Re: [PATCH] Add __builtin_stack_top

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

On Tue, Aug 4, 2015 at 1:50 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Aug 4, 2015 at 1:45 PM, Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
>> On Tue, Aug 04, 2015 at 01:00:32PM -0700, H.J. Lu wrote:
>>> There is another issue with x86, maybe other targets.  You
>>> can't get the real stack top when stack is realigned and
>>> -maccumulate-outgoing-args isn't used since ix86_expand_prologue
>>> will create and return another stack frame for
>>> __builtin_frame_address and __builtin_return_address.
>>> It will be wrong for __builtin_stack_top, which should
>>> return the real stack address.
>>
>> That's why I asked:
>>
>>> >> > You might have a reason why you want the entry stack address instead of the
>>> >> > frame address, but you didn't really explain I think?  Or I missed it.
>>
>> What would a C program do with this, that it cannot do with the frame
>> address, that would be useful and cannot be much better done in straight
>> assembler?  Do you actually want to expose the argument pointer, maybe?
>>
>
> Yes, we want to use the argument pointer as shown in testcases
> included in my patch.
>

Where do we stand on this?  We need the hard stack address at
function entry for x86 without using frame pointer.   I added
__builtin_stack_top since __builtin_frame_address can't give
us what we want.  Should __builtin_stack_top be added to
middle-end or x86 backend?

Thanks.

-- 
H.J.