[llvmlinux] Builtin: stack pointer

Andrew Pinski pinskia at gmail.com
Thu Mar 27 10:47:32 UTC 2014

On Thu, Mar 27, 2014 at 3:44 AM, Andrew Pinski <pinskia at gmail.com> wrote:
> On Thu, Mar 27, 2014 at 3:25 AM, Renato Golin <renato.golin at linaro.org> wrote:
>> On 27 March 2014 10:12, Andreas Schwab <schwab at suse.de> wrote:
>>> Can't you use __builtin_frame_address (0) instead?
>> That would give me the frame pointer, not the stack pointer, and the
>> user would have to calculate manually the offset to get the actual
>> stack pointer, which would be target-specific, possibly making it even
>> worse. Is that what you meant?
> So if we audit what the kernel uses this method.  There is stack
> tracing code which is highly target dependent.
> And then there is current_thread_info which is also highly target
> dependent code (though it can use __builtin_frame_address(0) there
> since it is doing an alignment of a huge value at that point).
> I don't see why there needs to be an builtin function for this case,
> adding one extra instruction inside an inline-asm for the moving from
> the stack point to a normal register should be good enough.
> Take arch/arm64/include/asm/thread_info.h for an example,
> __builtin_frame_address(0) should be good enough since the sp and fp
> should be on the same page (that is what THREAD_SIZE is about).
>> (so you need to split by arch with ifdefs),
> Except it is already in the kernel anyways; not even by ifdefs but by
> different files so the reasoning adding a new builtin is not useful.

Please don't add a close list to the CC of GCC lists it is annoying.

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> Thanks,
> Andrew Pinski
>> cheers,
>> --renato

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