re PR rtl-optimization/23941 (compress_float_constant creates denormals)

        PR 23941
        * real.c (exact_real_truncate): Return false if the format cannot
        represent the number as a normal.

From-SVN: r104424

gcc/ChangeLog

+2005-09-19  Richard Henderson  <rth@redhat.com>
+
+	PR 23941
+	* real.c (exact_real_truncate): Return false if the format cannot
+	represent the number as a normal.
+
 2005-09-19  Dorit Nuzman  <dorit@il.ibm.com>
 
 	* tree-ssa-operands.c (swap_tree_operands): Export.

gcc/real.c

@@ -2399,7 +2399,19 @@ real_value_truncate (enum machine_mode mode, REAL_VALUE_TYPE a)
 bool
 exact_real_truncate (enum machine_mode mode, const REAL_VALUE_TYPE *a)
 {
+  const struct real_format *fmt;
   REAL_VALUE_TYPE t;
+  int emin2m1;
+
+  fmt = REAL_MODE_FORMAT (mode);
+  gcc_assert (fmt);
+
+  /* Don't allow conversion to denormals.  */
+  emin2m1 = (fmt->emin - 1) * fmt->log2_b;
+  if (REAL_EXP (a) <= emin2m1)
+    return false;
+
+  /* After conversion to the new mode, the value must be identical.  */
   real_convert (&t, mode, a);
   return real_identical (&t, a);
 }

gcc/testsuite/gcc.c-torture/execute/pr23941.c

+extern void abort (void);
+double d = __FLT_MIN__ / 2.0;
+int main()
+{
+  double x = __FLT_MIN__ / 2.0;
+  if (x != d)
+    abort ();
+  return 0;
+}