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

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Message-ID: <530D58CD.4080202@mit.edu>
Date:	Tue, 25 Feb 2014 19:00:29 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>, 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 02/20/2014 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
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