[cfe-dev] A new builtin: __builtin_stack_pointer()

[Behan Webster]

On 11/08/13 11:34, Richard Smith wrote:
> On Thu, Nov 7, 2013 at 6:29 PM, Behan Webster
> <behanw at converseincode.com <mailto:behanw at converseincode.com>> wrote:
>
>     On 11/07/13 01:47, Joerg Sonnenberger wrote:
>     > On Wed, Nov 06, 2013 at 05:46:43PM -0800, Behan Webster wrote:
>     >> Another way would be to introduce a new builtin in parallel to
>     others
>     >> like __builtin_return_address(). Essentially adding a
>     >> __builtin_stack_pointer() which does what the above code does.
>     The idea
>     >> would be to get something like this added to both clang and gcc
>     in order
>     >> to make this work across compilers, and across arches.
>     > You still haven't said what the intended behavior of this builtin is
>     > when the function containing it is inlined.
>     The intent in all situations is for it to use the current value in the
>     stack pointer register. It doesn't matter whether it is inlined or
>     not.
>     It should be the immediate value of R13 on ARM or esp on X86_64
>     (etc for
>     other arches).
>
>     The idea is to preclude the need to use ASM to move the value in the
>     stack pointer to another register in order to use it in C code
>     (which is
>     inefficient).
>
>     Essentially I'm looking for a better solution than adding a C symbol
>     name to the stack register.
>
>     Does that make sense?
>
>
> Not yet. Why do you want this? What will you do with it? What
> semantics do you want the returned pointer to have? How can correct
> code ever do anything with this pointer? (Remember that the backend is
> in principle allowed to modify the stack pointer at any time,
> including between the point where you call this intrinsic and the
> point where you use its result.)
Its used by the Linux kernel in several situations, but mostly for
threading, and various debug and stack tracing code.

Essentially the kernel code primarily uses the stack pointer by
assigning it a C symbol name like this.

    register unsigned long current_sp asm ("sp");

The value of that named register is read and stored elsewhere, or is
used to calculate the beginning or end of the stack in order to find the
current threadinfo or the current pt_regs (previous dump of the CPU
registers). Essentially in the case of ARM, r13 is used directly in the
resulting code.

gcc allows you to do the above in order to access the value of the stack
pointer register (in the way I describe), however clang does not unless
you then add the following.

    register unsigned long current_sp asm ("sp");
    asm("" : "=r(current_sp));

... which works, but is ugly. Since my goal is to be able to compile the
Linux kernel with both clang and gcc, in a way which is the most
efficient with each compiler, and which doesn't make the code worse, the
above it's going to cut it.

Instead I'd like to see __builtin_stack_pointer() added to both clang
and gcc. It's easier to read, would provide read only access to the
register value (which is safer), and mirrors the other __builtin
functions (which in the Linux kernel are often called together:
__builtin_frame_address() and __builtin_return_address()

For instance:

        frame.fp = (unsigned long)__builtin_frame_address(0);
        frame.sp = current_sp;
        frame.lr = (unsigned long)__builtin_return_address(0);
        frame.pc = (unsigned long)return_address;

No doubt there is a more optimal way than the patch I sent.

Behan

-- 
Behan Webster
behanw at converseincode.com

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