Re: [PATCH 2/3] Make __float128 use the _Float128 type, PR target/107299

["Kewen.Lin" <linkw@linux.ibm.com>]

>> I bet the above workaround in generic code was added for a reason, it would
>> surprise me if _Float128 worked at all without that hack.
> 
> OK, I'll have a look at those nan failures soon.

By investigating the exposed NaN failures, I found it's due to that it wants
to convert _Float128 type constant to long double type constant, it goes
through function real_convert which clears the signalling bit in the context
of !HONOR_SNANS (arg).

  if (r->cl == rvc_nan)
    r->signalling = 0;

The test cases don't have the explicit option -fsignaling-nans, I'm inclined
to believe it's intentional since there is only a sNaN generation.  If so,
we don't want this kind of conversion which is useless and can clear signalling
bit unexpectedly, one shortcut is to just copy the corresponding REAL_VALUE_TYPE
and rebuild with the given type if the modes are the same.

-----
diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
index e80be8049e1..d036b09dc6f 100644
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -2178,6 +2178,14 @@ fold_convert_const_real_from_real (tree type, const_tree arg1)
   REAL_VALUE_TYPE value;
   tree t;

+  /* If the underlying modes are the same, just copy the
+     TREE_REAL_CST information and rebuild with 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)

-----

The above diff can fix all exposed NaN failures.

BR,
Kewen