linux-kernel - Re: [PATCH v2 2/2] x86: add prefetching to do

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Date:	Fri, 08 Nov 2013 11:17:41 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Neil Horman <nhorman@...driver.com>, Joe Perches <joe@...ches.com>
CC:	Dave Jones <davej@...hat.com>, linux-kernel@...r.kernel.org,
	sebastien.dugue@...l.net, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org
Subject: Re: [PATCH v2 2/2] x86: add prefetching to do_csum

On 11/08/2013 11:07 AM, Neil Horman wrote:
> On Fri, Nov 08, 2013 at 08:51:07AM -0800, Joe Perches wrote:
>> On Fri, 2013-11-08 at 11:25 -0500, Neil Horman wrote:
>>> On Wed, Nov 06, 2013 at 12:07:38PM -0800, Joe Perches wrote:
>>>> On Wed, 2013-11-06 at 15:02 -0500, Neil Horman wrote:
>>>>> On Wed, Nov 06, 2013 at 09:19:23AM -0800, Joe Perches wrote:
>>>> []
>>>>>> __always_inline instead of inline
>>>>>> static __always_inline void prefetch_lines(const void *addr, size_t len)
>>>>>> {
>>>>>> 	const void *end = addr + len;
>>>>>> ...
>>>>>>
>>>>>> buff doesn't need a void * cast in prefetch_lines
>>>>>>
>>>>> Actually I take back what I said here, we do need the cast, not for a conversion
>>>>> from unsigned char * to void *, but rather to discard the const qualifier
>>>>> without making the compiler complain.
>>>>
>>>> Not if the function is changed to const void *
>>>> and end is also const void * as shown.
>>>>
>>> Addr is incremented in the for loop, so it can't be const.  I could add a loop
>>> counter variable on the stack, but that doesn't seem like it would help anything
>>
>> Perhaps you meant
>> 	void * const addr;
>> but that's not what I wrote.
>>
> No, I meant smoething like:
> static __always_inline void prefetch_lines(const void * addr, size_t len)
> {
> 	const void *tmp = (void *)addr;
> 	...
> 	for(;tmp<end; tmp+=cache_line_size())
> 	...
> }
> 
>> Let me know if this doesn't compile.
>> It does here...
> Huh, it does.  But that makes very little sense to me.  by qualifying addr as
> const, how is the compiler not throwing a warning in the for loop about us
> incrementing that same variable?
> 

As Joe is pointing out, you are confusing "const foo *tmp" with "foo *
const tmp".  The former means: "tmp is a variable pointing to type const
foo".  The latter means: "tmp is a constant pointing to type foo".

There is no problem modifying tmp in the former case; it prohibits
modifying *tmp.  In the latter case modifying tmp is prohibited, but
modifying *tmp is just fine.

Now, "const char *" would arguably be more correct here since arithmetic
on void is a gcc extension, but the same argument applies there.

	-hpa


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