fma, fmaf, fmal

Parameters

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
  • if z is not a NaN, then NaN is returned and FE_INVALID is raised,
  • if 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 is not 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 specifies that the situation where the value x * y is invalid and z is a NaN is a domain error.

Due to its infinite intermediate precision, fma is a common building block of other correctly-rounded mathematical operations, such as 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 mutiply-add unless the #pragma STDC FP_CONTRACT is off.

Example

#include <fenv.h>
#include <float.h>
#include <math.h>
#include <stdio.h>
// #pragma STDC FENV_ACCESS ON
 
int main(void)
{
    // demo the difference between fma and built-in operators
    double in = 0.1;
    printf("0.1 double is %.23f (%a)\n", in, in);
    printf("0.1*10 is 1.0000000000000000555112 (0x8.0000000000002p-3),"
           " or 1.0 if rounded to double\n");
    double expr_result = 0.1 * 10 - 1;
    printf("0.1 * 10 - 1 = %g : 1 subtracted after "
           "intermediate rounding to 1.0\n", expr_result);
    double fma_result = fma(0.1, 10, -1);
    printf("fma(0.1, 10, -1) = %g (%a)\n", fma_result, fma_result);
 
    // fma use in double-double arithmetic
    printf("\nin double-double arithmetic, 0.1 * 10 is representable as ");
    double high = 0.1 * 10;
    double low = fma(0.1, 10, -high);
    printf("%g + %g\n\n", high, low);
 
    // error handling
    feclearexcept(FE_ALL_EXCEPT);
    printf("fma(+Inf, 10, -Inf) = %f\n", fma(INFINITY, 10, -INFINITY));
    if (fetestexcept(FE_INVALID))
        puts("    FE_INVALID raised");
}

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 to 1.0
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

References

See also