[Bug 295089] [libm] implementation of rsqrt, rsqrtf, and rsqrtl.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=295089

            Bug ID: 295089
           Summary: [libm] implementation of rsqrt, rsqrtf, and rsqrtl.
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: kargl@FreeBSD.org

Created attachment 270500
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=270500&action=edit
Patch with libm changes

The attached diff implements the inverse square root function, i.e,
rsqrt(x) = 1 / sqrt(x).  Exhaustive testing of the float version
suggests that it is correctly rounded in round-to-nearest for all
test values in the range [0x1p-127,0x1p126] except for 127 values.
Exhaustive testing of rsqrt and rsqrtl cannot be done, but 1100M
values of x for rsqrt and 400M values for rsqrtl were tested.  All
tested values were correctly rounded.

I do not have access to LD128 (i.e., IEEE 128-bit floating point)
hardware, so the implementation of rsqrtl() is untested.

The following is a summary of changes to source code.

* lib/msun/Makefile:
  . Add s_rsqrt.c and s_rsqrtf.c to COMMON_SRCS.
  . For non-53-bit long double targets, add s_rsqrtl.c to COMMON_SRCS.
  . Add MLINKS for rsqrt.3, rsqrtf.3, and rsqrtl.3 to sqrt.3.

* lib/msun/Symbol.map:
  . Add rsqrt, rsqrtf, and rsqrtl to the Symbol map for shared libm.so.

* lib/msun/man/sqrt.3:
  . Update the sqrt.3 manual page to include information for rsqrt[fl].
  . Note, these function come from ISO C23 (and IEEE-754 2008).

* lib/msun/src/math.h:
  . Add prototypes for new functions.

* lib/msun/src/math_private.h:
  . Add _SPLIT, _FAST2SUM, _SLOW2SUM, _XADD, _MUL, and _XMUL
    macros to perform type-type arthimetic (i.e., float-float).

* src/s_rsqrt.c:
  . New file with the implementation of 'double rsqrt(double)'.
  . For 53-bit long double targets, add a weak reference for rsqrtl.

* src/s_rsqrtf.c:
  . New file with the implementation of 'float rsqrt(float)'.

* src/s_rsqrtl.c
  . New file with the implementation of 'long double rsqrt(long double)'.
    Note, the LD80 version uses bit twiddling and LD128 version is a
    straight C language implementation.  The LD128 is untested due to
    lack of hardware.

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