

 
Manual Reference Pages  FMA (3)
NAME
fma,
fmaf,
fmal
 fused multiplyadd
CONTENTS
Library
Synopsis
Description
Implementation Notes
See Also
Standards
History
LIBRARY
.Lb libm
SYNOPSIS
.In 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
DESCRIPTION
The
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 oo due to overflow in the intermediate product, whereas
fma(1.2e100, 2.0e208, 1.4e308)
returns approximately 1.0e308.
The fused multiplyadd 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
.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.
IMPLEMENTATION NOTES
In general, these routines will behave as one would expect if
x * y + z
were computed with unbounded precision and range,
then rounded to the precision of the return type.
However, on some platforms, if
z
is NaN, these functions may not raise an exception even
when the computation of
x * y
would have otherwise generated an invalid exception.
SEE ALSO
fenv(3),
math(3)
STANDARDS
The
fma,
fmaf,
and
fmal
functions conform to
isoC99.
A fused multiplyadd operation with virtually identical
characteristics appears in IEEE draft standard 754R.
HISTORY
The
fma
and
fmaf
routines first appeared in
.Fx 5.4 ,
and
fmal
appeared in
.Fx 6.0 .
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