== "Cas" and "Zacas" Extensions for Atomic Compare-and-Swap Instructions, Version 1.0.0

=== Introduction

Compare-and-Swap (CAS) provides an easy and typically faster way to perform
thread synchronization operations when supported as a hardware instruction. CAS
is typically used by lock-free and wait-free algorithms. This extension proposes
CAS instructions to operate on 32-bit, 64-bit, and 128-bit (RV64 only) data
values. The CAS instruction supports the C++11 atomic compare and exchange
operation.

While compare-and-swap for XLEN wide data may be accomplished using LR/SC, the
CAS atomic instructions scale better to highly parallel systems than LR/SC.
Many lock-free algorithms, such as a lock-free queue, require manipulation of
pointer variables. A simple CAS operation may not be sufficient to guard against
what is commonly referred to as the ABA problem in such algorithms that
manipulate pointer variables. To avoid the ABA problem, the algorithms associate
a reference counter with the pointer variable and perform updates using a
quadword compare and swap (of both the pointer and the counter). The double and
quadword CAS instructions support implementation of algorithms for ABA problem
avoidance.

The Zacas extension depends upon the A extension cite:[unpriv].

[[chapter2]]
=== Word/Doubleword/Quadword CAS (AMOCAS.W/D/Q)

[wavedrom, , ] 
.... 
{reg: [
  {bits:  7, name: 'opcode', attr:'AMO'},
  {bits:  5, name: 'rd', attr:'dest'},
  {bits:  3, name: 'funct3', attr:['010', '011', '100']},
  {bits:  5, name: 'rs1', attr:'addr'},
  {bits:  5, name: 'rs2', attr:'src'},
  {bits:  1, name: 'rl'},
  {bits:  1, name: 'aq'},
  {bits:  5, name: '00101', attr:['AMOCAS.W', 'AMOCAS.D', 'AMOCAS.Q']},
], config:{lanes: 1, hspace:1024}}
....

For RV32, `AMOCAS.W` atomically loads a 32-bit data value from address in `rs1`,
compares the loaded value to the 32-bit value held in `rd`, and if the comparison
is bitwise equal, then stores the 32-bit value held in `rs2` to the original
address in `rs1`. The value loaded from memory is placed into register `rd`. The
operation performed by `AMOCAS.W` for RV32 is as follows:

[listing]
----
    temp = mem[X(rs1)]
    if ( temp == X(rd) )
        mem[X(rs1)] = X(rs2)
    X(rd) = temp
----

`AMOCAS.D` is similar to `AMOCAS.W` but operates on 64-bit data values.

For RV32, `AMOCAS.D` atomically loads 64-bits of a data value from address in
`rs1`, compares the loaded value to a 64-bit value held in a register pair
consisting of `rd` and `rd+1`, and if the comparison is bitwise equal, then
stores the 64-bit value held in the register pair `rs2` and `rs2+1` to the
original address in `rs1`. The value loaded from memory is placed into the
register pair `rd` and `rd+1`. The instruction requires the first register in
the pair to be even numbered; encodings with odd numbered registers specified
in `rs2` and `rd` are reserved. When the first register of a source register
pair is `x0`, then both halves of the pair read as zero. When the first
register of a destination register pair is `x0`, then the entire register
result is discarded and neither destination register is written.
The operation performed by `AMOCAS.D` for RV32 is as follows:
[listing]
    temp0 = mem[X(rs1)+0]
    temp1 = mem[X(rs1)+4]
    comp0 = (rd == x0)  ? 0 : X(rd)
    comp1 = (rd == x0)  ? 0 : X(rd+1)
    swap0 = (rs2 == x0) ? 0 : X(rs2)
    swap1 = (rs2 == x0) ? 0 : X(rs2+1)
    if ( temp0 == comp0 ) && ( temp1 == comp1 )
        mem[X(rs1)+0] = swap0
        mem[X(rs1)+4] = swap1
    endif
    if ( rd != x0 )
        X(rd)   = temp0
        X(rd+1) = temp1
    endif

