Index: gcc/target.def
===================================================================
--- gcc/target.def	(revision 262159)
+++ gcc/target.def	(working copy)
@@ -3310,6 +3310,16 @@ constants can be done inline.  The funct
  HOST_WIDE_INT, (const_tree constant, HOST_WIDE_INT basic_align),
  default_constant_alignment)
 
+DEFHOOK
+(translate_mode_attribute,
+ "Define this hook if the port should translate machine_mode @var{mode}\n\
+to another mode.  For example, rs6000's @code{KFmode}, when it is the same\n\
+as @code{TFmode}.\n\
+\n\
+The default version of the hook returns that mode that was passed in.",
+ machine_mode, (machine_mode mode),
+ default_translate_mode_attribute)
+
 /* True if MODE is valid for the target.  By "valid", we mean able to
    be manipulated in non-trivial ways.  In particular, this means all
    the arithmetic is supported.  */
Index: gcc/targhooks.c
===================================================================
--- gcc/targhooks.c	(revision 262159)
+++ gcc/targhooks.c	(working copy)
@@ -393,6 +393,14 @@ default_mangle_assembler_name (const cha
   return get_identifier (stripped);
 }
 
+/* The default implementation of TARGET_TRANSLATE_MODE_ATTRIBUTE.  */
+
+machine_mode
+default_translate_mode_attribute (machine_mode mode)
+{
+  return mode;
+}
+
 /* True if MODE is valid for the target.  By "valid", we mean able to
    be manipulated in non-trivial ways.  In particular, this means all
    the arithmetic is supported.
Index: gcc/targhooks.h
===================================================================
--- gcc/targhooks.h	(revision 262159)
+++ gcc/targhooks.h	(working copy)
@@ -72,6 +72,7 @@ extern void default_print_operand_addres
 extern bool default_print_operand_punct_valid_p (unsigned char);
 extern tree default_mangle_assembler_name (const char *);
 
+extern machine_mode default_translate_mode_attribute (machine_mode);
 extern bool default_scalar_mode_supported_p (scalar_mode);
 extern bool default_libgcc_floating_mode_supported_p (scalar_float_mode);
 extern opt_scalar_float_mode default_floatn_mode (int, bool);
Index: gcc/doc/tm.texi
===================================================================
--- gcc/doc/tm.texi	(revision 262159)
+++ gcc/doc/tm.texi	(working copy)
@@ -4255,6 +4255,14 @@ hook returns true for both @code{ptr_mod
 Define this to return nonzero if the memory reference @var{ref}  may alias with the system C library errno location.  The default  version of this hook assumes the system C library errno location  is either a declaration of type int or accessed by dereferencing  a pointer to int.
 @end deftypefn
 
+@deftypefn {Target Hook} machine_mode TARGET_TRANSLATE_MODE_ATTRIBUTE (machine_mode @var{mode})
+Define this hook if the port should translate machine_mode @var{mode}
+to another mode.  For example, rs6000's @code{KFmode}, when it is the same
+as @code{TFmode}.
+
+The default version of the hook returns that mode that was passed in.
+@end deftypefn
+
 @deftypefn {Target Hook} bool TARGET_SCALAR_MODE_SUPPORTED_P (scalar_mode @var{mode})
 Define this to return nonzero if the port is prepared to handle
 insns involving scalar mode @var{mode}.  For a scalar mode to be
Index: gcc/doc/tm.texi.in
===================================================================
--- gcc/doc/tm.texi.in	(revision 262159)
+++ gcc/doc/tm.texi.in	(working copy)
@@ -3336,6 +3336,8 @@ stack.
 
 @hook TARGET_REF_MAY_ALIAS_ERRNO
 
+@hook TARGET_TRANSLATE_MODE_ATTRIBUTE
+
 @hook TARGET_SCALAR_MODE_SUPPORTED_P
 
 @hook TARGET_VECTOR_MODE_SUPPORTED_P
Index: gcc/config/rs6000/rs6000.c
===================================================================
--- gcc/config/rs6000/rs6000.c	(revision 262159)
+++ gcc/config/rs6000/rs6000.c	(working copy)
@@ -1802,6 +1802,9 @@ static const struct attribute_spec rs600
 #undef TARGET_EH_RETURN_FILTER_MODE
 #define TARGET_EH_RETURN_FILTER_MODE rs6000_eh_return_filter_mode
 
+#undef TARGET_TRANSLATE_MODE_ATTRIBUTE
+#define TARGET_TRANSLATE_MODE_ATTRIBUTE rs6000_translate_mode_attribute
+
 #undef TARGET_SCALAR_MODE_SUPPORTED_P
 #define TARGET_SCALAR_MODE_SUPPORTED_P rs6000_scalar_mode_supported_p
 
@@ -35677,6 +35680,18 @@ rs6000_eh_return_filter_mode (void)
   return TARGET_32BIT ? SImode : word_mode;
 }
 
+/* Target hook for translate_mode_attribute.  */
+static machine_mode
+rs6000_translate_mode_attribute (machine_mode mode)
+{
+  if ((FLOAT128_IEEE_P (mode)
+       && ieee128_float_type_node == long_double_type_node)
+      || (FLOAT128_IBM_P (mode)
+	  && ibm128_float_type_node == long_double_type_node))
+    return COMPLEX_MODE_P (mode) ? E_TCmode : E_TFmode;
+  return mode;
+}
+
 /* Target hook for scalar_mode_supported_p.  */
 static bool
 rs6000_scalar_mode_supported_p (scalar_mode mode)
Index: gcc/c-family/c-attribs.c
===================================================================
--- gcc/c-family/c-attribs.c	(revision 262159)
+++ gcc/c-family/c-attribs.c	(working copy)
@@ -1517,6 +1517,10 @@ handle_mode_attribute (tree *node, tree
 	  return NULL_TREE;
 	}
 
+      /* Allow the target a chance to translate MODE into something supported.
+	 See PR86324.  */
+      mode = targetm.translate_mode_attribute (mode);
+
       valid_mode = false;
       switch (GET_MODE_CLASS (mode))
 	{
