# RFC ls003 Big Integer

**URLs**:

* <https://libre-soc.org/openpower/sv/biginteger/analysis/>
* <https://libre-soc.org/openpower/sv/rfc/ls003/>
* <https://bugs.libre-soc.org/show_bug.cgi?id=960>
* <https://git.openpower.foundation/isa/PowerISA/issues/91>

**Severity**: Major

**Status**: New

**Date**: 20 Oct 2022

**Target**: v3.2B

**Source**: v3.0B

**Books and Section affected**: **UPDATE**

```
    Book I 64-bit Fixed-Point Arithmetic Instructions 3.3.9.1
    Appendix E Power ISA sorted by opcode
    Appendix F Power ISA sorted by version
    Appendix G Power ISA sorted by Compliancy Subset
    Appendix H Power ISA sorted by mnemonic
```

**Summary**

Instructions added

```
    maddedu - Multiply-Add Extended Double Unsigned
    maddedus - Multiply-Add Extended Double Unsigned/Signed
    divmod2du - Divide/Modulo Quad-Double Unsigned
    dsld - Double Shift Left Doubleword
    dsrd - Double Shift Right Doubleword
```

**Submitter**: Luke Leighton (Libre-SOC)

**Requester**: Libre-SOC

**Impact on processor**:

```
    Addition of five new GPR-based instructions
```

**Impact on software**:

```
    Requires support for new instructions in assembler, debuggers,
    and related tools.
```

**Keywords**:

```
    GPR, Big-integer, Double-word
```

**Motivation**

* Similar to `maddhdu` and `maddld`, but allow for a big-integer rolling
  accumulation affect: `RC` effectively becomes a 64-bit carry in chains
  of highly-efficient loop-unrolled arbitrary-length big-integer operations.
* Similar to `divdeu`, and has similar advantages to `maddedu`,
  Modulo result is available with the quotient in a single instruction
  allowing highly-efficient arbitrary-length big-integer division.
* Combining at least three instructions into one, the `dsld` and `dsrd`
  instructions make shifting an arbitrary-length big-integer vector by
  a scalar 64-bit quantity highly efficient.

**Notes and Observations**:

1. It is not practical to add Rc=1 variants when VA-Form is used and
   there is a **pair** of results produced.
2. An overflow variant (XER.OV set) of `divmod2du` would be valuable
   but VA-Form EXT004 is under severe pressure.
3. Both `maddhdu` and `divmod2du` instructions have been present in Intel x86
   for several decades.  Likewise, `dsld` and `dsrd`.
4. None of these instruction is present in VSX: these are 128/64 whereas
   VSX is 128/128.
5. `maddedu` and `divmod2du` are full inverses of each other, including
  when used for arbitrary-length big-integer arithmetic.
6. These are all 3-in 2-out instructions. If Power ISA did not already
  have LD/ST-with-update instructions and instructions with `RAp`
  and `RTp` then these instructions would not be proposed.
7. `maddedus` is the first Scalar signed/unsigned multiply instruction. The
  only other signed/unsigned multiply instruction is the
  specialist `vmsummbm` (bytes only), requires VSX,
  and is unsuited for big-integer or other general arithmetic.
8. Unresolved: dsld/dsrd are 3-in 3-out (in the Rc=1 variants) where the
   normal threshold set is 3-in 2-out.

**Changes**

Add the following entries to:

* the Appendices of Book I
* Instructions of Book I added to Section 3.3.9.1
* VA2-Form of Book I Section 1.6.21.1 and 1.6.2

----------------

\newpage{}

# Multiply-Add Extended Double Unsigned

`maddedu RT, RA, RB, RC`

|  0-5  | 6-10 | 11-15 | 16-20 | 21-25 | 26-31 | Form    |
|-------|------|-------|-------|-------|-------|---------|
| EXT04 | RT   |  RA   |  RB   |   RC  |  XO   | VA-Form |

Pseudocode:

```
prod[0:127] <- (RA) * (RB)    # Multiply RA and RB, result 128-bit
sum[0:127] <- EXTZ(RC) + prod # Zero extend RC, add product
RT <- sum[64:127]             # Store low half in RT
RS <- sum[0:63]               # RS implicit register, equal to RC
```

Special registers altered:

    None

The 64-bit operands are (RA), (RB), and (RC).
RC is zero-extended (not shifted, not sign-extended).
The 128-bit product of the operands (RA) and (RB) is added to (RC).
The low-order 64 bits of the 128-bit sum are
placed into register RT.
The high-order 64 bits of the 128-bit sum are
placed into register RS.
RS is implicitly defined as the same register as RC.

