symver_attribute,
+       [cat > conftest.c <<EOF
+void foo (void) {}
+__typeof(foo) foo __attribute__ ((__symver__("foo@foo")));
+EOF
+       gcc_cv_symver_attribute=no
+       if ${CC-cc} -Werror -S conftest.c -o conftest.s >/dev/null 2>&1; then \
+           gcc_cv_symver_attribute=yes
+       fi
+       rm -f conftest.[cs]
+])
+AS_IF([test $gcc_cv_symver_attribute = yes],
+      [AC_DEFINE(HAVE_SYMVER_ATTRIBUTE, [], [GCC supports symver attribute])])
+
 AC_CONFIG_COMMANDS([include/attr],
        [dnl
        rm -rf include/attr
diff --git a/libattr/syscalls.c b/libattr/syscalls.c
index 907560a..7ee6d39 100644
--- a/libattr/syscalls.c
+++ b/libattr/syscalls.c
@@ -31,10 +31,10 @@
  * prefer symver attribute if available (since gcc 10),
  * fall back to traditional .symver asm directive otherwise.
  */
-#ifdef __has_attribute
-# if __has_attribute(__symver__)
-#  define SYMVER(cn, vn) __typeof(cn) cn __attribute__((__symver__(vn)))
-# elif __has_attribute(__no_reorder__)
+#if defined(HAVE_SYMVER_ATTRIBUTE)
+# define SYMVER(cn, vn) __typeof(cn) cn __attribute__((__symver__(vn)))
+#elif defined(__has_attribute)
+# if __has_attribute(__no_reorder__)
    /*
     * Avoid wrong partitioning with older gcc and LTO. May not work reliably
     * with all versions; use -flto-partition=none if you encounter problems.
-- 
2.34.1