On Fri, 11 Jul 2025, Jakub Jelinek wrote:
> Hi!
>
> As the following testcase shows e.g. on ia32, letting IPA opts change
> signature of functions which have [[{gnu,clang}::musttail]] calls
> can turn programs that would be compiled normally into something
> that is rejected because the caller has fewer argument stack slots
> than the function being tail called.
>
> The following patch prevents signature changes for such functions.
> It is perhaps too big hammer in some cases, but it might be hard
> to try to figure out what signature changes are still acceptable and which
> are not at IPA time.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk/15.2?
OK, but please give Martin/Honza a chance to comment.
Thanks,
Richard.
> 2025-07-11 Jakub Jelinek <ja...@redhat.com>
> Martin Jambor <mjam...@suse.cz>
>
> PR ipa/121023
> * ipa-fnsummary.cc (compute_fn_summary): Disallow signature changes
> on cfun->has_musttail functions.
>
> * c-c++-common/musttail32.c: New test.
>
> --- gcc/ipa-fnsummary.cc.jj 2025-07-04 09:01:47.507516910 +0200
> +++ gcc/ipa-fnsummary.cc 2025-07-10 14:00:19.488185173 +0200
> @@ -3421,6 +3421,21 @@ compute_fn_summary (struct cgraph_node *
> info->inlinable = tree_inlinable_function_p (node->decl);
>
> bool no_signature = false;
> +
> + /* Don't allow signature changes for functions which have
> + [[gnu::musttail]] or [[clang::musttail]] calls. Sometimes
> + (more often on targets which pass everything on the stack)
> + signature changes can result in tail calls being impossible
> + even when without the signature changes they would be ok.
> + See PR121023. */
> + if (cfun->has_musttail)
> + {
> + if (dump_file)
> + fprintf (dump_file, "No signature change:"
> + " function has calls with musttail attribute.\n");
> + no_signature = true;
> + }
> +
> /* Type attributes can use parameter indices to describe them.
> Special case fn spec since we can safely preserve them in
> modref summaries. */
> --- gcc/testsuite/c-c++-common/musttail32.c.jj 2025-07-10
> 14:00:56.760698477 +0200
> +++ gcc/testsuite/c-c++-common/musttail32.c 2025-07-10 14:02:21.945586151
> +0200
> @@ -0,0 +1,23 @@
> +/* PR ipa/121023 */
> +/* { dg-do compile { target musttail } } */
> +/* { dg-options "-O2" } */
> +
> +struct S { int a, b; };
> +
> +[[gnu::noipa]] int
> +foo (struct S x, int y, int z)
> +{
> + return x.a + y + z;
> +}
> +
> +[[gnu::noinline]] static int
> +bar (struct S x, int y, int z)
> +{
> + [[gnu::musttail]] return foo ((struct S) { x.a, 0 }, y, 1);
> +}
> +
> +int
> +baz (int x)
> +{
> + return bar ((struct S) { 1, 2 }, x, 2) + bar ((struct S) { 2, 3 }, x + 1,
> 2);
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
