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)[XLEN:(XLEN*2)-1]
  17     prod2 <- MULS(RB, diff)[XLEN:(XLEN*2)-1]
  18     if n = 0 then
  19         #round <- EXTS([0]*(XLEN-1) || [1]*1)
  20         #prod1 <- ROTL64(prod1, 1)
  21         #prod2 <- ROTL64(prod2, 1)
  22         #prod1 <- prod1 + round
  23         #prod2 <- prod2 + round
  24         #res1 <- ROTL64(prod1, XLEN-1)
  25         #res2 <- ROTL64(prod2, XLEN-1)
  26         #m <- MASK(1, (XLEN-1))
  27         RT <- prod1
  28         RS <- prod2
  29     else
  30         round <- EXTS([0]*(XLEN -n -1) || [1]*1 || [0]*(n-1))
  31         prod1 <- prod1 + round
  32         prod2 <- prod2 + round
  33         res1 <- ROTL64(prod1, XLEN-n)
  34         res2 <- ROTL64(prod2, XLEN-n)
  35         m <- MASK(n, (XLEN-1))
  36         signbit1 <- prod1[0]
  37         signbit2 <- prod2[0]
  38         smask1 <- ([signbit1]*XLEN) & ¬m
  39         smask2 <- ([signbit2]*XLEN) & ¬m
  40         RT <- (res1 & m | smask1)
  41         RS <- (res2 & m | smask2)
  42
  43 Special Registers Altered:
  44
  45     None