diff --git a/gcc/match.pd b/gcc/match.pd
index 97399e580a4..8b44b5cc92c 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -229,13 +229,16 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
 /* Simplify x - x.
    This is unsafe for certain floats even in non-IEEE formats.
    In IEEE, it is unsafe because it does wrong for NaNs.
+   PR middle-end/98420: x - x may be -0.0 with FE_DOWNWARD.
    Also note that operand_equal_p is always false if an operand
    is volatile.  */
 (simplify
  (minus @0 @0)
  (if (!FLOAT_TYPE_P (type)
       || (!tree_expr_maybe_nan_p (@0)
-	  && !tree_expr_maybe_infinite_p (@0)))
+	  && !tree_expr_maybe_infinite_p (@0)
+	  && (!HONOR_SIGN_DEPENDENT_ROUNDING (type)
+	      || !HONOR_SIGNED_ZEROS (type))))
   { build_zero_cst (type); }))
 (simplify
  (pointer_diff @@0 @0)
diff --git a/gcc/testsuite/gcc.dg/pr98420.c b/gcc/testsuite/gcc.dg/pr98420.c
new file mode 100644
index 00000000000..c289b845d63
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr98420.c
@@ -0,0 +1,8 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -ffinite-math-only -frounding-math -fdump-tree-optimized" } */
+double foo (double a)
+{
+  return a - a;
+}
+
+/* { dg-final { scan-tree-dump " = a_\[0-9\]\\(D\\) - a_\[0-9\]\\(D\\);" "optimized" } } */