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 > > > > _______________________________________________ > webkit-dev mailing list > [email protected] > http://lists.webkit.org/mailman/listinfo.cgi/webkit-dev > _______________________________________________ webkit-dev mailing list [email protected] http://lists.webkit.org/mailman/listinfo.cgi/webkit-dev
