[Arm][CMSE]Add warn_unused_return attribute to cmse functions

At present it is possible to call the CMSE functions for checking
addresses (such as cmse_check_address_range) and  forget to check/use
the return value. This patch makes the interfaces more robust against
programmer error by marking these functions with the warn_unused_result
attribute. With this set, any use of these functions that does not use
the result will produce a warning.

This produces a warning on default warn levels when the result of the
cmse functions is not used.

For the following function:
void foo()
{
     int *data;
     cmse_check_address_range((int*)data, 0, 0);
}
The following warning is emitted:
warning: ignoring return value of 'cmse_check_address_range' declared
with attribute 'warn_unused_result' [-Wunused-result]
     6 |  cmse_check_address_range((int*)data, 0, 0);
        |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

gcc/ChangeLog:

2019-07-31  Joel Hutton  <Joel.Hutton@arm.com>

         * config/arm/arm_cmse.h (cmse_nonsecure_caller): Add
warn_unused_result attribute.
         (cmse_check_address_range): Add warn_unused_result attribute.

libgcc/ChangeLog:

2019-07-31  Joel Hutton  <Joel.Hutton@arm.com>

         * config/arm/cmse.c (cmse_check_address_range): Add
warn_unused_result attribute.

2019-07-31  Joel Hutton  <Joel.Hutton@arm.com>

         * gcc.target/arm/cmse/cmse-17.c: New test.

From-SVN: r273924

gcc/ChangeLog

+2019-07-31  Joel Hutton  <Joel.Hutton@arm.com>
+
+	* config/arm/arm_cmse.h (cmse_nonsecure_caller): Add
+	warn_unused_result attribute.
+	(cmse_check_address_range): Add warn_unused_result attribute.
+
 2019-07-31  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/91257

gcc/config/arm/arm_cmse.h

@@ -164,6 +164,7 @@ __CMSE_TT_ASM (at)
 
 /* FIXME: diagnose use outside cmse_nonsecure_entry functions.  */
 __extension__ static __inline int __attribute__ ((__always_inline__))
+__attribute__ ((warn_unused_result))
 cmse_nonsecure_caller (void)
 {
   return __builtin_arm_cmse_nonsecure_caller ();
@@ -184,6 +185,7 @@ cmse_nonsecure_caller (void)
 #define CMSE_MPU_READ		8
 
 __extension__ void *
+__attribute__ ((warn_unused_result))
 cmse_check_address_range (void *, size_t, int);
 
 #define cmse_check_pointed_object(p, f) \

gcc/testsuite/ChangeLog

+2019-07-31  Joel Hutton  <Joel.Hutton@arm.com>
+
+	* gcc.target/arm/cmse/cmse-17.c: New test.
+
 2019-07-30  Martin Sebor  <msebor@redhat.com>
 
 	PR testsuite/91258

gcc/testsuite/gcc.target/arm/cmse/cmse-17.c

+/* { dg-do compile } */
+/* { dg-options "-mcmse"}  */
+
+#include <arm_cmse.h>
+
+void foo()
+{
+	int *data;
+	cmse_check_address_range((int*)data, 0, 0); /* { dg-warning "ignoring return value" } */
+}

libgcc/ChangeLog

+2019-07-31  Joel Hutton  <Joel.Hutton@arm.com>
+
+	* config/arm/cmse.c (cmse_check_address_range): Add
+	warn_unused_result attribute.
+
 2019-07-22  Martin Liska  <mliska@suse.cz>
 
 	* config/pa/stublib.c: Remove stub symbol __gnu_lto_v1.

libgcc/config/arm/cmse.c

@@ -30,6 +30,7 @@
    address range.  See ACLE changes for ARMv8-M.  */
 
 void *
+__attribute__ ((warn_unused_result))
 cmse_check_address_range (void *p, size_t size, int flags)
 {
   cmse_address_info_t permb, perme;