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std::fma, std::fmaf, std::fmal

Defined in header `<cmath>`
float fma ( float x, float y, float z );	(1)	(since C++11) (constexpr since C++23)
float fmaf( float x, float y, float z );	(2)	(since C++11) (constexpr since C++23)
double fma ( double x, double y, double z );	(3)	(since C++11) (constexpr since C++23)
long double fma ( long double x, long double y, long double z );	(4)	(since C++11) (constexpr since C++23)
long double fmal( long double x, long double y, long double z );	(5)	(since C++11) (constexpr since C++23)
Promoted fma ( Arithmetic1 x, Arithmetic2 y, Arithmetic3 z );	(6)	(since C++11) (constexpr since C++23)
#define FP_FAST_FMA /* implementation-defined */	(7)	(since C++11)
#define FP_FAST_FMAF /* implementation-defined */	(8)	(since C++11)
#define FP_FAST_FMAL /* implementation-defined */	(9)	(since C++11)

1-5) Computes (x*y) + z as if to infinite precision and rounded only once to fit the result type.

6) A set of overloads or a function template for all combinations of arguments of arithmetic type not covered by (1-5). If any argument has integral type, it is cast to double. If any other argument is long double, then the return type is long double, otherwise it is double.

7-9) If the macro constants FP_FAST_FMA, FP_FAST_FMAF, or FP_FAST_FMAL are defined, the function std::fma evaluates faster (in addition to being more precise) than the expression x*y+z for float, double, and long double arguments, respectively. If defined, these macros evaluate to integer 1.

Parameters

x, y, z

values of floating-point or integral types

Return value

If successful, returns the value of (x*y) + z as if calculated to infinite precision and rounded once to fit the result type (or, alternatively, calculated as a single ternary floating-point operation).

If a range error due to overflow occurs, ±HUGE_VAL, ±HUGE_VALF, or ±HUGE_VALL is returned.

If a range error due to underflow occurs, the correct value (after rounding) is returned.

Error handling

Errors are reported as specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

If x is zero and y is infinite or if x is infinite and y is zero, and z is not a NaN, then NaN is returned and FE_INVALID is raised
If x is zero and y is infinite or if x is infinite and y is zero, and z is a NaN, then NaN is returned and FE_INVALID may be raised
If x*y is an exact infinity and z is an infinity with the opposite sign, NaN is returned and FE_INVALID is raised
If x or y are NaN, NaN is returned
If z is NaN, and x*y aren't 0*Inf or Inf*0, then NaN is returned (without FE_INVALID).

Notes

This operation is commonly implemented in hardware as fused multiply-add CPU instruction. If supported by hardware, the appropriate FP_FAST_FMA? macros are expected to be defined, but many implementations make use of the CPU instruction even when the macros are not defined.

POSIX additionally specifies that the situations specified to return FE_INVALID are domain errors.

Due to its infinite intermediate precision, std::fma is a common building block of other correctly-rounded mathematical operations, such as std::sqrt or even the division (where not provided by the CPU, e.g. Itanium).

As with all floating-point expressions, the expression (x*y) + z may be compiled as a fused multiply-add unless the #pragma STDC FP_CONTRACT is off.

Example

#include <iostream>
#include <iomanip>
#include <cmath>
#include <cfenv>
#ifndef __GNUC__
#pragma STDC FENV_ACCESS ON
#endif
int main()
{
    // demo the difference between fma and built-in operators
    const double in = 0.1;
    std::cout << "0.1 double is " << std::setprecision(23) << in
              << " (" << std::hexfloat << in << std::defaultfloat << ")\n"
              << "0.1*10 is 1.0000000000000000555112 (0x8.0000000000002p-3), "
              << "or 1.0 if rounded to double\n";
    const double expr_result = 0.1 * 10 - 1;
    const double fma_result = std::fma(0.1, 10, -1);
    std::cout << "0.1 * 10 - 1 = " << expr_result
              << " : 1 subtracted after intermediate rounding\n"
              << "fma(0.1, 10, -1) = " << std::setprecision(6) << fma_result << " ("
              << std::hexfloat << fma_result << std::defaultfloat << ")\n\n";
 
    // fma is used in double-double arithmetic
    const double high = 0.1 * 10;
    const double low = std::fma(0.1, 10, -high);
    std::cout << "in double-double arithmetic, 0.1 * 10 is representable as "
              << high << " + " << low << "\n\n";
 
    // error handling 
    std::feclearexcept(FE_ALL_EXCEPT);
    std::cout << "fma(+Inf, 10, -Inf) = " << std::fma(INFINITY, 10, -INFINITY) << '\n';
    if(std::fetestexcept(FE_INVALID))
        std::cout << "    FE_INVALID raised\n";
}

Possible output:

0.1 double is 0.10000000000000000555112 (0x1.999999999999ap-4)
0.1*10 is 1.0000000000000000555112 (0x8.0000000000002p-3), or 1.0 if rounded to double
0.1 * 10 - 1 = 0 : 1 subtracted after intermediate rounding
fma(0.1, 10, -1) = 5.55112e-17 (0x1p-54)
 
in double-double arithmetic, 0.1 * 10 is representable as 1 + 5.55112e-17
 
fma(+Inf, 10, -Inf) = -nan
    FE_INVALID raised