On Fri, Nov 14, 2014 at 10:43:39PM +0600, Yerden Zhumabekov wrote: > > 14.11.2014 19:53, Neil Horman ?????: > > On Fri, Nov 14, 2014 at 05:57:51PM +0600, Yerden Zhumabekov wrote: > >> 14.11.2014 17:33, Neil Horman ?????: > >>> Not really. That covers the case of applications selecting the hash > >>> function > >>> using the DEFUALT_HASH_FUNC macro, but doesn't nothing for applications > >>> using > >>> the function directly. Test_hash_perf is an example of this, and > >>> ostensibly > >>> because of the behavior without SSE4.2 it defines these huge test tables > >>> twice > >>> based on the availability of SSE4.2. It would be better if we could allow > >>> applications to use rte_hash_crc regardless, and make the code it uses at > >>> run > >>> time configurable. > >> I see, then we have a problem here :) > >> > >> Actually, that was one of my concerns when developing these patches. I > >> looked through the source code of libs and examples and I saw the > >> '#ifdef..#include..#endif'-like appoach while selecting hash function > >> was common. So I organized patches to minimize the impact on API and not > >> to contradict this approach. > >> > > Thats a reasonable approach, but I really hate the idea of continuing this > > need > > to select cpu features at compile time if its not nececcesary. > > > >> If we prefer to change this approach then, I guess, we need to introduce > >> broader changes to rte_hash library and change other code which uses it. > >> If that's what's needed, then it'll take some time for me to rework > >> these patches. > >> > > Well, its possible you'll get lucky. crc is such a common operation, its > > entirely possible that the gcc intrinsic emits software based crc > > computation if > > the SSE4.2 instructions aren't enabled. I recommend modifying the > > test_hash_crc > > function to use rte_hash_crc with SSE4.2 disabled, and see if you get a > > crash. > > If you don't examine the disassembly of your new function and confirm that > > something reasonable that doesn't use SSE4.2 is emitted. If thats the case, > > your patch is fine, and we can focus on how to change the ifdefs in the > > existing > > code, as use of the rte_hash_crc functions should be safe. > > > > Unfortunately, it seems not to be the case. Trying to force compiling a > test program with _mm_crc32_u32 intrinsic on computer with no SSE4.2 > support leads to "Illegal instruction error". So it looks like GCC does > not fall back to crc32 software implementation. > Ok, but crc32 is pretty easy to implement in software. Just appropriate the calculate_crc32c function from the BSD or Linux kernels and if (unlikely(!support_sse42)) calculate_crc32 operation.
Neil > -- > Sincerely, > > Yerden Zhumabekov > State Technical Service > Astana, KZ > > >