S390: Implement math-barriers math_opt_barrier and math_force_eval.

This patch implements the s390 specific math barriers in order
to omit the store and load from stack if possible.

sysdeps/s390/fpu/math-barriers.h

@@ -0,0 +1,46 @@
+/* Control when floating-point expressions are evaluated.  s390 version.
+   Copyright (C) 2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef S390_MATH_BARRIERS_H
+#define S390_MATH_BARRIERS_H 1
+
+#ifdef HAVE_S390_VX_GCC_SUPPORT
+# define ASM_CONSTRAINT_VR "v"
+#else
+# define ASM_CONSTRAINT_VR
+#endif
+
+#define math_opt_barrier(x)						\
+  ({ __typeof (x) __x = (x);						\
+    if (__builtin_types_compatible_p (__typeof (x), _Float128))		\
+      __asm__ ("# math_opt_barrier_f128 %0" : "+fm" (__x));		\
+    else								\
+      __asm__ ("# math_opt_barrier %0"					\
+	       : "+f" ASM_CONSTRAINT_VR "m" (__x));			\
+    __x; })
+#define math_force_eval(x)						\
+  ({ __typeof (x) __x = (x);						\
+    if (__builtin_types_compatible_p (__typeof (x), _Float128))		\
+      __asm__ __volatile__ ("# math_force_eval_f128 %0"			\
+			    : : "fm" (__x));				\
+    else								\
+      __asm__ __volatile__ ("# math_force_eval %0"			\
+			    : : "f" ASM_CONSTRAINT_VR "m" (__x));	\
+  })
+
+#endif