On Mon, 9 Jun 2025, Jeremy Rifkin wrote:
> 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.
Thanks for the patch (and the ping)! Your patch looks fine to me,
though I can't formally approve it myself. CC'ing Jason
>
> 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 <jer...@rifkin.dev>
> ---
> gcc/cp/call.cc | 1 +
> gcc/cp/cp-gimplify.cc | 1 +
> gcc/gimplify.cc | 1 +
> .../c-c++-common/attr-warn-unused-result-2.c | 15 +++++++++++++++
> gcc/tree-cfg.cc | 2 ++
> 5 files changed, 20 insertions(+)
> create mode 100644 gcc/testsuite/c-c++-common/attr-warn-unused-result-2.c
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 2c3ef3dfc35..a70fc13c6a4 100644
> --- 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 0fcfa16d2c5..2a21e960994 100644
> --- 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 9f9ff92d064..fa9890e7cea 100644
> --- 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
> index 00000000000..09be1a933c9
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/attr-warn-unused-result-2.c
> @@ -0,0 +1,15 @@
> +// PR c/82134
> +/* { dg-do compile } */
> +
> +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 fad308e7f7b..5cd72ed1b3e 100644
> --- a/gcc/tree-cfg.cc
> +++ b/gcc/tree-cfg.cc
> @@ -9961,6 +9961,8 @@ 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
> --
> 2.43.0
>
>
