| FMA(3) | Library Functions Manual | FMA(3) | 
fma, fmaf,
  fmal —
#include <math.h>
double
  
  fma(double
    x, double y,
    double z);
float
  
  fmaf(float
    x, float y,
    float z);
long double
  
  fmal(long
    double x, long double
    y, long double
  z);
fma(), fmaf(), and
  fmal() functions return (x * y) +
  z, computed with only one rounding error. Using the ordinary
  multiplication and addition operators, by contrast, results in two roundings:
  one for the intermediate product and one for the final result.
For instance, the expression 1.2e100 * 2.0e208 - 1.4e308 produces infinity due to overflow in the intermediate product, whereas fma(1.2e100, 2.0e208, -1.4e308) returns approximately 1.0e308.
The fused multiply-add operation is often used to improve the
    accuracy of calculations such as dot products. It may also be used to
    improve performance on machines that implement it natively. The macros
    FP_FAST_FMA, FP_FAST_FMAF
    and FP_FAST_FMAL may be defined in
    <math.h> to indicate that
    fma(), fmaf(), and
    fmal() (respectively) have comparable or faster
    speed than a multiply operation followed by an add operation.
fma(), fmaf(), and
  fmal() functions conform to ISO/IEC
  9899:1999 (“ISO C99”). A fused multiply-add
  operation with virtually identical characteristics appears in IEEE draft
  standard 754R.
fma() and fmaf() routines
  first appeared in FreeBSD 5.4, and
  fmal() appeared in FreeBSD
  6.0. The fma(), fmaf()
  and fmal() routines were imported into
  NetBSD in NetBSD 7.0.
| September 27, 2017 | NetBSD 9.4 |