[LLVMdev] A new builtin: __builtin_stack_pointer()

[Jakob Stoklund Olesen]

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