[C PATCH] Prevent -Wunused-value warning with __atomic_fetch_* (PR c/69407)

[Marek Polacek]

This is not a regression but I thought I'd post this anyway.  Martin reported
that we generate -Wunused-value warnings on the attached testcase, which
arguable doesn't make sense.  Setting TREE_USED suppresses the warning.  Since
we already compute 'fetch_op' I used that.  (This warning doesn't trigger e.g.
for __atomic_load/store/compare.)

Bootstrapped/regtested on x86_64-linux, ok for trunk or gcc7?

2016-03-04  Marek Polacek  <pola...@redhat.com>

        PR c/69407
        * c-common.c (resolve_overloaded_builtin): Set TREE_USED for the fetch
        operations.

        * gcc.dg/atomic-op-6.c: New test.

diff --git gcc/c-family/c-common.c gcc/c-family/c-common.c
index 965cf49..25afa9c 100644
--- gcc/c-family/c-common.c
+++ gcc/c-family/c-common.c
@@ -11443,6 +11443,10 @@ resolve_overloaded_builtin (location_t loc, tree 
function,
            && orig_code != BUILT_IN_ATOMIC_STORE_N)
          result = sync_resolve_return (first_param, result, orig_format);
 
+       if (fetch_op)
+         /* Prevent -Wunused-value warning.  */
+         TREE_USED (result) = true;
+
        /* If new_return is set, assign function to that expr and cast the
           result to void since the generic interface returned void.  */
        if (new_return)
diff --git gcc/testsuite/gcc.dg/atomic-op-6.c gcc/testsuite/gcc.dg/atomic-op-6.c
index e69de29..c8d93a4 100644
--- gcc/testsuite/gcc.dg/atomic-op-6.c
+++ gcc/testsuite/gcc.dg/atomic-op-6.c
@@ -0,0 +1,11 @@
+/* Test we don't generate bogus warnings.  */
+/* PR c/69407 */
+/* { dg-do compile } */
+/* { dg-options "-Wall -Wextra" } */
+
+void
+foo (int *p, int a)
+{
+  __atomic_fetch_add (&p, a, 0);
+  __atomic_add_fetch (&p, a, 0);
+}

        Marek