openpower/isa/butterfly.mdwn

   1 <!-- SVP64 Butterfly DCT Instructions here described are based on -->
   2
   3 <!-- PLEASE NOTE THESE ARE UNAPPROVED AND DRAFT, NOT SUBMITTED TO OPF ISA WG -->
   4
   5 # [DRAFT] Integer Butterfly Multiply Add/Sub FFT/DCT
   6
   7 A-Form
   8
   9 * maddsubrs  RT,RA,SH,RB
  10
  11 Pseudo-code:
  12
  13     n <- SH
  14     sum <- (RT) + (RA)
  15     diff <- (RT) - (RA)
  16     prod1 <- MULS(RB, sum)
  17     prod1_lo <- prod1[XLEN:(XLEN*2)-1]
  18     prod2 <- MULS(RB, diff)
  19     prod2_lo <- prod2[XLEN:(XLEN*2)-1]
  20     if n = 0 then
  21         RT <- prod1_lo
  22         RS <- prod2_lo
  23     else
  24         if n = 1 then
  25             round <- [0]*(XLEN -n) || [1]*1
  26         else
  27             round <- [0]*(XLEN -n) || [1]*1 || [0]*(n-1)
  28         prod1_lo <- prod1_lo + round
  29         prod2_lo <- prod2_lo + round
  30         m <- MASK(n, (XLEN-1))
  31         res1 <- ROTL64(prod1_lo, XLEN-n) & m
  32         res2 <- ROTL64(prod2_lo, XLEN-n) & m
  33         signbit1 <- prod1_lo[0]
  34         signbit2 <- prod2_lo[0]
  35         smask1 <- ([signbit1]*XLEN) & ¬m
  36         smask2 <- ([signbit2]*XLEN) & ¬m
  37         RT <- (res1 | smask1)
  38         RS <- (res2 | smask2)
  39
  40 Special Registers Altered:
  41
  42     None