math: Add internal roundeven_finite

Some CORE-MATH routines uses roundeven and most of ISA do not have
an specific instruction for the operation.  In this case, the call
will be routed to generic implementation.

However, if the ISA does support round() and ctz() there is a better
alternative (as used by CORE-MATH).

This patch adds such optimization and also enables it on powerpc.
On a power10 it shows the following improvement:

expm1f                      master      patched       improvement
latency                     9.8574       7.0139            28.85%
reciprocal-throughput       4.3742       2.6592            39.21%

Checked on powerpc64le-linux-gnu and aarch64-linux-gnu.

Reviewed-by: DJ Delorie <dj@redhat.com>

sysdeps/ieee754/flt-32/e_gammaf_r.c

@@ -140,7 +140,7 @@ __ieee754_gammaf_r (float x, int *signgamp)
    };
 
   double m = z - 0x1.7p+1;
-  double i = roundeven (m);
+  double i = roundeven_finite (m);
   double step = copysign (1.0, i);
   double d = m - i, d2 = d * d, d4 = d2 * d2, d8 = d4 * d4;
   double f = (c[0] + d * c[1]) + d2 * (c[2] + d * c[3])

sysdeps/ieee754/flt-32/math_config.h

@@ -57,6 +57,33 @@ static inline int32_t
 converttoint (double_t x);
 #endif
 
+#ifndef ROUNDEVEN_INTRINSICS
+/* When set, roundeven_finite will route to the internal roundeven function.  */
+# define ROUNDEVEN_INTRINSICS 1
+#endif
+
+/* Round x to nearest integer value in floating-point format, rounding halfway
+  cases to even.  If the input is non finite the result is unspecified.  */
+static inline double
+roundeven_finite (double x)
+{
+  if (!isfinite (x))
+    __builtin_unreachable ();
+#if ROUNDEVEN_INTRINSICS
+  return roundeven (x);
+#else
+  double y = round (x);
+  if (fabs (x - y) == 0.5)
+    {
+      union { double f; uint64_t i; } u = {y};
+      union { double f; uint64_t i; } v = {y - copysign (1.0, x)};
+      if (__builtin_ctzll (v.i) > __builtin_ctzll (u.i))
+        y = v.f;
+    }
+  return y;
+#endif
+}
+
 static inline uint32_t
 asuint (float f)
 {

sysdeps/ieee754/flt-32/s_expm1f.c

@@ -95,7 +95,7 @@ __expm1f (float x)
       return __math_oflowf (0);
     }
   double a = iln2 * z;
-  double ia = roundeven (a);
+  double ia = roundeven_finite (a);
   double h = a - ia;
   double h2 = h * h;
   uint64_t u = asuint64 (ia + big);

sysdeps/ieee754/flt-32/s_tanf.c

@@ -38,7 +38,7 @@ rltl (float z, int *q)
   double x = z;
   double idl = -0x1.b1bbead603d8bp-32 * x;
   double idh = 0x1.45f306ep-1 * x;
-  double id = roundeven (idh);
+  double id = roundeven_finite (idh);
   *q = (int64_t) id;
   return (idh - id) + idl;
 }

sysdeps/powerpc/fpu/math_private.h

@@ -59,4 +59,9 @@ __ieee754_sqrtf128 (_Float128 __x)
 #define _GL_HAS_BUILTIN_ILOGB 0
 #endif
 
+#ifdef _ARCH_PWR6
+/* ISA 2.03 provides frin/round() and cntlzw/ctznll().  */
+# define ROUNDEVEN_INTRINSICS 0
+#endif
+
 #endif /* _PPC_MATH_PRIVATE_H_ */