fold-const: Treat fp conversion to a type with same mode as copy

In function fold_convert_const_real_from_real, when the modes of
two types involved in fp conversion are the same, we can simply
take it as copy, rebuild with the exactly same TREE_REAL_CST and
the target type.  It is more efficient and helps to avoid possible
unexpected signalling bit clearing in [1].

[1] https://gcc.gnu.org/pipermail/gcc-patches/2022-December/608533.html

gcc/ChangeLog:

	* fold-const.cc (fold_convert_const_real_from_real): Treat floating
	point conversion to a type with same mode as copy instead of normal
	convertFormat.

1 files changed, 9 insertions, 0 deletions

diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
index 00e2af0..d4ee3b3 100644
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -2178,6 +2178,15 @@ fold_convert_const_real_from_real (tree type, const_tree arg1)
   REAL_VALUE_TYPE value;
   tree t;
 
+  /* If the underlying modes are the same, simply treat it as
+     copy and rebuild with TREE_REAL_CST information and the
+     given type.  */
+  if (TYPE_MODE (type) == TYPE_MODE (TREE_TYPE (arg1)))
+    {
+      t = build_real (type, TREE_REAL_CST (arg1));
+      return t;
+    }
+
   /* Don't perform the operation if flag_signaling_nans is on
      and the operand is a signaling NaN.  */
   if (HONOR_SNANS (arg1)