What's up with gcc weird stack manipulation when it wants extra stack alignment?

Question

I've seen this r10 weirdness a few times, so let's see if anyone knows what's up.

Take this simple function:

#define SZ 4

void sink(uint64_t *p);

void andpop(const uint64_t* a) {
    uint64_t result[SZ];
    for (unsigned i = 0; i < SZ; i++) {
        result[i] = a[i] + 1;
    }

    sink(result);
}

It just adds 1 to each of the 4 64-bit elements of the passed-in array and stores it in a local and calls sink() on the result (to avoid the whole function being optimized away).

Here's the corresponding assembly:

andpop(unsigned long const*):
        lea     r10, [rsp+8]
        and     rsp, -32
        push    QWORD PTR [r10-8]
        push    rbp
        mov     rbp, rsp
        push    r10
        sub     rsp, 40
        vmovdqa ymm0, YMMWORD PTR .LC0[rip]
        vpaddq  ymm0, ymm0, YMMWORD PTR [rdi]
        lea     rdi, [rbp-48]
        vmovdqa YMMWORD PTR [rbp-48], ymm0
        vzeroupper
        call    sink(unsigned long*)
        add     rsp, 40
        pop     r10
        pop     rbp
        lea     rsp, [r10-8]
        ret

It's hard to understand almost everything that is going on with r10. First, r10 is set to point to rsp + 8, then push QWORD PTR [r10-8], which as far as I can tell pushes a copy of the return address on the stack. Following that, rbp is set up as normal and then finally r10 itself is pushed.

To unwind all this, r10 is popped off of the stack and used to restore rsp to its original value.

Some observations:

• Looking at the entire function, all of this seems like a totally roundabout way of simply restoring rsp to it's original value before ret - but the usual epilog of mov rsp, rpb would do just as well (see clang)!
• That said, the (expensive) push QWORD PTR [r10-8] doesn't even help in that mission: this value (the return address?) is apparently never used.
• Why is r10 pushed and popped at all? The value isn't clobbered in the very small function body and there is no register pressure.

What's up with that? I've seen it several times before, and it usually wants to use r10, sometimes r13. It seems likely that has something to do with aligning the stack to 32 bytes, since if you change SZ to be less than 4 it uses xmm ops and the issue disappears.

Here's SZ == 2 for example:

andpop(unsigned long const*):
        sub     rsp, 24
        vmovdqa xmm0, XMMWORD PTR .LC0[rip]
        vpaddq  xmm0, xmm0, XMMWORD PTR [rdi]
        mov     rdi, rsp
        vmovaps XMMWORD PTR [rsp], xmm0
        call    sink(unsigned long*)
        add     rsp, 24
        ret

Much nicer!

Answer 1

Well, you answered your question: The stack pointer needs to be aligned to 32 bytes before it can be accessed with aligned AVX2 loads and stores, but the ABI only provides 16 byte alignment. Since the compiler cannot know how much the alignment is off, it has to save the stack pointer in a scratch register and restore it afterwards. But the saved value has to outlive the function call, so it has to be put on the stack, and a stack frame has to be created.

Some x86-64 ABIs have a red zone (a region of the stack below the stack pointer which is not used by signal handlers), so it is feasible not to change the stack pointer at all for such short functions, but GCC apparently does not implement this optimization and it would not apply here anyway because of the function call at the end.

In addition, the default stack alignment implementation is rather poor. For this case, -maccumulate-outgoing-args results in better-looking code with GCC 6, just aligning RSP after saving RBP, instead of copying the return address before saving RBP:

andpop:
        pushq   %rbp
        movq    %rsp, %rbp            # make a traditional stack frame
        andq    $-32, %rsp            # reserve 0 or 16 bytes
        subq    $32, %rsp

        vmovdqu (%rdi), %xmm0         # split unaligned load from tune=generic
        vinserti128     $0x1, 16(%rdi), %ymm0, %ymm0   # use -march=haswell instead
        movq    %rsp, %rdi
        vpaddq  .LC0(%rip), %ymm0, %ymm0
        vmovdqa %ymm0, (%rsp)

        vzeroupper
        call    sink@PLT
        leave
        ret

(editor's note: gcc8 and later make asm like this by default (Godbolt compiler explorer with gcc8, clang7, ICC19, and MSVC), even without -maccumulate-outgoing-args)

---

This issue (GCC generating poor code for stack alignment) recently came up when we had to implement a workaround for GCC __tls_get_addr ABI bug, and we ended up writing the stack realignment by hand.

EDIT There is also another issue, related to RTL pass ordering: stack alignment is picked before the final determination whether the stack is actually needed, as BeeOnRope's second example shows.