fma(3) - NetBSD Manual Pages

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FMA(3)                  NetBSD Library Functions Manual                 FMA(3)


NAME

     fma, fmaf, fmal -- fused multiply-add


LIBRARY

     Math Library (libm, -lm)


SYNOPSIS

     #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);


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 intermedi-
     ate product and one for the final result.

     For instance, the expression 1.2e100 * 2.0e208 - 1.4e308 produces infin-
     ity 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 per-
     formance 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.


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 ISO/IEC 9899:1999
     (``ISO C99'').  A fused multiply-add operation with virtually identical
     characteristics appears in IEEE draft standard 754R.


HISTORY

     The 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.

NetBSD 10.0                   September 27, 2017                   NetBSD 10.0