c, c++: Fix unused result for empty types [PR82134]

Hi,
This fixes PR c/82134 which concerns gcc emitting an incorrect unused
result diagnostic for empty types. This diagnostic is emitted from
tree-cfg.cc because of a couple code paths which attempt to avoid
copying empty types, resulting in GIMPLE that isn't using the returned
value of a call. To fix this I've added suppress_warning in three locations
and a corresponding check in do_warn_unused_result.

Cheers,
Jeremy

	PR c/82134

gcc/cp/ChangeLog:

	* call.cc (build_call_a): Add suppress_warning
	* cp-gimplify.cc (cp_gimplify_expr): Add suppress_warning

gcc/ChangeLog:

	* gimplify.cc (gimplify_modify_expr): Add suppress_warning
	* tree-cfg.cc (do_warn_unused_result): Check warning_suppressed_p

gcc/testsuite/ChangeLog:

	* c-c++-common/attr-warn-unused-result-2.c: New test.

Signed-off-by: Jeremy Rifkin <jeremy@rifkin.dev>

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 26296255b1ece139a02a3136dcac8e7dcc041438..37ad0a977c22bd9097440351ee692ea7b9785754 100644 (file)
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -412,6 +412,7 @@ build_call_a (tree function, int n, tree *argarray)
              /* We're disconnecting the initializer from its target,
                 don't create a temporary.  */
              arg = TARGET_EXPR_INITIAL (arg);
+           suppress_warning (arg, OPT_Wunused_result);
            tree t = build0 (EMPTY_CLASS_EXPR, TREE_TYPE (arg));
            arg = build2 (COMPOUND_EXPR, TREE_TYPE (t), arg, t);
            CALL_EXPR_ARG (function, i) = arg;

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index d54fe347a6ca90c8a39daebb79b5ecfa2bfe5a9f..4ff8f36a9fb6d32c4c9da3fba2d54cb8dd60ddef 100644 (file)
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -690,6 +690,7 @@ cp_gimplify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p)
                    && (REFERENCE_CLASS_P (op1) || DECL_P (op1)))
                  op1 = build_fold_addr_expr (op1);
 
+               suppress_warning (op1, OPT_Wunused_result);
                gimplify_and_add (op1, pre_p);
              }
            gimplify_expr (&TREE_OPERAND (*expr_p, 0), pre_p, post_p,

diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
index 9f9ff92d0642f65536eacc41215830d54db34f48..fa9890e7cea08fdef3857d4ae452ce2d3a463784 100644 (file)
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -7305,6 +7305,7 @@ gimplify_modify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
       && !(TREE_ADDRESSABLE (TREE_TYPE (*from_p))
           && TREE_CODE (*from_p) == CALL_EXPR))
     {
+      suppress_warning (*from_p, OPT_Wunused_result);
       gimplify_stmt (from_p, pre_p);
       gimplify_stmt (to_p, pre_p);
       *expr_p = NULL_TREE;

diff --git a/gcc/testsuite/c-c++-common/attr-warn-unused-result-2.c b/gcc/testsuite/c-c++-common/attr-warn-unused-result-2.c
new file mode 100644 (file)
index 0000000..f35198d
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/attr-warn-unused-result-2.c
@@ -0,0 +1,16 @@
+/* PR c/82134 */
+/* { dg-do compile } */
+/* { dg-additional-options -Wno-c++-compat { target c } } */
+
+struct S {};
+
+__attribute__((warn_unused_result)) struct S foo();
+
+void use_s(struct S);
+
+void
+test (void)
+{
+  struct S s = foo(); /* { dg-bogus "ignoring return value of" } */
+  use_s(foo()); /* { dg-bogus "ignoring return value of" } */
+}

diff --git a/gcc/tree-cfg.cc b/gcc/tree-cfg.cc
index 3d23f178da7eccdb524bc0d6b3b629d3e8b6b7d8..275fc436d13511df4a357f880278ed02784b1f7d 100644 (file)
--- a/gcc/tree-cfg.cc
+++ b/gcc/tree-cfg.cc
@@ -9849,6 +9849,8 @@ do_warn_unused_result (gimple_seq seq)
            break;
          if (gimple_call_internal_p (g))
            break;
+         if (warning_suppressed_p (g, OPT_Wunused_result))
+           break;
 
          /* This is a naked call, as opposed to a GIMPLE_CALL with an
             LHS.  All calls whose value is ignored should be