The AArch64 processor (aka arm64), part 10: Loading constants

Since AArch64 uses fixed-size 32-bit instructions, you have to exercise some creativity to load a 64-bit constant.

    ; move wide with zero
    ; Rd = imm16 << n
    ; n can be 0, 16, 32, or 48
    movz    Rd, #imm16, LSL #n

    ; move wide with not
    ; Rd = ~(imm16 << n)
    ; n can be 0, 16, 32, or 48
    movn    Rd, #imm16, LSL #n

    ; move wide with keep
    ; Rd[n+15:n] = imm16
    movk    Rd, #imm16, LSL #n

The MOVZ instruction loads a 16-bit unsigned value into one of the four lanes of a 64-bit destination, or one of the two lanes of a 32-bit destination. All the remaining lanes are set to zero.

The MOVN instruction does the same thing as MOVZ, except the whole thing is bitwise negated. (Be careful not to confuse MOVN with MVN.)

The MOVK instruction does the same thing as MOVZ, except that instead of setting the other lanes to zero, the other lanes are left unchanged.

Loading a 32-bit value can be done in two instructions by using MOVZ to load 16 bits into half of the register, than the MOVK into the other half.

    movz    r0, #0x1234             ; r0 = 0x00001234
    movk    r0, #0xABCD, LSL #16    ; r0 = 0xABCD1234

This technique can be extended to load a 64-bit value in four steps, but that’s getting quite unwieldy. The compiler is more likely to store the value in the code segment and use a pc-relative addressing mode to load it.

    ; special syntax for pc-relative loads
    ldr     x0, =0x123456789ABCDEF0 ; load 64-bit value
    ldr     w0, =0x12345678         ; load 32-bit value

As I noted in the discussion of addressing modes, the assembler and disassembler use this special equals-sign notation to represent a pc-relative load. It means that the value is stored in a literal pool in the code segment, and a pc-relative load is being used to fetch it. The assembler batches up all of these literals and emits them between functions. The pc-relative load has a reach of ±1MB, so you are unlikely to run into the problem that you had on AArch32, where the reach was only ±4KB, and you had to find a safe place to dump the literals in the middle of the function.

There are quite a number of instructions that generate constants, and if you use the MOV pseudo-instruction, the assembler will try to find one that works.

    ; load up a constant somehow
    mov     Rd, #imm

A common type of sort-of constant is the address of a global variable. It’s a constant whose value isn’t discovered until runtime. We’ll look at those next time.