Code demonstrating hand-crafted and compiler-generated migration of loop-invariant code out of the loop

loop-invariant-migration.c

void migration(int *destination, int const *source, int factor, int limit) {
    for (int i = 0; i < limit; i++) {
        destination[factor * limit + i] = source[i];
    }
}

loop-invariant-migration_arm64.asm

// -fmove-loop-invariants

migration:
        mov     w4, 0
        mul     w2, w3, w2
        b       .L2
.L3:
        add     w5, w2, w4
        ldr     w6, [x1, w4, sxtw 2]
        str     w6, [x0, w5, sxtw 2]
        add     w4, w4, 1
.L2:
        cmp     w4, w3
        blt     .L3
        ret

loop-invariant-migration_handcrafted.c

void after_migration(int *destination, int const *source, int factor, int limit) {
    int step = factor * limit;
    for (int i = 0; i < limit; i++) {
        destination[step + i] = source[i];
    }
}

loop-invariant-migration_x86-64.asm

# -fmove-loop-invariants

migration:
        movl    $0, %eax
        imull   %ecx, %edx
        jmp     .L2
.L3:
        leal    (%rdx,%rax), %r8d
        movslq  %r8d, %r8
        movslq  %eax, %r9
        movl    (%rsi,%r9,4), %r9d
        movl    %r9d, (%rdi,%r8,4)
        addl    $1, %eax
.L2:
        cmpl    %ecx, %eax
        jl      .L3
        ret