openpower/sv/twin_butterfly.mdwn

   1 * <https://bugs.libre-soc.org/show_bug.cgi?id=1074>
   2 * <https://libre-soc.org/openpower/sv/biginteger/> for format and
   3   information about implicit RS/FRS
   4 * <https://git.libre-soc.org/?p=openpower-isa.git;a=blob;f=src/openpower/decoder/isa/test_caller_svp64_dct.py;hb=HEAD>
   5 * [[openpower/isa/svfparith]]
   6
   7 # [DRAFT] Twin Butterfly DCT Instruction(s)
   8
   9 The goal is to implement instructions that calculate the expression:
  10
  11 ```
  12 fdct_round_shift((a +/- b) * c)
  13 ```
  14
  15 For the single-coefficient butterfly instruction, and:
  16
  17 ```
  18  fdct_round_shift(a * c1  +/- b * c2)
  19 ```
  20
  21 For the double-coefficient butterfly instruction.
  22
  23 `fdct_round_shift` is defined as `ROUND_POWER_OF_TWO(x, 14)`
  24
  25 ```
  26 #define ROUND_POWER_OF_TWO(value, n) (((value) + (1 << ((n)-1))) >> (n))
  27 ```
  28
  29 These instructions are at the core of **ALL** FDCT calculations in many major video codecs, including -but not limited to- VP8/VP9, AV1, etc.
  30 Arm includes special instructions to optimize these operations, although they are limited in precision: `vqrdmulhq_s16`/`vqrdmulhq_s32`.
  31
  32 The suggestion is to have a single instruction to calculate both values `((a + b) * c) >> N`, and `((a - b) * c) >> N`.
  33 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.
  34
  35 # [DRAFT] Integer Butterfly Multiply Add/Sub FFT/DCT
  36
  37 A-Form
  38
  39 * maddsubrs  RT,RA,RB,SH
  40
  41 Pseudo-code:
  42
  43 ```
  44     sum <- (RT) + (RA)      # RT = a, RA = b
  45     diff <- (RT) - (RA)
  46     prod1 <- MUL(RB, sum)   # RB = c
  47     prod2 <- MUL(RB, diff)  # TODO: Pick high half?
  48     res1 <- ROTL64(prod1, XLEN-SH)
  49     res2 <- ROTL64(prod2, XLEN-SH)
  50     RT <- (RT) + res1
  51     RS <- (RS) + res2
  52 ```
  53
  54 Special Registers Altered:
  55
  56 ```
  57     None
  58 ```
  59
  60 Where we have added this variant in A-Form (defined in fields.txt):
  61
  62 ```
  63 # # 1.6.17 A-FORM
  64     |0     |6     |11      |16     |21      |26    |31 |
  65     ...
  66     | PO   |  RT  |   RA   |   RB  |   SH   |   XO |Rc |
  67
  68 ```
  69
  70 The instruction has been added to `minor_22.csv`:
  71
  72 ```
  73 ------01000,ALU,OP_MADDSUBRS,RT,CONST_SH,RB,RT,NONE,CR0,0,0,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
  74 ```
  75