For RV64, `AMOCAS.W` atomically loads a 32-bit data value from address in
`rs1`, compares the loaded value to the lower 32 bits of the value held in `rd`,
and if the comparison is bitwise equal, then stores the lower 32 bits of the
value held in `rs2` to the original address in `rs1`. The 32-bit value loaded
from memory is sign-extended and is placed into register `rd`. The operation
performed by `AMOCAS.W` for RV64 is as follows:

[listing]
    temp[31:0] = mem[X(rs1)]
    if ( temp[31:0] == X(rd)[31:0] )
        mem[X(rs1)] = X(rs2)[31:0]
    X(rd) = SignExtend(temp[31:0])

For RV64, `AMOCAS.D` atomically loads 64-bits of a data value from address in
`rs1`, compares the loaded value to a 64-bit value held in `rd`, and if the
comparison is bitwise equal, then stores the 64-bit value held in `rs2` to the
original address in `rs1`. The value loaded from memory is placed into register
`rd`. The operation performed by `AMOCAS.D` for RV64 is as follows:
[listing]
    temp = mem[X(rs1)]
    if ( temp == X(rd) )
        mem[X(rs1)] = X(rs2)
    X(rd) = temp

`AMOCAS.Q` (RV64 only) atomically loads 128-bits of a data value from address in
`rs1`, compares the loaded value to a 128-bit value held in a register pair
consisting of `rd` and `rd+1`, and if the comparison is bitwise equal, then
stores the 128-bit value held in the register pair `rs2` and `rs2+1` to the
original address in `rs1`. The value loaded from memory is placed into the
register pair `rd` and `rd+1`. The instruction requires the first register in
the pair to be even numbered; encodings with odd numbered registers specified in
`rs2` and `rd` are reserved. When the first register of a source register pair
is `x0`, then both halves of the pair read as zero. When the first register of a
destination register pair is `x0`, then the entire register result is discarded
and neither destination register is written. The operation performed by
`AMOCAS.Q` is as follows:
[listing]
    temp0 = mem[X(rs1)+0]
    temp1 = mem[X(rs1)+8]
    comp0 = (rd == x0)  ? 0 : X(rd)
    comp1 = (rd == x0)  ? 0 : X(rd+1)
    swap0 = (rs2 == x0) ? 0 : X(rs2)
    swap1 = (rs2 == x0) ? 0 : X(rs2+1)
    if ( temp0 == comp0 ) && ( temp1 == comp1 )
        mem[X(rs1)+0] = swap0
        mem[X(rs1)+8] = swap1
    endif
    if ( rd != x0 )
        X(rd)   = temp0
        X(rd+1) = temp1
    endif

[NOTE]
====
For a future RV128 extension, `AMOCAS.Q` would encode a single XLEN=128 register
in `rs2` and `rd`.
====

[NOTE]
====
Some algorithms may load the previous data value of a memory location into the
register used as the compare data value source by a Zacas instruction. When
using a Zacas instruction that uses a register pair to source the compare value,
the two registers may be loaded using two individual loads. The two individual
loads may read an inconsistent pair of values but that is not an issue since the
`AMOCAS` operation itself uses an atomic load-pair from memory to obtain the
data value for its comparison.

The following example code sequence illustrates the use of `AMOCAS.D` in a RV32
implementation to atomically increment a 64-bit counter.
[listing]
# a0 - address of the counter.
increment:
  lw   a2, (a0)      # Load current counter value using
  lw   a3, 4(a0)     # two individual loads.
retry:
  mv   a6, a2        # Save the low 32 bits of the current value.
  mv   a7, a3        # Save the high 32 bits of the current value.
  addi a4, a2, 1     # Increment the low 32 bits.
  sltu a1, a4, a2    # Determine if there is a carry out.
  add  a5, a3, a1    # Add the carry if any to high 32 bits.
  amocas.d.aqrl a2, a4, (a0)
  bne  a2, a6, retry # If amocas.d failed then retry
  bne  a3, a7, retry # using current values loaded by amocas.d.
  ret
====

Just as for AMOs in the A extension, `AMOCAS.W/D/Q` requires that the address
held in `rs1` be naturally aligned to the size of the operand (i.e., 16-byte
aligned for _quadwords_, eight-byte aligned for _doublewords_, and four-byte
aligned for _words_). And the same exception options apply if the address
is not naturally aligned.

