The AArch64 processor (aka arm64), part 23: Common patterns

Let’s look at some common patterns in compiler-generated code. We’ll start with a simple function call.

extern DWORD CreateWidget(WIDGETINFO const* info, int flags, HWIDGET* widget);
extern WIDGETINFO c_info;

if (CreateWidget(&c_info, WidgetFlags::FailIfExists,
                 &widget) != NO_ERROR) ...

    mov     w1, #1          ; WidgetFlags::FailIfExists
    adrp    x0, unrelated_global ; top 52 bits of pointer to c_info
    add     x0, x0, #0x320  ; lower 12 bits of pointer to c_info
    add     x2, sp, #0x40   ; x2 -> widget
    bl      CreateWidget    ; call it
    cbnz    w0, error       ; branch if nonzero return value

The parameters are loaded into the x0 through x2 registers, though not necessarily in that order. In this case, w1 is the flags parameter, and it gets a hard-coded constant.

The info parameter is a pointer to a global, so we use the ADRP + ADD sequence to get its address. Note that the name of the c_info variable appears nowhere in the disassembly. We just have to realize that c_info is 0x320 bytes after unrelated_global.

The last parameter is a pointer to a local variable, so we calculate its address by adding the appropriate offset to the stack pointer.

After the function returns, we branch if it returned a nonzero value in w0, which is the return value register for 32-bit integers.

If Create­Widget is a naïvely-imported function, then that BL will call the import stub, which looks like this:

CreateWidget:
    adrp        xip0, _imp_ResetDoodad
    ldr         xip0, [xip0, #0x8E8]
    br          xip0

This is an import stub that uses the xip0 scratch register to look up the import address entry for _imp_CreateWidget by loading the doubleword that is 0x8E8 bytes after _imp_ResetDoodad. Again, since we are building the address in two parts, the actual destination variable is not visible in the disassembly.

If the Create­Widget function had been declared with __declspec(dllimport), then the compiler would call indirectly through the import address table:

    mov     w1, #1          ; WidgetFlags::FailIfExists
    adrp    x0, unrelated_global ; top 52 bits of pointer to c_info
    add     x0, x0, #0x320  ; lower 12 bits of pointer to c_info
    add     x2, sp, #0x40   ; x2 -> widget

    adrp    x8, _imp_ResetDoodad
    ldr     x8, [x8, #0x8E8] ; load CreateWidget function pointer
    blr     x8              ; call it

    cbnz    w0, error       ; branch if nonzero return value

Virtual method calls also require obtaining the destination function pointer at runtime, this time from the vtable.

p->method(42);

    ; assume x19 holds "p"

    mov     x0, x19         ; x0 = this
    ldr     x8, [x19]       ; x8 -> vtable
    mov     w1, #42         ; parameter 1
    ldr     x8, [x8, #8]    ; load function pointer for p->method
    br      x8              ; call it

If control flow guard is active, then there will be a call to validate the call target before using it.

    ldr     x8, [x19]       ; x8 -> vtable
    ldr     x20, [x8, #8]   ; x20 = function pointer for p->method
    adrp    x8, unrelated_symbol+0x4280 ; page that contains __guard_check_icall_fptr
    ldr     x8, [x8, #0x820] ; x8 -> __guard_check_icall_fptr
    mov     x15, x20        ; x15 = address to check
    
    mov     x0, x19         ; x0 = this
    mov     w1, #42         ; parameter 1
    br      x20             ; call the function

The __guard_check_icall_fptr function uses a nonstandard calling convention: It takes the pointer to be checked in the x15 register instead of x0.

The last interesting code generation is the table-based dispatch for dense switch statements.

    ; switch on value in w19
    cmp     w19, #9         ; beyond end of table?
    bhi     do_default      ; Y: then go to default case
    adr     x9, switch_table
    ldrsw   x8, [x9, w19 uxtw #2] ; load offset from table
    adr     x9, some_code   ; some code address in the middle of the cases
    add     x8, x9, x8, lsl #2 ; move forward by this many instructions
    br      x8              ; and jump there

First, we reject values which don’t correspond to an entry in our table. In more complex scenarios, the BHI might take us to code that tests some straggler values, or possible even tests a different jump table.

If the value has an entry in our switch table, we use ADR to get the address of the table, which is stored in the code segment somewhere nearby (probably after the end of the function). Then we use LDRSW to load a signed word from the table, using the value in w19 as an unsigned index, shifted left by 2, which makes it a word index.

Okay, so we now have an offset loaded from the table.

Next, we set x9 to point to some code and use the offset as an instruction count (shift left by 2 since each instruction is 4 bytes) relative to the code address. That produces a new code address which we branch to.

Depending on how much code exists in each of the cases, the jump table could be a table of bytes, halfwords, or (in this case) words.

Sometimes the compiler is super-clever, and it puts the jump table close to the code. That way, it doesn’t need to load an anchor code address. The jump table itself serves as the anchor.

    ; switch on value in w19
    cmp     w19, #9         ; beyond end of table?
    bhi     do_default      ; Y: then go to default case
    adr     x9, switch_table
    ldrsw   x8, [x9, w19 uxtw #2] ; load offset from table
;   don't need to reload x9
    add     x8, x9, x8, lsl #2 ; move forward by this many instructions relative to table
    br      x8              ; and jump there

In principle, the compiler could have a jump table of code pointers rather than a jump table of instruction offsets. Although it costs an extra instruction or two (to add the offset to an anchor code address), it does allow for a smaller table, since each entry is only a word, or possibly as small as a byte. It also makes the code position-independent, which means fewer relocations are needed.

We’ll wrap up the series with the traditional line-by-line walkthrough of a simple function.