[CRT:MATH] Import frexp from musl

sdk/lib/crt/math/frexp.c

···
-#include <math.h>
-#include <stdlib.h>
-#include <internal/ieee.h>
-
 /*
- * @implemented
+ * PROJECT:     ReactOS CRT
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Implementation of frexp
+ * COPYRIGHT:   Imported from musl libc
+ *              https://git.musl-libc.org/cgit/musl/tree/src/math/frexp.c
+ *              blob: 27b6266ed0c1d7c5dadd06ecc186a994fdcd1c52
+ *              See https://git.musl-libc.org/cgit/musl/tree/COPYRIGHT
  */
-double
-frexp(double __x, int *exptr)
-{
-	union
-	{
-		double*   __x;
-		double_s*   x;
-	} x;
 
-	x.__x = &__x;
-
-	if ( exptr != NULL )
-		*exptr = x.x->exponent - 0x3FE;
+#include <math.h>
+#include <stdint.h>
 
+double frexp(double x, int *e)
+{
+	union { double d; uint64_t i; } y = { x };
+	int ee = y.i>>52 & 0x7ff;
 
-	x.x->exponent = 0x3FE;
+	if (!ee) {
+		if (x) {
+			x = frexp(x*0x1p64, e);
+			*e -= 64;
+		} else *e = 0;
+		return x;
+	} else if (ee == 0x7ff) {
+		return x;
+	}
 
-	return __x;
+	*e = ee - 0x3fe;
+	y.i &= 0x800fffffffffffffull;
+	y.i |= 0x3fe0000000000000ull;
+	return y.d;
 }
-
-
-