[U-Boot] [PATCH v2] ARM: Align stack to 8 bytes

[Vitaly Kuzmichev]

I would like to add some explanations:

This is the issue gone from GCC behavior on VLA allocation. I did a
simple test with VLA, and the following snippet from its ASM listing may
clarify the root cause of issue:

VLA allocation start.
R1 is initialized by the length of VLA.
>80080030:       e281300f        add     r3, r1, #15     ; 0xf
>80080034:       e2033f7e        and     r3, r3, #504    ; 0x1f8
Align VLA size.

>80080038:       e1a0500d        mov     r5, sp
Save SP to recover it when VLA becomes needless.

>8008003c:       e04dd003        sub     sp, sp, r3
Allocate R3 bytes on stack.

>80080040:       e1a0300d        mov     r3, sp
Store VLA address in R3.

>80080044:       e1a0c1a3        lsr     ip, r3, #3
>80080048:       e1a0218c        lsl     r2, ip, #3
Here VLA address is aligned by 8 bytes.

If SP is either 0xYYYYYYY4 or 0xZZZZZZZC, r2 will lose significant digit
and will become 0xYYYYYYY0/0xZZZZZZZ8 (VLA=SP-4). It will less than SP,
so the next 'push' (alias to STMDB) will decrement SP by 4 and will
store register at the top of the stack, so this will overwrite first 4
bytes of VLA.


On 06/15/2010 10:18 PM, Vitaly Kuzmichev wrote:
> The ARM ABI requires that the stack be aligned to 8 bytes as it is noted
> in Procedure Call Standard for the ARM Architecture:
> http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042d/index.html
> 
> Unaligned SP also causes the problem with variable-length arrays
> allocation when VLA address becomes less than stack pointer during
> aligning of this address, so the next 'push' in the stack overwrites
> first 4 bytes of VLA.
> 
> Signed-off-by: Vitaly Kuzmichev <vkuzmichev at mvista.com>