mips: Emit psabi diagnostic for return values affected by C++ zero-width bit-field ABI change [PR 102024]

diff --git a/gcc/config/mips/mips.cc b/gcc/config/mips/mips.cc
index a1c4b437cd4..3284cf71f6f 100644
--- a/gcc/config/mips/mips.cc
+++ b/gcc/config/mips/mips.cc
@@ -6274,10 +6274,17 @@  mips_callee_copies (cumulative_args_t, const function_arg_info &arg)
 
    For n32 & n64, a structure with one or two fields is returned in
    floating-point registers as long as every field has a floating-point
-   type.  */
+   type.
+
+   The C++ FE used to remove zero-width bit-fields in GCC 11 and earlier.
+   To make a proper diagnostic, this function will set HAS_ZERO_WIDTH_BF
+   to 1 once a C++ zero-width bit-field shows up, and then ignore it.
+   Then the caller can determine if this zero-width bit-field will make a
+   difference and emit a -Wpsabi inform.  */
 
 static int
-mips_fpr_return_fields (const_tree valtype, tree *fields)
+mips_fpr_return_fields (const_tree valtype, tree *fields,
+			int *has_zero_width_bf)
 {
   tree field;
   int i;
@@ -6294,6 +6301,12 @@  mips_fpr_return_fields (const_tree valtype, tree *fields)
       if (TREE_CODE (field) != FIELD_DECL)
 	continue;
 
+      if (DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD (field))
+	{
+	  *has_zero_width_bf = 1;
+	  continue;
+	}
+
       if (!SCALAR_FLOAT_TYPE_P (TREE_TYPE (field)))
 	return 0;
 
@@ -6319,11 +6332,14 @@  static bool
 mips_return_in_msb (const_tree valtype)
 {
   tree fields[2];
+  int has_zero_width_bf = 0;
+  int use_fpr = mips_fpr_return_fields (valtype, fields,
+					&has_zero_width_bf);
 
   return (TARGET_NEWABI
 	  && TARGET_BIG_ENDIAN
 	  && AGGREGATE_TYPE_P (valtype)
-	  && mips_fpr_return_fields (valtype, fields) == 0);
+	  && (use_fpr == 0 || has_zero_width_bf));
 }
 
 /* Return true if the function return value MODE will get returned in a
@@ -6418,8 +6434,35 @@  mips_function_value_1 (const_tree valtype, const_tree fn_decl_or_type,
 	 return values, promote the mode here too.  */
       mode = promote_function_mode (valtype, mode, &unsigned_p, func, 1);
 
+      int has_zero_width_bf = 0;
+      int use_fpr = mips_fpr_return_fields (valtype, fields,
+					    &has_zero_width_bf);
+      if (TARGET_HARD_FLOAT &&
+	  warn_psabi &&
+	  has_zero_width_bf &&
+	  use_fpr != 0)
+	{
+	  static unsigned last_reported_type_uid;
+	  unsigned uid = TYPE_UID (TYPE_MAIN_VARIANT (valtype));
+	  if (uid != last_reported_type_uid)
+	    {
+	      static const char *url =
+		CHANGES_ROOT_URL
+		"gcc-12/changes.html#zero_width_bitfields";
+	      inform (input_location,
+		      "the ABI for returning a value containing "
+		      "zero-width bit-fields but otherwise an aggregate "
+		      "with only one or two floating-point fields was "
+		      "retconned in GCC %{12.1%}", url);
+	      last_reported_type_uid = uid;
+	    }
+	}
+
+      if (has_zero_width_bf)
+	use_fpr = 0;
+
       /* Handle structures whose fields are returned in $f0/$f2.  */
-      switch (mips_fpr_return_fields (valtype, fields))
+      switch (use_fpr)
 	{
 	case 1:
 	  return mips_return_fpr_single (mode,
diff --git a/gcc/testsuite/g++.target/mips/mips.exp b/gcc/testsuite/g++.target/mips/mips.exp
new file mode 100644
index 00000000000..9fa7e771b4d
--- /dev/null
+++ b/gcc/testsuite/g++.target/mips/mips.exp
@@ -0,0 +1,34 @@ 
+# Copyright (C) 2019-2022 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with GCC; see the file COPYING3.  If not see
+# <http://www.gnu.org/licenses/>.
+
+# GCC testsuite that uses the `dg.exp' driver.
+
+# Exit immediately if this isn't a MIPS target.
+if ![istarget mips*-*-*] then {
+  return
+}
+
+# Load support procs.
+load_lib g++-dg.exp
+
+# Initialize `dg'.
+dg-init
+
+# Main loop.
+dg-runtest [lsort [glob -nocomplain $srcdir/$subdir/*.C]] "" ""
+
+# All done.
+dg-finish
diff --git a/gcc/testsuite/g++.target/mips/pr102024.C b/gcc/testsuite/g++.target/mips/pr102024.C
new file mode 100644
index 00000000000..a4b99c79be0
--- /dev/null
+++ b/gcc/testsuite/g++.target/mips/pr102024.C
@@ -0,0 +1,20 @@ 
+// PR target/102024
+// { dg-do compile }
+// { dg-options "-O2 -mabi=64 -mhard-float" }
+// { dg-final { scan-assembler-not "\\\$f0" } }
+
+struct foo
+{
+  double a;
+  int : 0;
+  double b;
+};
+
+struct foo
+make_foo(void) // { dg-message "the ABI for returning a value containing zero-width bit-fields but otherwise an aggregate with only one or two floating-point fields was retconned in GCC 12.1" }
+{
+  struct foo ret;
+  ret.a = 114;
+  ret.b = 514;
+  return ret;
+}