All three operands and the result are interpreted as
unsigned integers.

The differences here to `maddhdu` are that `maddhdu` stores the upper
half in RT, where `maddedu` stores the upper half in RS.

The value stored in RT is exactly equivalent to `maddld` despite `maddld`
performing sign-extension on RC, because RT is the full mathematical result
modulo 2^64 and sign/zero extension from 64 to 128 bits produces identical
results modulo 2^64. This is why there is no maddldu instruction.

*Programmer's Note:
To achieve a big-integer rolling-accumulation effect:
assuming the scalar to multiply is in r0, and r3 is
used (effectively) as a 64-bit carry,
the vector to multiply by starts at r4 and the result vector
in r20, instructions may be issued `maddedu r20,r4,r0,r3`
`maddedu r21,r5,r0,r3` etc. where the first `maddedu` will have
stored the upper half of the 128-bit multiply into r3, such
that it may be picked up by the second `maddedu`. Repeat inline
to construct a larger bigint scalar-vector multiply,
as Scalar GPR register file space permits. If register
spill is required then r3, as the effective 64-bit carry,
continues the chain.*

Examples:

```
# (r0 * r1) + r2, store lower in r4, upper in r2
maddedu r4, r0, r1, r2

# Chaining together for larger bigint (see Programmer's Note above)
# r3 starts with zero (no carry-in)
maddedu r20,r4,r0,r3
maddedu r21,r5,r0,r3
maddedu r22,r6,r0,r3
```

----------

\newpage{}

# Multiply-Add Extended Double Unsigned/Signed

`maddedus RT, RA, RB, RC`

|  0-5  | 6-10 | 11-15 | 16-20 | 21-25 | 26-31 | Form    |
|-------|------|-------|-------|-------|-------|---------|
| EXT04 | RT   |  RA   |  RB   |   RC  |  XO   | VA-Form |

Pseudocode:

```
if (RB)[0] != 0 then                 # workaround no unsigned-signed mul op
    prod[0:127] <- -((RA) * -(RB))
else
    prod[0:127] <- (RA) * (RB)
sum[0:127] <- prod + EXTS128((RC))
RT <- sum[64:127]                    # Store low half in RT
RS <- sum[0:63]                      # RS implicit register, equal to RC
```

Special registers altered:

    None

The 64-bit operands are (RA), (RB), and (RC).
(RC) is sign-extended to 128-bits and then summed with the
128-bit product of zero-extended (RA) and sign-extended (RB).
The low-order 64 bits of the 128-bit sum are
placed into register RT.
The high-order 64 bits of the 128-bit sum are
placed into register RS.
RS is implicitly defined as the same register as RC.

*Programmer's Note:
To achieve a big-integer rolling-accumulation effect:
assuming the signed scalar to multiply is in r0, and r3 is
used (effectively) as a 64-bit carry,
the unsigned vector to multiply by starts at r4 and the signed result vector
in r20, instructions may be issued `maddedus r20,r4,r0,r3`
`maddedus r21,r5,r0,r3` etc. where the first `maddedus` will have
stored the upper half of the 128-bit multiply into r3, such
that it may be picked up by the second `maddedus`. Repeat inline
to construct a larger bigint scalar-vector multiply,
as Scalar GPR register file space permits. If register
spill is required then r3, as the effective 64-bit carry,
continues the chain.*

Examples:

```
# (r0 * r1) + r2, store lower in r4, upper in r2
maddedus r4, r0, r1, r2

# Chaining together for larger bigint (see Programmer's Note above)
# r3 starts with zero (no carry-in)
maddedus r20,r4,r0,r3
maddedus r21,r5,r0,r3
maddedus r22,r6,r0,r3
```

