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

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Message-Id: <530B32A4020000780011EACB@nat28.tlf.novell.com>
Date:	Mon, 24 Feb 2014 10:53:08 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Daniel Borkmann" <dborkman@...hat.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 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.

Jan

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