[LLVMdev] A new builtin: __builtin_stack_pointer()

[Behan Webster]

On 11/05/13 00:20, Alp Toker wrote:
> On 20/10/2013 16:22, Behan Webster wrote:
>> On 10/10/13 22:01, Jakob Stoklund Olesen wrote:
>>> On Oct 10, 2013, at 12:32 PM, Behan Webster <behanw at converseincode.com> wrote:
>>>
>>>> One of the issues the LLVMLinux project is having is with the use of
>>>> named registers in the Linux kernel code. The kernel uses something like
>>>> this in order to assign a C variable name to a register (one for each
>>>> kernel arch).
>>>>
>>>>    register unsigned long current_stack_pointer asm("esp");
>>>>
>>>> clang doesn't allow this kind of thing which required a patch which less
>>>> efficient:
>>>>
>>>> #define current_stack_pointer ({ \
>>>>       unsigned long esp; \
>>>>       asm("mov %%esp, %0" : "=r"(esp)); \
>>>>       esp; \
>>>> })
>>>>
>>>> This works for both gcc and clang, but often adds in 3 extra
>>>> instructions since you need to copy the stack pointer to another
>>>> register, but first that register needs to be saved to the stack and
>>>> then restored after the stackpointer has been used; inefficient.
>>> #define current_stack_pointer ({ \
>>> 	register unsigned long esp asm("esp"); \
>>> 	asm("" : "=r"(esp)); \
>>> 	esp; \
>>>     })
>>>
>>> /jakob
>> That seems to work! Though I'm still testing it in all our situations
>> for LLVMLinux. It looks like I can get this to work with LLVM 3.3 and
>> the LLVMLinux kernel patches. Thanks!
>>
>> However, I'd still like to see __builtin_stack_pointer() added since it
>> is simple, obvious, and mirrors other builtins like
>> __builtin_frame_address().
> Ping on this?
Sorry. Been away from my desk for 2 weeks (Linux Con, Embedded Linux
Con, and a week teaching). Just catching up this week.

The suggested work around DOES work for many situations, but is ugly.
However it allows the LLVMLinux patches to work with clang/LLVM 3.3
right now which is important. But I'd love something better for LLVM 3.4

> It seems like a legitimate builtin to have in clang, if only for
> compatibility and the nicer IR representation.

Personally, I'd still like to see __builtin_stack_pointer() added to
clang/LLVM (and gcc too for that matter, but that's still a work in
progress). It would be very useful for our efforts in getting generic
code to work in the Linux kernel.

Thanks,

Behan

-- 
Behan Webster
behanw at converseincode.com