[LLVMdev] A new builtin: __builtin_stack_pointer()

[Renato Golin]

Hi Behan,

Only saw this thread now. I think having the built-in makes sense, but you
sent the description and patch to the LLVM list, not the Clang list. ;)

I'd copy the cfe-dev, but I think now the thread is more complex than
cross-posting would allow. So, maybe it'd be better if you posted there
again, your original email, the patch and some of the comments on this
thread.

cheers,
--renato


On 5 November 2013 16:15, Behan Webster <behanw at converseincode.com> wrote:

>  On 11/05/13 11:30, Jean-Daniel Dupas wrote:
>
>
>  Le 5 nov. 2013 à 19:00, Behan Webster <behanw at converseincode.com> a
> écrit :
>
>  On 11/05/13 09:26, Konstantin Tokarev wrote:
>
>
> 11.10.2013, 01:39, "Jakob Stoklund Olesen" <stoklund at 2pi.dk>:
>
> 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; \
>    })
>
> And #ifdef it for each arch? Builtin would be much more handy.
>
> Actually no #ifdef is required with Jakob's suggestion (works the same
> in both clang and gcc).
>
>
>  It may works for both GCC and clang, it does not means it will works
> with every arch linux supports (ARM, PPC, AArch, …)
>
> True enough. Which is indeed another argument for
> __builtin_stack_pointer().
>
> That point is precisely one of the reasons that we're pushing for this
> addition.
>
>
> Behan
>
> --
> Behan Websterbehanw at converseincode.com
>
>
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>
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