[En-Nut-Discussion] ARM Alignment for Threads

[Ole Reinhardt]

Hi Bob!

> It seems that setting the alignment to 8 in the load script is not
> the solution to getting 8 byte stack alignment. Thread stacks can end
> up on a 4 byte boundary, thus screwing up va_arg().

Which platform are you working on? I reviewed Cortex context.c, and
there Uwe already implemented 8 byte alignment in rev. r5341, for AT91 I
have implemented it in rev. 5395.

> When a thread is started, it's stack is set via NutHeapAlloc(). There
> are two problems: 1) NUTMEM_ALIGNMENT defaults to 4, and 2) the
> pointer returned is set to the address of HEAPNODE->hn_next, which is
> at a 4 byte offset from the address of the heap node allocated.

I took over Uwes implementation from Cortex to AT91. There the allocated
memory is aligned _after_ allocation. In worst case 8 byes more than
requested will be allocated.

> So, even if you set NUTMEM_ALIGNMENT to 8, you still can get a stack
> pointer on a 4 byte boundary.

I took a short look to NutHeapAlloc(). Looks like NUTMEM_ALIGNMENT isn't
taken into account at all, beside of reserving anough extra bytes for
alignment?

@Harald: Could you look to the code please as well? Looks like the idea
         was to allow alignment, but finally it's not implemented? Have
         I overseen anything?

Bye,

Ole

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