<!-- SVP64 Butterfly DCT Instructions here described are based on -->

<!-- PLEASE NOTE THESE ARE UNAPPROVED AND DRAFT, NOT SUBMITTED TO OPF ISA WG -->

# [DRAFT] Integer Butterfly Multiply Add/Sub FFT/DCT

A-Form

* maddsubrs  RT,RA,SH,RB

Pseudo-code:

    n <- SH
    sum <- (RT) + (RA)
    diff <- (RT) - (RA)
    prod1 <- MULS(RB, sum)[XLEN:(XLEN*2)-1]
    prod2 <- MULS(RB, diff)[XLEN:(XLEN*2)-1]
    res1 <- ROTL64(prod1, XLEN-n)
    res2 <- ROTL64(prod2, XLEN-n)
    m <- MASK(n, (XLEN-1))
    signbit1 <- res1[0]
    signbit2 <- res2[0]
    smask1 <- ([signbit1]*XLEN) & ¬m
    smask2 <- ([signbit2]*XLEN) & ¬m
    s64_1 <- [0]*(XLEN-1) || signbit1
    s64_2 <- [0]*(XLEN-1) || signbit2
    RT <- (res1 & m | smask1) + s64_1
    RS <- (res2 & m | smask2) + s64_2

Special Registers Altered:

    None