[webkit-dev] Using memcmp

Patrick Hanna phanna at email.unc.edu
Wed Jul 1 08:52:19 PDT 2009

I think WebCore::equal already has a patch for SH4 and ARM. I was only  
going to replicate that same code in StringHash::equal. Are you  
talking about testing for alignment and then doing the 32-bit loop  
over the aligned segment?

On Jul 1, 2009, at 10:31 AM, Simone Fiorentino wrote:

> Hi Patrick,
> I think WebKit code can't rely on system memcmp() alignment  
> functionality
> since its implementation may change.
> I think a better solution should be to write code (inside WebKit)  
> that use
> aligned access in memory (as done in WebCore::equal)...for platforms  
> that
> needs aligned accesses.
> StringHash::equal() needs a patch (as done for WebCore::equal)...at  
> least for
> SH4 platform.
> If you prefer I can submit such a patch for SH4 platform.
> Tnx.
> Best regards,
> S.
> On Wed July 1 2009, Patrick Hanna wrote:
>> On Jun 30, 2009, at 2:53 PM, Maciej Stachowiak wrote:
>>> On Jun 30, 2009, at 9:57 AM, Patrick Hanna wrote:
>>>> I have noticed that rather than using memcmp for comparing strings,
>>>> webkit likes to cast to uint32_t* and compare in a for loop. For
>>>> example, WebCore::equal in AtomicString.cpp and StringHash::equal
>>>> in StringHash.h. Is there any reason not to memcmp? I am assuming
>>>> that most implementations of memcmp will do the word alignment and
>>>> basically do the same thing.
>>> We found the copy loop to be a little bit faster than memcmp on the
>>> platforms where we tested. memcmp tends not to do as well for short
>>> strings because it doesn't know that the start and end have some
>>> alignment guarantees. Also there is the function call overhead.
>>>> The reason I ask is because StringHash::equal does not check for
>>>> PLATFORM(ARM) or PLATFORM(SH4) so I was going to submit a patch to
>>>> use memcmp for ARM/SH4 but didn't know if that was frowned upon
>>>> (and WebCore::equal does a loop for ARM/SH4 as well).
>>> Is there a reason you'd want to make the code use a different
>>> implementation depending on the CPU? (It's not obvious to me from
>>> your message).
>> Sorry, I did not mention the problem. On some arm platforms,  
>> unaligned
>> access causes a bus error so we cannot reinterpret_cast from UChar*  
>> to
>> uint32_t*. Our memcmp does the alignment  check for us and is  
>> inlined.
>> When I do the patch, I will do the loop to be consistent with the
>> AtomicString implementation. Thanks for the info.
>>> Regards,
>>> Maciej
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