<!-- 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]
    if n = 0 then
        #round <- EXTS([0]*(XLEN-1) || [1]*1)
        #prod1 <- ROTL64(prod1, 1)
        #prod2 <- ROTL64(prod2, 1)
        #prod1 <- prod1 + round
        #prod2 <- prod2 + round
        #res1 <- ROTL64(prod1, XLEN-1)
        #res2 <- ROTL64(prod2, XLEN-1)
        #m <- MASK(1, (XLEN-1))
        RT <- prod1
        RS <- prod2
    else
        round <- EXTS([0]*(XLEN -n -1) || [1]*1 || [0]*(n-1))
        prod1 <- prod1 + round
        prod2 <- prod2 + round
        res1 <- ROTL64(prod1, XLEN-n)
        res2 <- ROTL64(prod2, XLEN-n)
        m <- MASK(n, (XLEN-1))
        signbit1 <- prod1[0]
        signbit2 <- prod2[0]
        smask1 <- ([signbit1]*XLEN) & ¬m
        smask2 <- ([signbit2]*XLEN) & ¬m
        RT <- (res1 & m | smask1)
        RS <- (res2 & m | smask2)

Special Registers Altered:

    None