Just as for AMOs in the A extension, the `AMOCAS.W/D/Q` optionally provide
release consistency semantics, using the `aq` and `rl` bits, to help implement
multiprocessor synchronization. The memory operation performed by an
`AMOCAS.W/D/Q`, when successful, has acquire semantics if `aq` bit is 1 and has
release semantics if `rl` bit is 1. The memory operation performed by an
`AMOCAS.W/D/Q`, when not successful, has acquire semantics if `aq` bit is 1 but
does not have release semantics, regardless of `rl`.

A FENCE instruction may be used to order the memory read access and, if
produced, the memory write access by an `AMOCAS.W/D/Q` instruction.

[NOTE]
====
An unsuccessful `AMOCAS.W/D/Q` may either not perform a memory write or may
write back the old value loaded from memory. The memory write, if produced, does
not have release semantics, regardless of `rl`.
====

An `AMOCAS.W/D/Q` instruction always requires write permissions.

<<<

[NOTE]
====
The following example code sequence illustrates the use of `AMOCAS.Q` to
implement the _enqueue_ operation for a non-blocking concurrent queue using the
algorithm outlined in cite:[queue]. The algorithm atomically operates on a
pointer and its associated modification counter using the `AMOCAS.Q` instruction
to avoid the ABA problem.

[listing]
# Enqueue operation of a non-blocking concurrent queue.
# Data structures used by the queue:
#   structure pointer_t {ptr:   node_t *, count: uint64_t}
#   structure node_t    {next: pointer_t, value: data type}
#   structure queue_t   {Head: pointer_t, Tail:  pointer_t}
# Inputs to the procedure:
#   a0 - address of Tail variable
#   a4 - address of a new node to insert at tail
enqueue:
  ld   a6, (a0)          # a6 = Tail.ptr
  ld   a7, 8(a0)         # a7 = Tail.count
  ld   a2, (a6)          # a2 = Tail.ptr->next.ptr
  ld   a3, 8(a6)         # a3 = Tail.ptr->next.count
  ld   t1, (a0)
  ld   t2, 8(a0)
  bne  a6, t1, enqueue   # Retry if Tail & next are not consistent
  bne  a7, t2, enqueue   # Retry if Tail & next are not consistent
  bne  a2, x0, move_tail # Was tail pointing to the last node?
  mv   t1, a2            # Save Tail.ptr->next.ptr
  mv   t2, a3            # Save Tail.ptr->next.count
  addi a5, a3, 1         # Link the node at the end of the list
  amocas.q.aqrl a2, a4, (a6)
  bne  a2, t1, enqueue   # Retry if CAS failed
  bne  a3, t2, enqueue   # Retry if CAS failed
  addi a5, a7, 1         # Update Tail to the inserted node
  amocas.q.aqrl a6, a4, (a0)
  ret                    # Enqueue done
move_tail:               # Tail was not pointing to the last node
  addi a3, a7, 1         # Try to swing Tail to the next node
  amocas.q.aqrl a6, a2, (a0)
  j    enqueue           # Retry

====

=== Additional AMO PMAs

There are four levels of PMA support defined for AMOs in the A extension. Zacas
defines three additional levels of support: `AMOCASW`, `AMOCASD`, and `AMOCASQ`.

`AMOCASW` indicates that in addition to instructions indicated by `AMOArithmetic`
level support, the `AMOCAS.W` instruction is supported. `AMOCASD` indicates that
in addition to instructions indicated by `AMOCASW` level support, the `AMOCAS.D`
instruction is supported. `AMOCASQ` indicates that in addition to instructions
indicated by `AMOCASD` level support, the `AMOCAS.Q` instruction is supported.

[NOTE]
====
`AMOCASW/D/Q` require `AMOArithmetic` level support as the `AMOCAS.W/D/Q`
instructions require ability to perform an arithmetic comparison and a swap
operation. 
====