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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/103406] [12 Regression] gcc -O0 behaves differently
 on "DBL_MAX related operations" than gcc -O1 and above
Date: Wed, 24 Nov 2021 13:02:06 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103406

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
And the -nan vs. nan is because we fold x + -y to x - y:

/* We can't reassociate at all for saturating types.  */
(if (!TYPE_SATURATING (type))

 /* Contract negates.  */
 /* A + (-B) -> A - B */
 (simplify
  (plus:c @0 (convert? (negate @1)))
  /* Apply STRIP_NOPS on the negate.  */
  (if (tree_nop_conversion_p (type, TREE_TYPE (@1))
       && !TYPE_OVERFLOW_SANITIZED (type))
   (with
    {
     tree t1 =3D type;
     if (INTEGRAL_TYPE_P (type)
         && TYPE_OVERFLOW_WRAPS (type) !=3D TYPE_OVERFLOW_WRAPS (TREE_TYPE =
(@1)))
       t1 =3D TYPE_OVERFLOW_WRAPS (type) ? type : TREE_TYPE (@1);
    }
    (convert (minus (convert:t1 @0) (convert:t1 @1))))))

all the negate contracting misses HONOR_SIGNED_ZEROS/HONOR_NANS checking?
Not sure if they are really a problem for signed zeros?  If so we should
try to get a testcase for that as well.

diff --git a/gcc/match.pd b/gcc/match.pd
index 5adcd6bd02c..9cdd113e02c 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -2471,7 +2473,8 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
   (plus:c @0 (convert? (negate @1)))
   /* Apply STRIP_NOPS on the negate.  */
   (if (tree_nop_conversion_p (type, TREE_TYPE (@1))
-       && !TYPE_OVERFLOW_SANITIZED (type))
+       && !TYPE_OVERFLOW_SANITIZED (type)
+       && (!FLOAT_TYPE_P (type) || !tree_expr_maybe_nan_p (@1)))
    (with
     {
      tree t1 =3D type;
@@ -2484,7 +2487,8 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
  (simplify
   (minus @0 (convert? (negate @1)))
   (if (tree_nop_conversion_p (type, TREE_TYPE (@1))
-       && !TYPE_OVERFLOW_SANITIZED (type))
+       && !TYPE_OVERFLOW_SANITIZED (type)
+       && (!FLOAT_TYPE_P (type) || !tree_expr_maybe_nan_p (@1)))
    (with
     {
      tree t1 =3D type;

btw, that doesn't fix it since combine will happily contract the negate as
well:

-   10: {r88:DF=3D-r83:DF;use r89:V2DF;clobber flags:CC;}
-      REG_DEAD r89:V2DF
-      REG_UNUSED flags:CC
-   11: r90:DF=3Dr88:DF+r83:DF
-      REG_DEAD r88:DF
+    9: NOTE_INSN_DELETED
+   10: NOTE_INSN_DELETED
+   11: r90:DF=3Dr83:DF-r83:DF=