----------

\newpage{}

# Divide/Modulo Quad-Double Unsigned

`divmod2du RT,RA,RB,RC`

|  0-5  | 6-10 | 11-15 | 16-20 | 21-25 | 26-31 | Form    |
|-------|------|-------|-------|-------|-------|---------|
| EXT04 | RT   |  RA   |  RB   |   RC  |  XO   | VA-Form |

Pseudo-code:

```
if ((RA) <u (RB)) & ((RB) != [0]*64) then  # Check RA<RB, for divide-by-0
    dividend[0:127] <- (RA) || (RC)        # Combine RA/RC as 128-bit
    divisor[0:127] <- [0]*64 || (RB)       # Extend RB to 128-bit
    result <- dividend / divisor           # Unsigned Division
    modulo <- dividend % divisor           # Unsigned Modulo
    RT <- result[64:127]                   # Store result in RT
    RS <- modulo[64:127]                   # Modulo in RC, implicit
else                                       # In case of error
    RT <- [1]*64                           # RT all 1's
    RS <- [0]*64                           # RS all 0's
```

Special registers altered:

    None

The 128-bit dividend is (RA) || (RC). The 64-bit divisor is
(RB). If the quotient can be represented in 64 bits, it is
placed into register RT. The modulo is placed into register RS.
RS is implicitly defined as the same register as RC, similarly to maddedu.

The quotient can be represented in 64-bits when both these conditions
are true:

* (RA) < (RB) (unsigned comparison)
* (RB) is NOT 0 (not divide-by-0)

If these conditions are not met, RT is set to all 1's, RS to all 0's.

All operands, quotient, and modulo are interpreted as unsigned integers.

Divide/Modulo Quad-Double Unsigned is a VA-Form instruction
that is near-identical to `divdeu` except that:

* the lower 64 bits of the dividend, instead of being zero, contain a
  register, RC.
* it performs a fused divide and modulo in a single instruction, storing
  the modulo in an implicit RS (similar to `maddedu`)
* There is no `UNDEFINED` behaviour.

RB, the divisor, remains 64 bit.  The instruction is therefore a 128/64
division, producing a (pair) of 64 bit result(s), in the same way that
Intel [divq](https://www.felixcloutier.com/x86/div) works.
Overflow conditions
are detected in exactly the same fashion as `divdeu`, except that rather
than have `UNDEFINED` behaviour, RT is set to all ones and RS set to all
zeros on overflow.

*Programmer's note: there are no Rc variants of any of these VA-Form
instructions. `cmpi` will need to be used to detect overflow conditions:
the saving in instruction count is that both RT and RS will have already
been set to useful values (all 1s and all zeros respectively)
needed as part of implementing Knuth's Algorithm D*

For Scalar usage, just as for `maddedu`, `RS=RC`
Examples:

```
# ((r0 << 64) + r2) / r1, store in r4
# ((r0 << 64) + r2) % r1, store in r2
divmod2du r4, r0, r1, r2
```

----------

\newpage{}

# Double-Shift Left Doubleword

`dsld RT,RA,RB,RC`

|  0-5  | 6-10 | 11-15 | 16-20 | 21-25 | 26-30 | 31 | Form     |
|-------|------|-------|-------|-------|-------|----|----------|
| EXT04 | RT   |  RA   |  RB   |   RC  |  XO   | Rc | VA2-Form |

Pseudo-code:

    n <- (RB)[58:63]                        # Use lower 6-bits for shift
    v <- ROTL64((RA), n)                    # Rotate RA 64-bit left by n bits
    mask <- MASK(64, 63-n)                  # 1s mask in MSBs
    RT <- (v[0:63] & mask) | ((RC) & ¬mask) # mask-in RC into RT
    RS <- v[0:63] & ¬mask                   # part normally lost into RC
    overflow = 0                            # Clear overflow flag
    if RS != [0]*64:                        # Check if RS is NOT zero
        overflow = 1                        # Set the overflow flag

Special Registers Altered:

    CR0                    (if Rc=1)

The contents of register RA are shifted left the number
of bits specified by (RB) 58:63. The same number of
shifted bits are taken from the **right** (LSB) end of register
RC and placed into the **rightmost** (LSB) end of the result, RT.
Additionally, the MSB (leftmost) bits of register RA that would normally
be discarded by a 64-bit left shift are placed into the
LSBs of RS.

