calls: When bypassing emit_push_insn for 0 sized arg, emit at least anti_adjust_stack for alignment pad if needed [PR104558]

The following testcase ICEs on x86_64 when asked to use the pre-GCC 8
ABI where zero sized arguments weren't ignored.
In GCC 7 the emit_push_insn calls in store_one_arg were unconditional,
it is true that they didn't actually push anything because it had zero
size, but because arg->locate.alignment_pad is 8 in this case,
emit_push_insn at the end performs
          if (alignment_pad && args_addr == 0)
            anti_adjust_stack (alignment_pad);
and an assert larger on is upset if we don't do it.
The following patch keeps the emit_push_insn conditional but calls
the anti_adjust_stack when needed by hand for the zero sized arguments.
For the new x86_64 ABI where zero sized arguments are ignored
arg->locate.alignment_pad is 0 in this case, so nothing changes
- we in that case really do ignore it.

There is another emit_push_insn call earlier in store_one_arg, also made
conditional on non-zero size by Marek in GCC 8, but that one is for
arguments with non-BLKmode and the only way those can be zero size is
if they are TYPE_EMPTY_P aka when they are completely ignored.  But
I believe arg->locate.alignment_pad should be 0 in that case, so IMHO
there is no need to do anything in the second spot.

2022-03-03  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/104558
	* calls.cc (store_one_arg): When not calling emit_push_insn
	because size_rtx is const0_rtx, call at least anti_adjust_stack
	on arg->locate.alignment_pad if !argblock and the alignment might
	be non-zero.

	* gcc.dg/pr104558.c: New test.

gcc/calls.cc

@@ -5139,6 +5139,13 @@ store_one_arg (struct arg_data *arg, rtx argblock, int flags,
 			ARGS_SIZE_RTX (arg->locate.offset),
 			reg_parm_stack_space,
 			ARGS_SIZE_RTX (arg->locate.alignment_pad), false);
+      /* If we bypass emit_push_insn because it is a zero sized argument,
+	 we still might need to adjust stack if such argument requires
+	 extra alignment.  See PR104558.  */
+      else if ((arg->locate.alignment_pad.var
+		|| maybe_ne (arg->locate.alignment_pad.constant, 0))
+	       && !argblock)
+	anti_adjust_stack (ARGS_SIZE_RTX (arg->locate.alignment_pad));
 
       /* Unless this is a partially-in-register argument, the argument is now
 	 in the stack.

gcc/testsuite/gcc.dg/pr104558.c

@@ -0,0 +1,15 @@
+/* PR middle-end/104558 */
+/* { dg-do compile } */
+/* { dg-options "-fabi-version=9" } */
+
+struct __attribute__ ((aligned)) A {};
+
+struct A a;
+
+void bar (int, int, int, int, int, int, int, struct A);
+
+void
+foo (void)
+{
+  bar (0, 1, 2, 3, 4, 5, 6, a);
+}