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 9.0 |