# _mm_fmaddsub_pd, _mm256_fmaddsub_pd

Multiply-adds and subtracts packed double-precision floating-point values using three float64 vectors. The corresponding FMA instruction is VFMADDSUB<XXX>PD, where XXX could be 132, 213, or 231.

## Syntax

For 128-bit vector

extern __m128d _mm_fmaddsub_pd(__m128d a, __m128d b, __m128d c);

For 256-bit vector

extern __m256d _mm256_fmaddsub_pd(__m256d a, __m256d b, __m256d c);

## Arguments

a | float64 vector used for the operation
b | float64 vector also used for the operation
c | float64 vector also used for the operation

## Description

Performs a set of SIMD multiply-add-subtract computation on packed double-precision floating-point values using three source vectors/operands, `a`,`b`, and `c`. Corresponding values in two operands, `a` and `b`, are multiplied and infinite precision intermediate results are obtained. The odd values in the third operand, `c`, are added to the intermediate results while the even values are subtracted from them. The final results are rounded to the nearest float64 values.

The compiler defaults to using the VFMADDSUB213PD instruction and uses the other forms VFMADDSUB132PD or VFMADDSUB231PD only if a low-level optimization decides it is useful or necessary. For example, the compiler could change the default if it finds that another instruction form saves a register or eliminates a move.

## Returns

Result of the multiply-add-subtract operation.

Parent topic: Intrinsics for Fused Multiply Add Operations