Re: [x32] PATCH: Remove ix86_promote_function_mode

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Jun 20, 2011 at 7:40 AM, Jeff Law <law@redhat.com> wrote:
>
>>> Peter, do you think it is safe to assume upper 32bits are zero in
>>> user space for x32? Kernel isn't a problem since pointer is 64bit
>>> in kernel and we don't pass pointers on stack to kernel.
>>
>> As I have already stated, if we *cannot* require pointers to be
>> zero-extended on entry to the kernel, we're going to have to have
>> special entry points for all the x32 system calls except the ones that
>> don't take pointers.asdfasfd
> BTW (and feel free to respond off-list), what's the rationale behind
> zero-extending values in x32 from 32 bits to 64 bits rather than the
> more traditional sign-extending?
>

Since hardware zero-extends 32-bit result to 64-bit in the destination
general-purpose register, we can load addresses into 32bit (ptr_mode)
register and use the full 64bit (Pmode) register without any additional
instructions. As far as the processor is concerned, x32 process is the
same as x86-64 process.  The only difference is x32 won't go over
4GB.

-- 
H.J.