Re: [PATCH] x86: LLVMLinux: Reimplement current_stack_pointer without register usage.

[Andy Lutomirski <luto@amacapital.net>]

On 02/20/2014 08:55 PM, H. Peter Anvin wrote:
> This seems like really deep magic when looking at it... at the very
> least, this needs to be very carefully commented, including why it works
> on the various platforms.
> 
> How much does this actually affect the output?  I only see three uses of
> current_stack_pointer:
> 
> /* how to get the thread information struct from C */
> static inline struct thread_info *current_thread_info(void)
> {
>         return (struct thread_info *)
>                 (current_stack_pointer & ~(THREAD_SIZE - 1));
> }
> 
> ... here we need the mov anyway, because we have to then AND it with a
> mask, which we obviously can't do inside the stack pointer.

No clue what code is actually generated, but the new code could generate:

mov $MASK, %rax;
and %esp, %rax;

Admittedly, I can't see any reason why this would be an improvement.

--Andy