When Rc=1, the overflow flag in CR0 is set if RS is nonzero,
or cleared if it is zero; all other bits of CR0 are set from RT as normal. 
XER.OV and XER.SO remain unchanged.

*Programmer's note:
similar to maddedu and divmod2du, dsld can be chained (using RC),
effectively using RC as a 64-bit carry-in and carry-out. Arbitrary
length Scalar-Vector shift may be performed without the additional
masking instructions normally needed.*

----------

# Double-Shift Right Doubleword

`dsrd RT,RA,RB,RC`

|  0-5  | 6-10 | 11-15 | 16-20 | 21-25 | 26-30 | 31 | Form     |
|-------|------|-------|-------|-------|-------|----|----------|
| EXT04 | RT   |  RA   |  RB   |   RC  |  XO   | Rc | VA2-Form |

Pseudo-code:

    n <- (RB)[58:63]                        # Take lower 6-bits for shift
    v <- ROTL64((RA), 64-n)                 # Rotate RA 64-bit left by 64-n bits
    mask <- MASK(n, 63)                     # 0's mask, set mask[n:63] to 1'
    RT <- (v[0:63] & mask) | ((RC) & ¬mask) #
    RS <- v[0:63] & ¬mask
    overflow = 0
    if RS != [0]*64:
        overflow = 1

Special Registers Altered:

    CR0                    (if Rc=1)

The contents of register RA are shifted right the number
of bits specified by (RB) 58:63. The same number of
shifted bits are taken from the **left** (MSB) end of register RC
and placed into the **leftmost** (MSB) end of the result, RT.
Additionally, the LSB (rightmost) bits of register RA that would normally
be discarded by a 64-bit right shift are placed into the
MSBs of RS.

When Rc=1, the overflow flag in CR0 is set if RS is nonzero,
or cleared if it is zero; all other bits of CR0 are set from RT as normal. 
XER.OV and XER.SO remain unchanged.

*Programmer's note:
similar to maddedu and divmod2du, dsrd can be chained (using RC),
effectively using RC as a 64-bit carry-in and carry-out. Arbitrary
length Scalar-Vector shift may be performed without the additional
masking instructions normally needed.*

----------

\newpage{}

# VA2-Form

Add the following to Book I, 1.6.21.1, VA2-Form

```
|0      |6     |11     |16     |21  |24|26  |31  |
| PO    |  RT  |   RA  |   RB  | RC    | XO | Rc |
```

Add 'VA2-Form' to `RA` thru `XO` Field in Book I, 1.6.2

```
RA (11:15)
    Field used to specify a GPR to be used as a
    source or as a target.
    Formats: ... VA2, ...

RB (16:20)
    Field used to specify a GPR to be used as a
    source.
    Formats: ... VA2, ...

RC (21:25)
    Field used to specify a GPR to be used as a
    source.
    Formats: ... VA2, ...

Rc (31)
    RECORD bit.
    0    Do not alter the Condition Register.
    1    Set Condition Register Field 0 or Field 1 as
         described in Section 2.3.1, 'Condition Regis-
         ter' on page 30.
    Formats: ... VA2, ...

RT (6:10)
    Field used to specify a GPR to be used as a target.
    Formats: ... VA2, ...

XO (26:30)
    Extended opcode field.
    Formats: ... VA2, ...
```

----------

# Appendices

    Appendix E Power ISA sorted by opcode
    Appendix F Power ISA sorted by version
    Appendix G Power ISA sorted by Compliancy Subset
    Appendix H Power ISA sorted by mnemonic

|Form| Book | Page | Version | mnemonic | Description |
|----|------|------|---------|----------|-------------|
|VA  | I    | #    | 3.2B    |maddedu   | Multiply-Add Extend Double Unsigned |
|VA  | I    | #    | 3.2B    |maddedus  | Multiply-Add Extend Double Unsigned Signed |
|VA  | I    | #    | 3.2B    |divmod2du | Divide/Modulo Quad-Double Unsigned |
|VA2 | I    | #    | 3.2B    |dsld      | Double-Shift Left Doubleword |
|VA2 | I    | #    | 3.2B    |dsrd      | Double-Shift Right Doubleword |

----------------

[[!tag opf_rfc]]