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     res1 <- ROTL64(prod1, XLEN-n)
  19     res2 <- ROTL64(prod2, XLEN-n)
  20     m <- MASK(n, (XLEN-1))
  21     s1 <- res1[0]
  22     s2 <- res2[0]
  23     smask1 <- ([s1]*XLEN) & ¬m
  24     smask2 <- ([s2]*XLEN) & ¬m
  25     RT <- res1 & m | smask1
  26     RS <- res2 & m | smask2
  27
  28 Special Registers Altered:
  29
  30     None