Improve check for a function attribute support.

The compiler may advertise function attribute support
with __has_attribute operator even though it does
not implement the feature on some architecture.

This fixes the issue with  GCC 11 on ppc64le with
__attribute__((zero_call_used_regs("used"))).

Fixes: #959.

Author: oniko

configure.ac

@@ -680,6 +680,27 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
 ])
 CFLAGS=$saved_CFLAGS
 
+dnl Force compiler to use zero_call_used_regs("used") to check for the function attribute support.
+dnl Otherwise the compiler may falsely advertise it with __has_attribute operator, even though
+dnl it does not implement it on some archs.
+AC_MSG_CHECKING([for zero_call_used_regs(user)])
+saved_CFLAGS=$CFLAGS
+CFLAGS="-O0 -Werror"
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+	void _test_function(void);
+	__attribute__((zero_call_used_regs("used"))) void _test_function(void) {
+		volatile int *i; volatile int j = 0; if (j) *i = 0;
+	}
+]],
+[[ _test_function() ]]
+)],[
+	AC_DEFINE([HAVE_ATTRIBUTE_ZEROCALLUSEDREGS], 1, [Define to 1 to use __attribute__((zero_call_used_regs("used")))])
+	AC_MSG_RESULT([yes])
+], [
+	AC_MSG_RESULT([no])
+])
+CFLAGS=$saved_CFLAGS
+
 AC_MSG_CHECKING([for systemd tmpfiles config directory])
 if test "x$prefix" != "xNONE"; then
 	saved_PKG_CONFIG=$PKG_CONFIG

lib/crypto_backend/memutils.c

@@ -9,11 +9,9 @@
 
 #define ATTR_NOINLINE __attribute__ ((noinline))
 #define ATTR_ZERO_REGS
-#if defined __has_attribute
-#  if __has_attribute (zero_call_used_regs)
+#if HAVE_ATTRIBUTE_ZEROCALLUSEDREGS
 #    undef ATTR_ZERO_REGS
 #    define ATTR_ZERO_REGS __attribute__ ((zero_call_used_regs("used")))
-#  endif
 #endif
 
 /* Workaround for https://github.com/google/sanitizers/issues/1507 */

meson.build

@@ -697,6 +697,21 @@ if cc.links(
         description: 'Define to 1 to use __attribute__((symver))')
 endif
 
+# ==========================================================================
+# Check compiler support for zero_called_used_regs("used") function attribute
+if cc.links(
+        '''void _test_fn(void);
+
+        __attribute__((zero_call_used_regs("used"))) void _test_fn(void) {
+		volatile int *i; volatile int j = 0; if (j) *i = 0;
+        }
+        int main(void) { _test_fn(); return 0; }''',
+        args: ['-O0', '-Werror' ],
+        name: 'for zero_call_used_regs("used") attribute support')
+    conf.set10('HAVE_ATTRIBUTE_ZEROCALLUSEDREGS', true,
+        description: 'Define to 1 to use __attribute__((zero_call_used_regs("used")))')
+endif
+
 # ==========================================================================
 
 if get_option('dev-random')