VAX: Handle subtracting from self with QMATH DImode add/sub

Remove an assertion the failure of which has not been actually observed,
but which appears clearly dangerous, for when the QMATH DImode add/sub
handler is invoked with the subtrahend and the minuend both the same.
Instead handle the operation by emitting a move of constant 0 to the
output operand.  Adjust the relevant inline comment accordingly.

	gcc/
	* config/vax/vax.c (vax_expand_addsub_di_operands): Handle equal
	input operands with subtraction.

1 files changed, 6 insertions, 4 deletions

diff --git a/gcc/config/vax/vax.c b/gcc/config/vax/vax.c
index 2274cce..07c499c 100644
--- a/gcc/config/vax/vax.c
+++ b/gcc/config/vax/vax.c
@@ -2042,12 +2042,14 @@ vax_expand_addsub_di_operands (rtx * operands, enum rtx_code code)
     }
   else
     {
-      /* If are adding the same value together, that's really a multiply by 2,
-	 and that's just a left shift of 1.  */
+      /* If we are adding a value to itself, that's really a multiply by 2,
+	 and that's just a left shift by 1.  If subtracting, it's just 0.  */
       if (rtx_equal_p (operands[1], operands[2]))
 	{
-	  gcc_assert (code != MINUS);
-	  emit_insn (gen_ashldi3 (operands[0], operands[1], const1_rtx));
+	  if (code == PLUS)
+	    emit_insn (gen_ashldi3 (operands[0], operands[1], const1_rtx));
+	  else
+	    emit_move_insn (operands[0], const0_rtx);
 	  return;
 	}