Message-ID: <530D5989.5060001@zytor.com>
Date: Tue, 25 Feb 2014 19:03:37 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0
MIME-Version: 1.0
To: Andy Lutomirski <luto@amacapital.net>, behanw@converseincode.com,
        tglx@linutronix.de, mingo@redhat.com, x86@kernel.org,
        peterz@infradead.org, ak@linux.intel.com, oleg@redhat.com
CC: akpm@linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86: LLVMLinux: Reimplement current_stack_pointer without
 register usage.
References: <1392957882-24105-1-git-send-email-behanw@converseincode.com> <5306DC49.4060603@zytor.com> <530D58CD.4080202@mit.edu>
In-Reply-To: <530D58CD.4080202@mit.edu>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org

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


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/