Re: x86-64: Maintain 16-byte stack alignment

[Herbert Xu]

On Thu, Jan 12, 2017 at 08:01:51AM +0000, Ard Biesheuvel wrote:
> 
> [From memory] the arm64 ELF psABI mandates a 16 byte stack alignment
> at function entry, and 8 byte alignment at all other times. This means
> compiled code will typically preserve 16 byte alignment, and
> __aligned(16) on a stack variable will likely not result in an
> explicit alignment of the stack pointer *. But the arm64 ISA does not
> have any load/store instructions that would trigger a misalignment
> fault on an address that is 8 byte aligned but not 16 byte aligned, so
> the situation is very different from x86 (assuming I am correct in
> asserting that there are no failure modes resulting from a misaligned
> stack other than this one and a potential performance hit)

OK, sounds like we're already using 16-byte aligned stacks on
ARM64.  So unless x86-64 stacks are much smaller, I don't see
the need to use 8-byte aligned stacks at least from a stack space
point-of-view.

Thanks,
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt