Fix uninitialised use in mips_split_move

While testing the fix for PR91068, I hit an rtl checking failure
while building newlib.  mips_split_move was decomposing an address that
happened to be symbolic and then tried to access the REGNO of the base
register field, which wasn't initialised but which by chance pointed to
valid memory.

2019-07-07  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/mips/mips.c (mips_split_move): Zero-initialize addr
	and check whether addr.reg is nonnull before using it.

From-SVN: r273174

gcc/ChangeLog

+2019-07-07  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* config/mips/mips.c (mips_split_move): Zero-initialize addr
+	and check whether addr.reg is nonnull before using it.
+
 2019-07-06  Jakub Jelinek  <jakub@redhat.com>
 
 	* omp-low.c (lower_rec_input_clauses): For lastprivate clauses in

gcc/config/mips/mips.c

@@ -4849,7 +4849,7 @@ mips_split_move (rtx dest, rtx src, enum mips_split_type split_type, rtx insn_)
      can forward SRC for DEST.  This is most useful if the next insn is a
      simple store.   */
   rtx_insn *insn = (rtx_insn *)insn_;
-  struct mips_address_info addr;
+  struct mips_address_info addr = {};
   if (insn)
     {
       rtx_insn *next = next_nonnote_nondebug_insn_bb (insn);
@@ -4862,7 +4862,7 @@ mips_split_move (rtx dest, rtx src, enum mips_split_type split_type, rtx insn_)
 		{
 		  rtx tmp = XEXP (src, 0);
 		  mips_classify_address (&addr, tmp, GET_MODE (tmp), true);
-		  if (REGNO (addr.reg) != REGNO (dest))
+		  if (addr.reg && REGNO (addr.reg) != REGNO (dest))
 		    validate_change (next, &SET_SRC (set), src, false);
 		}
 	      else