[Bug target/119873] New: s390x musttail call failure on s390x

[jakub at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119873

            Bug ID: 119873
           Summary: s390x musttail call failure on s390x
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

typedef unsigned char uint8_t;
typedef unsigned short uint16_t;
typedef unsigned long long uint64_t;
struct TcFieldData {
  constexpr TcFieldData() : data(0) {}
  constexpr TcFieldData(uint16_t coded_tag, uint8_t hasbit_idx, uint8_t
aux_idx,
                        uint16_t offset)
      : data(uint64_t{offset} << 48 | uint64_t{aux_idx} << 24 |
uint64_t{hasbit_idx} << 16 | uint64_t{coded_tag}) {}
  uint64_t data;
};

const char *foo (void *, void *, void *, void *, uint64_t, TcFieldData);
[[gnu::noinline]] const char *bar (void *, void *, void *, void *, uint64_t,
TcFieldData) { return nullptr; }

const char *
baz (void *a, void *b, void *c, void *d, uint64_t e, TcFieldData f)
{
  [[clang::musttail]] return bar (a, b, c, d, e, f);
}

const char *
qux (void *a, void *b, void *c, void *d, uint64_t e, TcFieldData f)
{
  [[clang::musttail]] return foo (a, b, c, d, e, f);
}

from protobuf fails to compile on s390x-linux e.g. at -O2, with
/tmp/0.C: In function ‘const char* qux(void*, void*, void*, void*, uint64_t,
TcFieldData)’:
/tmp/0.C:24:34: error: cannot tail-call: target is not able to optimize the
call into a sibling call
   24 |   [[clang::musttail]] return foo (a, b, c, d, e, f);
      |                              ~~~~^~~~~~~~~~~~~~~~~~

If I understand it right, this is because
  /* Register 6 on s390 is available as an argument register but unfortunately
     "caller saved". This makes functions needing this register for arguments
     not suitable for sibcalls.  */
  return !s390_call_saved_register_used (exp);

I wonder if it couldn't be allowed if the argument passed in %r6 register is
passed through from the caller to the callee unmodified,
i.e. if in exp the argument in %r6 register is SSA_NAME which is
SSA_NAME_IS_DEFAULT_DEF with PARM_DECL which has %r6 as DECL_INCOMING_RTL.
Of course, if one passes something else, I can understand why it can't be tail
called.

Note, looking at what clang does, it silently ignores the musttail attribute,
calls the function normally, without diagnosing it.
I think the gcc behavior is better, at least the user is told that it can't be
tail called.  clang does the same for many reasons, but not in this case.