rs6000: Fix C++14 vs. C++17 ABI bug on powerpc64le [PR94707]

As mentioned in the PR and on IRC, the recently added struct-layout-1.exp
new tests FAIL on powerpc64le-linux (among other targets).
FAIL: tmpdir-g++.dg-struct-layout-1/t032 cp_compat_x_tst.o-cp_compat_y_tst.o execute
FAIL: tmpdir-g++.dg-struct-layout-1/t058 cp_compat_x_tst.o-cp_compat_y_tst.o execute
FAIL: tmpdir-g++.dg-struct-layout-1/t059 cp_compat_x_tst.o-cp_compat_y_tst.o execute
in particular.  The problem is that the presence or absence of the C++17
artificial empty base fields, which have non-zero TYPE_SIZE, but zero
DECL_SIZE, change the ABI decisions, if it is present (-std=c++17), the type
might not be considered homogeneous, while if it is absent (-std=c++14), it
can be.

The following patch fixes that and emits a -Wpsabi inform; perhaps more
often than it could, because the fact that rs6000_discover_homogeneous_aggregate
returns true when it didn't in in GCC 7/8/9 with -std=c++17 doesn't still
mean it will make a different ABI decision, but the warning triggered only
on the test I've changed (the struct-layout-1.exp tests use -w -Wno-psabi
already).

2020-04-23  Jakub Jelinek  <jakub@redhat.com>

	PR target/94707
	* config/rs6000/rs6000-call.c (rs6000_aggregate_candidate): Add
	cxx17_empty_base_seen argument.  Pass it to recursive calls.
	Ignore cxx17_empty_base_field_p fields after setting
	*cxx17_empty_base_seen to true.
	(rs6000_discover_homogeneous_aggregate): Adjust
	rs6000_aggregate_candidate caller.  With -Wpsabi, diagnose homogeneous
	aggregates with C++17 empty base fields.

	* g++.dg/tree-ssa/pr27830.C: Use -Wpsabi -w for -std=c++17 and higher.

1 files changed, 29 insertions, 5 deletions

diff --git a/gcc/config/rs6000/rs6000-call.c b/gcc/config/rs6000/rs6000-call.c
index e08621a..a9ae7ab 100644
--- a/gcc/config/rs6000/rs6000-call.c
+++ b/gcc/config/rs6000/rs6000-call.c
@@ -5528,7 +5528,8 @@ const struct altivec_builtin_types altivec_overloaded_builtins[] = {
    sub-tree.  */
 
 static int
-rs6000_aggregate_candidate (const_tree type, machine_mode *modep)
+rs6000_aggregate_candidate (const_tree type, machine_mode *modep,
+			    bool *cxx17_empty_base_seen)
 {
   machine_mode mode;
   HOST_WIDE_INT size;
@@ -5598,7 +5599,8 @@ rs6000_aggregate_candidate (const_tree type, machine_mode *modep)
 	    || TREE_CODE (TYPE_SIZE (type)) != INTEGER_CST)
 	  return -1;
 
-	count = rs6000_aggregate_candidate (TREE_TYPE (type), modep);
+	count = rs6000_aggregate_candidate (TREE_TYPE (type), modep,
+					    cxx17_empty_base_seen);
 	if (count == -1
 	    || !index
 	    || !TYPE_MAX_VALUE (index)
@@ -5636,7 +5638,14 @@ rs6000_aggregate_candidate (const_tree type, machine_mode *modep)
 	    if (TREE_CODE (field) != FIELD_DECL)
 	      continue;
 
-	    sub_count = rs6000_aggregate_candidate (TREE_TYPE (field), modep);
+	    if (cxx17_empty_base_field_p (field))
+	      {
+		*cxx17_empty_base_seen = true;
+		continue;
+	      }
+
+	    sub_count = rs6000_aggregate_candidate (TREE_TYPE (field), modep,
+						    cxx17_empty_base_seen);
 	    if (sub_count < 0)
 	      return -1;
 	    count += sub_count;
@@ -5669,7 +5678,8 @@ rs6000_aggregate_candidate (const_tree type, machine_mode *modep)
 	    if (TREE_CODE (field) != FIELD_DECL)
 	      continue;
 
-	    sub_count = rs6000_aggregate_candidate (TREE_TYPE (field), modep);
+	    sub_count = rs6000_aggregate_candidate (TREE_TYPE (field), modep,
+						    cxx17_empty_base_seen);
 	    if (sub_count < 0)
 	      return -1;
 	    count = count > sub_count ? count : sub_count;
@@ -5710,7 +5720,9 @@ rs6000_discover_homogeneous_aggregate (machine_mode mode, const_tree type,
       && AGGREGATE_TYPE_P (type))
     {
       machine_mode field_mode = VOIDmode;
-      int field_count = rs6000_aggregate_candidate (type, &field_mode);
+      bool cxx17_empty_base_seen = false;
+      int field_count = rs6000_aggregate_candidate (type, &field_mode,
+						    &cxx17_empty_base_seen);
 
       if (field_count > 0)
 	{
@@ -5725,6 +5737,18 @@ rs6000_discover_homogeneous_aggregate (machine_mode mode, const_tree type,
 		*elt_mode = field_mode;
 	      if (n_elts)
 		*n_elts = field_count;
+	      if (cxx17_empty_base_seen && warn_psabi)
+		{
+		  static const_tree last_reported_type;
+		  if (type != last_reported_type)
+		    {
+		      inform (input_location,
+			      "parameter passing for argument of type %qT "
+			      "when C++17 is enabled changed to match C++14 "
+			      "in GCC 10.1", type);
+		      last_reported_type = type;
+		    }
+		}
 	      return true;
 	    }
 	}