* <https://bugs.libre-soc.org/show_bug.cgi?id=1074>
* <https://libre-soc.org/openpower/sv/biginteger/> for format and
  information about implicit RS/FRS
* <https://git.libre-soc.org/?p=openpower-isa.git;a=blob;f=src/openpower/decoder/isa/test_caller_svp64_dct.py;hb=HEAD>

# [DRAFT] Twin Butterfly DCT Instruction(s)

The goal is to implement instructions that calculate the expression:

```
fdct_round_shift((a +/- b) * c)
```

For the single-coefficient butterfly instruction, and:

```
 fdct_round_shift(a * c1  +/- b * c2)
```

For the double-coefficient butterfly instruction.

`fdct_round_shift` is defined as `ROUND_POWER_OF_TWO(x, 14)`

```
#define ROUND_POWER_OF_TWO(value, n) (((value) + (1 << ((n)-1))) >> (n))
```

These instructions are at the core of **ALL** FDCT calculations in many major video codecs, including -but not limited to- VP8/VP9, AV1, etc.
Arm includes special instructions to optimize these operations, although they are limited in precision: `vqrdmulhq_s16`/`vqrdmulhq_s32`.

The suggestion is to have a single instruction to calculate both values `((a + b) * c) >> N`, and `((a - b) * c) >> N`.
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.

```
# [DRAFT] Integer Butterfly Multiply Add/Sub FFT/DCT

BF-Form

* maddsubrs  RT,RA,RB,RC,SH

Pseudo-code:

    RT2 <- RT + 1
    sum <- (RA) + (RB)
    diff <- (RA) - (RB)
    prod1 <- MUL(RC, sum)
    prod2 <- MUL(RC, diff)
    res1 <- ROTL64(prod1, SH)
    res2 <- ROTL64(prod2, SH)
    RT <- (RT) + res1
    RT2 <- (RT2) + res2

Special Registers Altered:

    None
```

Where BF-Form is defined in fields.txt:

```
# 1.6.39 BF-FORM
    |0     | 6   |11   |16   |21   | 25  |30  |31  |
    | PO   | RT  | RA  | RB  | RC  | SH  | XO | Rc |

```

The instruction has been added to `minor_22.csv`:
```
------01000,ALU,OP_MADDSUBRS,RA,RB,RC,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
```