VRSQRT_(3MVEC) Vector Math Library Functions VRSQRT_(3MVEC)

## NAME

vrsqrt_, vrsqrtf_ - vector reciprocal square root functions

## SYNOPSIS

cc [ flag... ] file... -lmvec [ library... ]

void vrsqrt_(int *n, double * restrict x, int *stridex,
double * restrict y, int *stridey);

void vrsqrtf_(int *n, float * restrict x, int *stridex,
float * restrict y, int *stridey);

## DESCRIPTION

These functions evaluate the function rsqrt(x), defined by rsqrt(x) = 1 /
sqrt(x), for an entire vector of values at once. The first parameter
specifies the number of values to compute. Subsequent parameters specify
the argument and result vectors. Each vector is described by a pointer to
the first element and a stride, which is the increment between successive
elements.

Specifically, vrsqrt_(n, x, sx, y, sy) computes y[i * *sy] = rsqrt(x[i *
*sx]) for each i = 0, 1, ..., *n - 1. The vrsqrtf_() function performs
the same computation for single precision data.

These functions are not guaranteed to deliver results that are identical
to the results of evaluating 1.0 / sqrt(x) given the same arguments.
Non-exceptional results, however, are accurate to within a unit in the
last place.

## USAGE

The element count *n must be greater than zero. The strides for the
argument and result arrays can be arbitrary integers, but the arrays
themselves must not be the same or overlap. A zero stride effectively
collapses an entire vector into a single element. A negative stride
causes a vector to be accessed in descending memory order, but note that
the corresponding pointer must still point to the first element of the
vector to be used; if the stride is negative, this will be the highest-
addressed element in memory. This convention differs from the Level 1
BLAS, in which array parameters always refer to the lowest-addressed
element in memory even when negative increments are used.

These functions assume that the default round-to-nearest rounding
direction mode is in effect. On x86, these functions also assume that the
default round-to-64-bit rounding precision mode is in effect. The result
of calling a vector function with a non-default rounding mode in effect
is undefined.

These functions handle special cases and exceptions in the spirit of IEEE
754. In particular,

o if x < 0, rsqrt(x) is NaN, and an invalid operation exception
is raised,

o rsqrt(NaN) is NaN,

o rsqrt(+Inf) is +0,

o rsqrt(+-0) is +-Inf, and a division-by-zero exception is
raised.

An application wanting to check for exceptions should call
feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return,
if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW)
is non-zero, an exception has been raised. The application can then
examine the result or argument vectors for exceptional values. Some
vector functions can raise the inexact exception even if all elements of
the argument array are such that the numerical results are exact.

## ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

+----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+----------------------------+-----------------------------+
|Interface Stability | Committed |
+----------------------------+-----------------------------+
|MT-Level | MT-Safe |
+----------------------------+-----------------------------+