openpower/sv/twin_butterfly.mdwn

   1 <https://bugs.libre-soc.org/show_bug.cgi?id=1074>
   2
   3 # [DRAFT] Twin Butterfly DCT Instruction(s)
   4
   5 The goal is to implement instructions that calculate the expression:
   6
   7 ```
   8 fdct_round_shift((a +/- b) * c)
   9 ```
  10
  11 For the single-coefficient butterfly instruction, and:
  12
  13 ```
  14  fdct_round_shift(a * c1  +/- b * c2)
  15 ```
  16
  17 For the double-coefficient butterfly instruction.
  18
  19 `fdct_round_shift` is defined as `ROUND_POWER_OF_TWO(x, 14)`
  20
  21 ```
  22 #define ROUND_POWER_OF_TWO(value, n) (((value) + (1 << ((n)-1))) >> (n))
  23 ```
  24
  25 The suggestion is to have a single instruction to calculate both values `((a + b) * c) >> N`, and `((a - b) * c) >> N`.
  26 The instruction will run in accumulate mode, so in order to calculate the 2-coeff version one would just have to call the same instruction with different order a, b and a different constant c.
  27
  28 ```
  29 # [DRAFT] Integer Butterfly Multiply Add/Sub FFT/DCT
  30
  31 BF-Form
  32
  33 * maddsubrs  RT,RA,RB,RC,SH
  34
  35 Pseudo-code:
  36
  37     RT2 <- RT + 1
  38     sum <- (RA) + (RB)
  39     diff <- (RA) - (RB)
  40     prod1 <- MUL(RC, sum)
  41     prod2 <- MUL(RC, diff)
  42     res1 <- ROTL64(prod1, SH)
  43     res2 <- ROTL64(prod2, SH)
  44     RT <- (RT) + res1
  45     RT2 <- (RT2) + res2
  46
  47 Special Registers Altered:
  48
  49     None
  50 ```
  51
  52 Where BF-Form is defined in fields.txt:
  53
  54 ```
  55 # 1.6.39 BF-FORM
  56     |0     | 6   |11   |16   |21   | 25  |30  |31  |
  57     | PO   | RT  | RA  | RB  | RC  | SH  | XO | Rc |
  58
  59 ```
  60
  61 The resulting autogenerated code is:
  62
  63 ```
  64 class butterfly:
  65     @inject()
  66     def op_maddsubrs(self, RA, RB, RC, RT):
  67         RT2 = copy_assign_rhs(RT + 1)
  68         sum = copy_assign_rhs(RA + RB)
  69         diff = copy_assign_rhs(RA - RB)
  70         prod1 = copy_assign_rhs(self.MUL(RC, sum))
  71         prod2 = copy_assign_rhs(self.MUL(RC, diff))
  72         res1 = copy_assign_rhs(self.ROTL64(prod1, SH))
  73         res2 = copy_assign_rhs(self.ROTL64(prod2, SH))
  74         RT = copy_assign_rhs(RT + res1)
  75         RT2 = copy_assign_rhs(RT2 + res2)
  76         return (RT,)
  77 ```
  78
  79 The instruction has been added to `minor_59.csv`:
  80 ```
  81 1111011111,ALU,OP_MADDSUBRS,RA,RB,RC,RT,NONE,CR1,0,0,ZERO,0,NONE,0,0,0,0,1,0,RC_ONLY,0,0,maddsubrs,A,,1,unofficial until submitted and approved/renumbered by the opf isa wg
  82 ```
  83