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

[Jakub Jelinek]

Hi!

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).

Bootstrapped/regtested on powerpc64le-linux, bootstrapped on powerpc64-linux
where regtest is still pending, but homogeneous aggregates are an ELFv2
thing, so I don't expect it to change anything (and so far no such messages
appear in the testsuite log files).

Ok for trunk?

2020-04-22  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.

--- gcc/config/rs6000/rs6000-call.c.jj	2020-03-30 22:53:40.746640328 +0200
+++ gcc/config/rs6000/rs6000-call.c	2020-04-22 13:05:07.947809888 +0200
@@ -5528,7 +5528,8 @@ const struct altivec_builtin_types altiv
    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 t
 	    || 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 t
 	    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 t
 	    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 (m
       && 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 +5737,17 @@ rs6000_discover_homogeneous_aggregate (m
 		*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,
+			      "prior to GCC 10, parameters of type "
+			      "%qT were passed incorrectly for C++17", type);
+		      last_reported_type = type;
+		    }
+		}
 	      return true;
 	    }
 	}
--- gcc/testsuite/g++.dg/tree-ssa/pr27830.C.jj	2020-01-11 16:31:54.844296359 +0100
+++ gcc/testsuite/g++.dg/tree-ssa/pr27830.C	2020-04-22 15:01:37.949881630 +0200
@@ -1,5 +1,7 @@
 /* { dg-do compile } */
 /* { dg-options "-O" } */
+/* Ignore ABI warnings for C++17 and later.  */
+/* { dg-additional-options "-Wno-psabi -w" { target c++17 } } */
 
 struct gc{};
 struct transform:public gc

	Jakub