Messages in this thread | | | Date | Tue, 25 Feb 2014 19:03:37 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] x86: LLVMLinux: Reimplement current_stack_pointer without register usage. |
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On 02/25/2014 07:00 PM, Andy Lutomirski wrote: >> >> 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. >
You have to generate one of the code sequences:
mov $MASK, %eax and %esp, %eax
... or ...
mov %esp, %eax and $MASK, %eax
No real difference either way.
-hpa
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