linux-kernel - Re: [PATCH 2/3] x86/hash: swap parameters of crc32

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Message-ID: <530B3102.4050102@redhat.com>
Date:	Mon, 24 Feb 2014 12:46:10 +0100
From:	Daniel Borkmann <dborkman@...hat.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	davem@...emloft.net, mingo@...e.hu, tglx@...utronix.de,
	ffusco@...hat.com, tgraf@...hat.com, linux-kernel@...r.kernel.org,
	hpa@...or.com
Subject: Re: [PATCH 2/3] x86/hash: swap parameters of crc32_u32()

On 02/24/2014 11:53 AM, Jan Beulich wrote:
>>>> On 24.02.14 at 11:22, Daniel Borkmann <dborkman@...hat.com> wrote:
>> On 02/24/2014 09:03 AM, Jan Beulich wrote:
>>>>>> On 22.02.14 at 13:09, Daniel Borkmann <dborkman@...hat.com> wrote:
>>>> On 02/21/2014 11:33 AM, Jan Beulich wrote:
>>>>> ... to match its two callers (i.e. the alternative would have been to
>>>>> swap the arguments at the call sites).
>>>>>
>>>>> Signed-off-by: Jan Beulich <jbeulich@...e.com>
>>>>> Cc: Francesco Fusco <ffusco@...hat.com>
>>>>> Cc: Daniel Borkmann <dborkman@...hat.com>
>>>>> Cc: Thomas Graf <tgraf@...hat.com>
>>>>> Cc: David S. Miller <davem@...emloft.net>
>>>>> ---
>>>>>     arch/x86/lib/hash.c |    2 +-
>>>>>     1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> --- 3.14-rc3-x86-hash-crc32.orig/arch/x86/lib/hash.c
>>>>> +++ 3.14-rc3-x86-hash-crc32/arch/x86/lib/hash.c
>>>>> @@ -37,7 +37,7 @@
>>>>>     #include <asm/cpufeature.h>
>>>>>     #include <asm/hash.h>
>>>>>
>>>>> -static inline u32 crc32_u32(u32 crc, u32 val)
>>>>> +static inline u32 crc32_u32(u32 val, u32 crc)
>>>>>     {
>>>>>     #ifdef CONFIG_AS_CRC32
>>>>>     	asm ("crc32l %1,%0\n" : "+r" (crc) : "rm" (val));
>>>>
>>>> Can you elaborate?
>>>>
>>>> Sorry, I need to ask here (even if it's a stupid question ;)) if this
>>>> change is safe to do; are referring to a cleanup or fixing a concrete
>>>> bug? The code is a modified version of the DPDK hash which you can find
>>>> in [1]. Arguments of the caller are in the correct order, afaik.
>>>>
>>>>      [1] http://dpdk.org/browse/dpdk/tree/lib/librte_hash/rte_hash_crc.h
>>>
>>> Yes, that file appears to be correct:
>>>
>>> rte_hash_crc_4byte(uint32_t data, uint32_t init_val)
>>>
>>> as opposed to
>>>
>>> static inline u32 crc32_u32(u32 crc, u32 val)
>>>
>>> (quite obviously data <-> val and crc <-> init_val, supported
>>> by the second argument in each caller being named "seed").
>>
>> If you want a more descriptive name, feel free to rename these vars,
>> but check it yourself, it's not a bug as you claim; results are the
>> same:
>
> Even if the results are the same (operands being symmetric?), check
> the generated code for your version and the fixed up one: The crc32
> instruction allows one of its operands to be in memory for a reason.

I'm fine with that. But then, please reflect these details in your
commit message.

Thanks !
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