https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119873
--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-15 branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:a36dd9ee5bb1d2f2f19b8d935db29468a35bb47e commit r15-9587-ga36dd9ee5bb1d2f2f19b8d935db29468a35bb47e Author: Jakub Jelinek <ja...@redhat.com> Date: Thu Apr 24 23:44:28 2025 +0200 s390: Allow 5+ argument tail-calls in some special cases [PR119873] protobuf (and therefore firefox too) currently doesn't build on s390*-linux. The problem is that it uses [[clang::musttail]] attribute heavily, and in llvm (IMHO llvm bug) [[clang::musttail]] calls with 5+ arguments on s390*-linux are silently accepted and result in a normal non-tail call. In GCC we just reject those because the target hook refuses to tail call it (IMHO the right behavior). Now, the reason why that happens is as s390_function_ok_for_sibcall attempts to explain, the 5th argument (assuming normal <= wordsize integer or pointer arguments, nothing that needs 2+ registers) is passed in %r6 which is not call clobbered, so we can't do tail call when we'd have to change content of that register and then caller would assume %r6 content didn't change and use it again. In the protobuf case though, the 5th argument is always passed through from the caller to the musttail callee unmodified, so one can actually emit just jg tail_called_function or perhaps tweak some registers but keep %r6 untouched, and in that case I think it is just fine to tail call it (at least unless the stack slots used for 6+ argument can't be modified by the callee in the ABI and nothing checks for that). So, the following patch checks for this special case, where the argument which uses %r6 is passed in a single register and it is passed default definition of SSA_NAME of a PARM_DECL with the same DECL_INCOMING_RTL. It won't really work at -O0 but should work for -O1 and above, at least when one doesn't really try to modify the parameter conditionally and hope it will be optimized away in the end. 2025-04-24 Jakub Jelinek <ja...@redhat.com> Stefan Schulze Frielinghaus <stefa...@gcc.gnu.org> PR target/119873 * config/s390/s390.cc (s390_call_saved_register_used): Don't return true if default definition of PARM_DECL SSA_NAME of the same register is passed in call saved register. (s390_function_ok_for_sibcall): Adjust comment. * gcc.target/s390/pr119873-1.c: New test. * gcc.target/s390/pr119873-2.c: New test. * gcc.target/s390/pr119873-3.c: New test. * gcc.target/s390/pr119873-4.c: New test. (cherry picked from commit 567684733e0c4f28158e19b72f4e7f660ad81561)
