Hi,
On Fri, Jul 11 2025, Richard Biener wrote:
> 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.
I agree.
Martin
>
> 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)
