From: "H. Peter Anvin" <hpa@zytor.com>
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.
Date: Tue, 25 Feb 2014 19:03:37 -0800 [thread overview]
Message-ID: <530D5989.5060001@zytor.com> (raw)
In-Reply-To: <530D58CD.4080202@mit.edu>
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
prev parent reply other threads:[~2014-02-26 3:14 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-21 4:44 [PATCH] x86: LLVMLinux: Reimplement current_stack_pointer without register usage behanw
2014-02-21 4:55 ` H. Peter Anvin
2014-02-26 3:00 ` Andy Lutomirski
2014-02-26 3:03 ` H. Peter Anvin [this message]
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