middle-end/94614 - avoid multiword moves to nothing

This adjusts emit_move_multi_word to handle moves into paradoxical
subregs parts that are not there and adjusts lower-subregs
CLOBBER resolving to deal with those as well.

2020-04-16  Richard Biener  <rguenther@suse.de>

	PR middle-end/94614
	* expr.c (emit_move_multi_word): Do not generate code when
	the destination part is undefined_operand_subword_p.
	* lower-subreg.c (resolve_clobber): Look through a paradoxica
	subreg.

gcc/ChangeLog

+2020-04-16  Richard Biener  <rguenther@suse.de>
+
+	PR middle-end/94614
+	* expr.c (emit_move_multi_word): Do not generate code when
+	the destination part is undefined_operand_subword_p.
+	* lower-subreg.c (resolve_clobber): Look through a paradoxica
+	subreg.
+
 2020-04-16  Martin Jambor  <mjambor@suse.cz>
 
 	PR tree-optimization/94598

gcc/expr.c

@@ -3692,6 +3692,11 @@ emit_move_multi_word (machine_mode mode, rtx x, rtx y)
   need_clobber = false;
   for (i = 0; i < CEIL (mode_size, UNITS_PER_WORD); i++)
     {
+      /* Do not generate code for a move if it would go entirely
+	 to the non-existing bits of a paradoxical subreg.  */
+      if (undefined_operand_subword_p (x, i))
+	continue;
+
       rtx xpart = operand_subword (x, i, 1, mode);
       rtx ypart;
 

gcc/lower-subreg.c

@@ -1150,6 +1150,10 @@ resolve_clobber (rtx pat, rtx_insn *insn)
   int ret;
 
   reg = XEXP (pat, 0);
+  /* For clobbers we can look through paradoxical subregs which
+     we do not handle in simplify_gen_subreg_concatn.  */
+  if (paradoxical_subreg_p (reg))
+    reg = SUBREG_REG (reg);
   if (!resolve_reg_p (reg) && !resolve_subreg_p (reg))
     return false;