linux-kernel - Re: [PATCH] x86: LLVMLinux: Reimplement current_stack

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Message-ID: <530D5989.5060001@zytor.com>
Date:	Tue, 25 Feb 2014 19:03:37 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>, behanw@...verseincode.com,
	tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
	peterz@...radead.org, ak@...ux.intel.com, oleg@...hat.com
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: LLVMLinux: Reimplement current_stack_pointer without
 register usage.

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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