Don't include subst attributes in "@" md helpers

In a later patch, I need to add "@" to a pattern that uses subst
attributes.  This combination is problematic for two reasons:

(1) define_substs are applied and filtered at a later stage than the
    handling of "@" patterns, so that the handling of "@" patterns
    doesn't know which subst variants are valid and which will later be
    dropped.  Just adding a "@" therefore triggers a build error due to
    references to non-existent patterns.

(2) Currently, the code will treat a single "@" pattern as contributing
    to a single set of overloaded functions.  These overloaded functions
    will have an integer argument for every subst attribute.  For example,
    the vczle and vczbe in:

      "@aarch64_rev<REVERSE:rev_op><mode><vczle><vczbe>"

    are subst attributes, and so currently we'd try to generate a
    single set of overloads that take four arguments: one for rev_op,
    one for the mode, one for vczle, and one for vczbe.  The gen_*
    and maybe_gen_* functions will also have one rtx argument for
    each operand in the original pattern.

    This model doesn't really make sense for define_substs, since
    define_substs are allowed to add extra operands to an instruction.
    The number of rtx operands to the generators would then be
    incorrect.

I think a more sensible way of handling define_substs would be to
apply them first (and thus expand things like <vczle> and <vczbe>
above) and then apply "@".  However, that's a relatively invasive
change and not suitable for stage 3.

This patch instead skips over subst attributes and restricts "@"
overload handling to the cases where no define_subst is applied.
I looked through all uses of "@" names in target code and there
seemed to be only one current use of "@" with define_substs,
in x86 vector code.  The current behaviour seemed to be unwanted there,
and the x86 code was having to work around it.

gcc/
	* read-rtl.cc (md_reader::handle_overloaded_name): Don't add
	arguments for uses of subst attributes.
	(apply_iterators): Only add instructions to an overloaded helper
	if they use the default subst iterator values.
	* doc/md.texi: Update documentation accordingly.
	* config/i386/i386-expand.cc (expand_vec_perm_broadcast_1): Update
	accordingly.

gcc/config/i386/i386-expand.cc

@@ -23884,7 +23884,7 @@ expand_vec_perm_broadcast_1 (struct expand_vec_perm_d *d)
       if (d->testing_p)
 	return true;
 
-      rtx (*gen_interleave) (machine_mode, int, rtx, rtx, rtx);
+      rtx (*gen_interleave) (machine_mode, rtx, rtx, rtx);
       if (elt >= nelt2)
 	{
 	  gen_interleave = gen_vec_interleave_high;
@@ -23895,7 +23895,7 @@ expand_vec_perm_broadcast_1 (struct expand_vec_perm_d *d)
       nelt2 /= 2;
 
       dest = gen_reg_rtx (vmode);
-      emit_insn (gen_interleave (vmode, 1, dest, op0, op0));
+      emit_insn (gen_interleave (vmode, dest, op0, op0));
 
       vmode = V4SImode;
       op0 = gen_lowpart (vmode, dest);

gcc/doc/md.texi

@@ -12405,4 +12405,9 @@ output and three inputs).  This combination would produce separate
 each operand count, but it would still produce a single
 @samp{maybe_code_for_@var{name}} and a single @samp{code_for_@var{name}}.
 
+Currently, these @code{@@} patterns only take into account patterns for
+which no @code{define_subst} has been applied (@pxref{Define Subst}).
+Any @samp{<@dots{}>} placeholders that refer to subst attributes
+(@pxref{Subst Iterators}) are ignored.
+
 @end ifset

gcc/read-rtl.cc

@@ -814,9 +814,14 @@ md_reader::handle_overloaded_name (rtx original, vec<mapping *> *iterators)
 	  pending_underscore_p = false;
 	}
 
-      /* Record an argument for ITERATOR.  */
-      iterators->safe_push (iterator);
-      tmp_oname.arg_types.safe_push (iterator->group->type);
+      /* Skip define_subst iterators, since define_substs are allowed to
+	 add new match_operands in their output templates.  */
+      if (iterator->group != &substs)
+	{
+	  /* Record an argument for ITERATOR.  */
+	  iterators->safe_push (iterator);
+	  tmp_oname.arg_types.safe_push (iterator->group->type);
+	}
     }
   if (base == copy)
     fatal_with_file_and_line ("no iterator attributes in name `%s'", name);
@@ -951,6 +956,7 @@ apply_iterators (rtx original, vec<rtx> *queue)
       /* We apply subst iterator after RTL-template is copied, as during
 	 subst-iterator processing, we could add an attribute to the
 	 RTL-template, and we don't want to do it in the original one.  */
+      bool add_oname = true;
       FOR_EACH_VEC_ELT (iterator_uses, i, iuse)
 	{
 	  v = iuse->iterator->current_value;
@@ -961,10 +967,14 @@ apply_iterators (rtx original, vec<rtx> *queue)
 	      current_iterator_name = iuse->iterator->name;
 	      iuse->iterator->group->apply_iterator (iuse->x, iuse->index,
 						     v->number);
+	      /* Only handle '@' overloading for the default value.
+		 See handle_overloaded_name for details.  */
+	      if (v != iuse->iterator->values)
+		add_oname = false;
 	    }
 	}
 
-      if (oname)
+      if (oname && add_oname)
 	add_overload_instance (oname, iterators, x);
 
       /* Add the new rtx to the end of the queue.  */