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  <[email protected]>
	    Stefan Schulze Frielinghaus  <[email protected]>

	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.

Author: jakubjelinek

gcc/config/s390/s390.cc

@@ -14496,7 +14496,21 @@ s390_call_saved_register_used (tree call_expr)
 
 	  for (reg = 0; reg < nregs; reg++)
 	    if (!call_used_or_fixed_reg_p (reg + REGNO (parm_rtx)))
-	      return true;
+	      {
+		rtx parm;
+		/* Allow passing through unmodified value from caller,
+		   see PR119873.  */
+		if (TREE_CODE (parameter) == SSA_NAME
+		    && SSA_NAME_IS_DEFAULT_DEF (parameter)
+		    && SSA_NAME_VAR (parameter)
+		    && TREE_CODE (SSA_NAME_VAR (parameter)) == PARM_DECL
+		    && (parm = DECL_INCOMING_RTL (SSA_NAME_VAR (parameter)))
+		    && REG_P (parm)
+		    && REGNO (parm) == REGNO (parm_rtx)
+		    && REG_NREGS (parm) == REG_NREGS (parm_rtx))
+		  break;
+		return true;
+	      }
 	}
       else if (GET_CODE (parm_rtx) == PARALLEL)
 	{
@@ -14543,8 +14557,9 @@ s390_function_ok_for_sibcall (tree decl, tree exp)
     return false;
 
   /* 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.  */
+     "caller saved".  This makes functions needing this register for arguments
+     not suitable for sibcalls, unless the same value is passed from the
+     caller.  */
   return !s390_call_saved_register_used (exp);
 }
 

gcc/testsuite/gcc.target/s390/pr119873-1.c

@@ -0,0 +1,11 @@
+/* PR target/119873 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+const char *foo (void *, void *, void *, void *, unsigned long, unsigned long);
+
+const char *
+bar (void *a, void *b, void *c, void *d, unsigned long e, unsigned long f)
+{
+  [[gnu::musttail]] return foo (a, b, c, d, e, f);
+}

gcc/testsuite/gcc.target/s390/pr119873-2.c

@@ -0,0 +1,17 @@
+/* PR target/119873 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+const char *foo (void *, void *, void *, void *, unsigned long, unsigned long);
+
+const char *
+bar (void *a, void *b, void *c, void *d, unsigned long e, unsigned long f)
+{
+  [[gnu::musttail]] return foo (a, b, c, d, e + 1, f);	/* { dg-error "cannot tail-call: target is not able to optimize the call into a sibling call" } */
+}
+
+const char *
+baz (void *a, void *b, void *c, void *d, unsigned long e, unsigned long f)
+{
+  [[gnu::musttail]] return foo (a, b, c, d, f, e);	/* { dg-error "cannot tail-call: target is not able to optimize the call into a sibling call" } */
+}

gcc/testsuite/gcc.target/s390/pr119873-3.c

@@ -0,0 +1,27 @@
+/* PR target/119873 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+extern int foo (int, int, int, long long, int);
+
+int
+bar (int u, int v, int w, long long x, int y)
+{
+  [[gnu::musttail]] return foo (u, v, w, x, y);
+}
+
+extern int baz (int, int, int, int, int);
+
+int
+qux (int u, int v, int w, int x, int y)
+{
+  [[gnu::musttail]] return baz (u, v, w, x, y);
+}
+
+extern int corge (int, int, int, int, unsigned short);
+
+int
+garply (int u, int v, int w, int x, unsigned short y)
+{
+  [[gnu::musttail]] return corge (u, v, w, x, y);
+}

gcc/testsuite/gcc.target/s390/pr119873-4.c

@@ -0,0 +1,27 @@
+/* PR target/119873 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+extern int foo (int, int, int, long long, int);
+
+int
+bar (int u, int v, int w, long long x, int y)
+{
+  [[gnu::musttail]] return foo (u, v, w, x + 1, y - 1);	/* { dg-error "cannot tail-call: target is not able to optimize the call into a sibling call" } */
+}
+
+extern int baz (int, int, int, int, int);
+
+int
+qux (int u, int v, int w, int x, int y)
+{
+  [[gnu::musttail]] return baz (u, v, w, x, y + 1);	/* { dg-error "cannot tail-call: target is not able to optimize the call into a sibling call" } */
+}
+
+extern int corge (int, int, int, int, unsigned short);
+
+int
+garply (int u, int v, int w, int x, unsigned short y)
+{
+  [[gnu::musttail]] return corge (u, v, w, x, y + 1);	/* { dg-error "cannot tail-call: target is not able to optimize the call into a sibling call" } */
+}