Use GCC builtins for nearbyint functions if desired.

This patch is using the corresponding GCC builtin for nearbyintf, nearbyint,
nearbintl and nearbyintf128 if the USE_FUNCTION_BUILTIN macros are defined to one
in math-use-builtins.h.

This is the case for s390 if build with at least --march=z196 --mzarch.
Otherwise the generic implementation is used.  The code of the generic
implementation is not changed.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

sysdeps/generic/math-use-builtins.h

@@ -0,0 +1,29 @@
+/* Using math gcc builtins instead of generic implementation.  Generic 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 MATH_USE_BUILTINS_H
+#define MATH_USE_BUILTINS_H	1
+
+/* Define these macros to 1 to use __builtin_xyz instead of the
+   generic implementation.  */
+#define USE_NEARBYINT_BUILTIN 0
+#define USE_NEARBYINTF_BUILTIN 0
+#define USE_NEARBYINTL_BUILTIN 0
+#define USE_NEARBYINTF128_BUILTIN 0
+
+#endif /* math-use-builtins.h */

sysdeps/ieee754/dbl-64/s_nearbyint.c

@@ -26,16 +26,20 @@
 #include <math_private.h>
 #include <fenv_private.h>
 #include <libm-alias-double.h>
-
+#include <math-use-builtins.h>
-static const double
-TWO52[2] = {
-	    4.50359962737049600000e+15, /* 0x43300000, 0x00000000 */
-	    -4.50359962737049600000e+15, /* 0xC3300000, 0x00000000 */
-};
 
 double
 __nearbyint (double x)
 {
+#if USE_NEARBYINT_BUILTIN
+  return __builtin_nearbyint (x);
+#else
+  /* Use generic implementation.  */
+  static const double
+    TWO52[2] = {
+		4.50359962737049600000e+15, /* 0x43300000, 0x00000000 */
+		-4.50359962737049600000e+15, /* 0xC3300000, 0x00000000 */
+  };
   fenv_t env;
   int64_t i0, sx;
   int32_t j0;
@@ -67,5 +71,6 @@ __nearbyint (double x)
   math_force_eval (t);
   libc_fesetenv (&env);
   return t;
+#endif /* ! USE_NEARBYINT_BUILTIN  */
 }
 libm_alias_double (__nearbyint, nearbyint)

sysdeps/ieee754/float128/float128_private.h

@@ -139,6 +139,9 @@
 #undef libm_alias_double_ldouble
 #define libm_alias_double_ldouble(func) libm_alias_float64_float128 (func)
 
+#include <math-use-builtins.h>
+#undef USE_NEARBYINTL_BUILTIN
+#define USE_NEARBYINTL_BUILTIN USE_NEARBYINTF128_BUILTIN
 
 /* IEEE function renames.  */
 #define __ieee754_acoshl __ieee754_acoshf128
@@ -342,6 +345,7 @@
 /* Builtin renames.  */
 #define __builtin_copysignl __builtin_copysignf128
 #define __builtin_signbitl __builtin_signbit
+#define __builtin_nearbyintl __builtin_nearbyintf128
 
 /* Get the constant suffix from bits/floatn-compat.h.  */
 #define L(x) __f128 (x)

sysdeps/ieee754/flt-32/s_nearbyintf.c

@@ -21,16 +21,20 @@
 #include <math_private.h>
 #include <fenv_private.h>
 #include <libm-alias-float.h>
-
+#include <math-use-builtins.h>
-static const float
-TWO23[2]={
-  8.3886080000e+06, /* 0x4b000000 */
- -8.3886080000e+06, /* 0xcb000000 */
-};
 
 float
 __nearbyintf(float x)
 {
+#if USE_NEARBYINTF_BUILTIN
+  return __builtin_nearbyintf (x);
+#else
+  /* Use generic implementation.  */
+  static const float
+    TWO23[2] = {
+		8.3886080000e+06, /* 0x4b000000 */
+		-8.3886080000e+06, /* 0xcb000000 */
+  };
 	fenv_t env;
 	int32_t i0,j0,sx;
 	float w,t;
@@ -58,5 +62,6 @@ __nearbyintf(float x)
 	math_force_eval (t);
 	libc_fesetenvf (&env);
 	return t;
+#endif /* ! USE_NEARBYINT_BUILTIN  */
 }
 libm_alias_float (__nearbyint, nearbyint)

sysdeps/ieee754/ldbl-128/s_nearbyintl.c

@@ -28,15 +28,19 @@
 #include <math-barriers.h>
 #include <math_private.h>
 #include <libm-alias-ldouble.h>
-
+#include <math-use-builtins.h>
-static const _Float128
-TWO112[2]={
-  L(5.19229685853482762853049632922009600E+33), /* 0x406F000000000000, 0 */
- L(-5.19229685853482762853049632922009600E+33)  /* 0xC06F000000000000, 0 */
-};
 
 _Float128 __nearbyintl(_Float128 x)
 {
+#if USE_NEARBYINTL_BUILTIN
+  return __builtin_nearbyintl (x);
+#else
+  /* Use generic implementation.  */
+  static const _Float128
+    TWO112[2] = {
+		 L(5.19229685853482762853049632922009600E+33), /* 0x406F000000000000, 0 */
+		 L(-5.19229685853482762853049632922009600E+33)  /* 0xC06F000000000000, 0 */
+  };
 	fenv_t env;
 	int64_t i0,j0,sx;
 	uint64_t i1 __attribute__ ((unused));
@@ -65,5 +69,6 @@ _Float128 __nearbyintl(_Float128 x)
 	math_force_eval (t);
 	fesetenv (&env);
 	return t;
+#endif /* ! USE_NEARBYINTL_BUILTIN  */
 }
 libm_alias_ldouble (__nearbyint, nearbyint)

sysdeps/s390/fpu/math-use-builtins.h

@@ -0,0 +1,49 @@
+/* Using math gcc builtins instead of generic implementation.  s390/s390x 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 MATH_USE_BUILTINS_S390_H
+#define MATH_USE_BUILTINS_S390_H	1
+
+#ifdef HAVE_S390_MIN_Z196_ZARCH_ASM_SUPPORT
+
+# include <features.h> /* For __GNUC_PREREQ.  */
+
+/* GCC emits the z196 zarch "load fp integer" instructions for these
+   builtins if build with at least --march=z196 -mzarch.  Otherwise a
+   function call to libc is emitted.  */
+# define USE_NEARBYINT_BUILTIN 1
+# define USE_NEARBYINTF_BUILTIN 1
+# define USE_NEARBYINTL_BUILTIN 1
+
+# if __GNUC_PREREQ (8, 0)
+#  define USE_NEARBYINTF128_BUILTIN 1
+# else
+#  define USE_NEARBYINTF128_BUILTIN 0
+# endif
+
+#else
+
+/* Disable the builtins if we do not have the z196 zarch instructions.  */
+# define USE_NEARBYINT_BUILTIN 0
+# define USE_NEARBYINTF_BUILTIN 0
+# define USE_NEARBYINTL_BUILTIN 0
+# define USE_NEARBYINTF128_BUILTIN 0
+
+#endif /* ! HAVE_S390_MIN_Z196_ZARCH_ASM_SUPPORT  */
+
+#endif /* math-use-builtins.h */