The AArch64 processor (aka arm64), part 13: Atomic access

Raymond Chen

Atomic operations are performed by the traditional RISC-style load locked/store conditional pattern.

    ; load exclusive register byte
    ldxrb   Rd/zr, [Xn/sp]

    ; load exclusive register halfword
    ldxrh   Rd/zr, [Xn/sp]

    ; load exclusive register
    ldxr    Rd/zr, [Xn/sp]

    ; load exclusive register pair
    ldxp    Rd1/zr, Rd2/zr, [Xn/sp]

These instructions atomically load a byte, halfword, word, doubleword, or pair of registers from memory. The instruction also tells the processor to monitor the memory address to see if any other processor writes to that same address, or addresses in the same “exclusive reservation granule”. (Implementations are allowed to have granules as large as 2KB.)

Note that the atomicity guarantee is only partial if you use LDXP to load a pair of 64-bit registers.¹ The entire 128-bit value is not loaded atomically; instead, each 64-bit portion is loaded atomically separately. You can still get tearing between the two registers.

The only supported addressing mode is register indirect. No offsets or indexes allowed.

After an exclusive load, you can attempt to store a value back to the same address:

    ; store exclusive register byte
    stxrb   Rs/zr, Rt/zr, [Xn/sp]

    ; store exclusive register halfword
    stxrh   Rs/zr, Rt/zr, [Xn/sp]

    ; store exclusive register
    stxr    Rs/zr, Rt/zr, [Xn/sp]

    ; store exclusive register pair
    stxp    Rs/zr, Rt1/zr, Rt2/zr, [Xn/sp]

If the reservation obtained by the previous LDX instruction is still valid, then the value in Rt/zr is stored to memory, and Rs is set to 0. Otherwise, no store is performed, and Rs is set to 1.

Whether the store succeeds or fails, the STX instructions clears the reservation.

For these exclusive load and store instructions, the address must be a multiple of the number of bytes being loaded. If not, then the behavior is undefined: There is no requirement that an exception be raised.

So don’t do that.

It is also required that the STX match the LDX both in address and operand sizes. You cannot perform an LDX for one address and follow up with a STX to a different address. You also cannot perform a LDXR and follow up with a STXRH to the same address. You aren’t even allowed to do a LDXP with two 32-bit registers and follow up with a STXR with a single 64-bit register. Again, the behavior is undefined if you break this rule.

The last instruction allows you to hit the reset button:

    ; clear exclusive

The CLREX discards any active reservation, and forces any subsequent STX to fail. This typically happens as part of interrupt handling or context switching to ensure that undefined behavior doesn’t occur if the thread was interrupted while it was in the middle of a LDX/STX sequence.

These instructions are usually coupled with memory barriers, which we’ll look at soon, but the next entry will be a little diversion.

Bonus chatter: There is an optional instruction set extension (mandatory starting in version 8.4) which includes a large set of atomic read-modify-write operations.

    ; atomic read-modify-write operation
    ; Rt = previous value of [Xr]
    ; [Xr] = Rt op Rs
    ldadd   Rs/zr, Rt/zr, [Xr/sp]       ; add
    ldclr   Rs/zr, Rt/zr, [Xr/sp]       ; and not
    ldeor   Rs/zr, Rt/zr, [Xr/sp]       ; exclusive or
    ldset   Rs/zr, Rt/zr, [Xr/sp]       ; or
    ldsmax  Rs/zr, Rt/zr, [Xr/sp]       ; signed maximum
    ldsmin  Rs/zr, Rt/zr, [Xr/sp]       ; signed minimum
    ldumax  Rs/zr, Rt/zr, [Xr/sp]       ; unsigned maximum
    ldumin  Rs/zr, Rt/zr, [Xr/sp]       ; unsigned minimum

By default, there is no memory ordering. You can add the suffix a to load with acquire, the suffix l to store with release, or the suffix al to get both. Note, however, that the acquire suffix is ignored if the destination register Rt is zr.

Furthermore, you can suffix b for byte memory access or h for halfword memory access.

The overall syntax is therefore

Prefix Op Acquire Release Size
ld add

For example, the instruction ldclrlh means

  • ld: Atomic load/modify/store
  • clr: Clear bits
  • (blank): No acquire on load
  • l: Release on store
  • h: Halfword size.

If you don’t care about the previous value, then you can use a pseudo-instruction that uses zr as the destination.

    ; atomic read-modify-write operation
    ; [Xr] = [Xr] op Rs
    stadd   Rs/zr, [Xr/sp]       ; add
    stclr   Rs/zr, [Xr/sp]       ; and not
    steor   Rs/zr, [Xr/sp]       ; exclusive or
    stset   Rs/zr, [Xr/sp]       ; or
    stsmax  Rs/zr, [Xr/sp]       ; signed maximum
    stsmin  Rs/zr, [Xr/sp]       ; signed minimum
    stumax  Rs/zr, [Xr/sp]       ; unsigned maximum
    stumin  Rs/zr, [Xr/sp]       ; unsigned minimum

You can add the l suffix for store with release, and you can add b and h suffixes to operate on smaller sizes. You cannot request acquire on load for these instructions because the acquire is ignored due to the destination being zr.

The optional instruction set extension also provides for atomic exchanges:

    ; swap
    ; write Rs and return previous value in Rt (atomic)
    swp     Rs/zr, Rt/zr, [Xn/sp]       ; word or doubleword
    swpb    Ws/zr, Wt/zr, [Xn/sp]       ; byte
    swph    Ws/zr, Wt/zr, [Xn/sp]       ; halfword

    ; compare and swap
    ; if value is Rs, then write Rt; Rs receives previous value
    ; (atomic)
    cas     Rs/zr, Rt/zr, [Xn/sp]       ; word or doubleword
    casb    Ws/zr, Wt/zr, [Xn/sp]       ; byte
    cash    Ws/zr, Wt/zr, [Xn/sp]       ; halfword
    casp    Rs/zr, Rt/zr, [Xn/sp]       ; register pair
                                        ; Rs,R(s+1) and Rt,R(t+1)

    ; also a, l, and al versions for acquire/release semantics

The memory order modifiers go between the swp/cas prefix and the size suffix, except that they go after the p. So you have casab (compare and swap with acquire, byte size) but caspa (compare and swap pair with acquire).

As with the ld instructions, requests to aquire on load are ignored if the destination register is zr.

The memory operand must be writable, even if the comparison fails. If no value is stored, then any requested release semantics are ignored.

Bonus reading: Atomics in AArch64.

¹ The load is required to be fully atomic starting with version 8.4 of the AArch64. On older processors, Windows uses CASP instead of LDXP/STXP.