Hacker News Reader

In other words, how about that the people who claim that operations that do extra work are as fast as the ones that don't prove it? Instead of the burden of proof falling on people that don't have such an opinion/experience ? Then i will bow my head and say "You are right. Thank you for pointing that out". But alas google-ing for 10min and have found no such benchmark anywhere. And writing such a test isn't hard, not in the slightest.

I tend to the opposite view: those saying "do not do X" are in fact obligated to explain why X should be avoided. But perhaps this is just a difference in worldview.

One good reference for the "alignment doesn't matter" view is an earlier post on the same blog that is being discussed here: http://lemire.me/blog/2012/05/31/data-alignment-for-speed-my...

I linked elsewhere in the thread to my more detailed experiments regarding unaligned vector access on Haswell and Skylake: http://www.agner.org/optimize/blog/read.php?i=415#423. This is the source of my conclusion that alignment is not a significant factor when reading from L3 or memory, but does matter when attempting multiple reads per cycle from L1.

Both of these link to code that can be run for further tests. If you find an example of an unaligned access that is significantly slower than an aligned on a recent processor (and they certainly may exist) I'll nudge Daniel into writing an update to his blog post.

View on HN

typedef float sse_a __attribute__ ((vector_size(16), aligned(16))); typedef float sse_u __attribute__ ((vector_size(16), aligned(1))); void c_is_faster_than_asm_a(sse_a *dst, sse_a *src, int count) { for (int i = 0; i < count/sizeof(sse_a); i += 8) { dst[i] = src[i+0]; dst[i] = src[i+1]; dst[i] = src[i+2]; dst[i] = src[i+3]; dst[i] = src[i+4]; dst[i] = src[i+5]; dst[i] = src[i+6]; dst[i] = src[i+7]; } } void c_is_faster_than_asm_u(sse_u *dst, sse_u *src, int count) { // ditto

./test $(( 512*1024+1024*0 )) aligned: 0 sec, 134539 nsec unaligned on aligned data: 0 sec, 101471 nsec unaligned on one byte unaligned data: 0 sec, 190368 nsec unaligned on three bytes unaligned data: 0 sec, 181823 nsec aligned nontemporal: 0 sec, 359920 nsec naive: 0 sec, 214007 nsec c_is_faster_than_asm_a: 0 sec, 92437 nsec c_is_faster_than_asm_u: 0 sec, 92643 nsec c_is_faster_than_asm_u+1: 0 sec, 156574 nsec c_is_faster_than_asm_u+3: 0 sec, 156359 nsec c_is_faster_than_asm_u+4: 0 sec, 154932 nsec c_is_faster_than_asm_u+8: 0 sec, 155784 nsec ./test $(( 512*1024+1024*1 )) aligned: 0 sec, 107036 nsec unaligned on aligned data: 0 sec, 94861 nsec unaligned on one byte unaligned data: 0 sec, 114444 nsec unaligned on three bytes unaligned data: 0 sec, 115915 nsec aligned nontemporal: 0 sec, 407951 nsec naive: 0 sec, 219215 nsec c_is_faster_than_asm_a: 0 sec, 82474 nsec c_is_faster_than_asm_u: 0 sec, 82554 nsec c_is_faster_than_asm_u+1: 0 sec, 112544 nsec c_is_faster_than_asm_u+3: 0 sec, 115159 nsec c_is_faster_than_asm_u+4: 0 sec, 198434 nsec c_is_faster_than_asm_u+8: 0 sec, 118952 nsec

./test $(( 512*1024+1024*4 )) aligned: 0 sec, 107576 nsec unaligned on aligned data: 0 sec, 94010 nsec unaligned on one byte unaligned data: 0 sec, 140534 nsec unaligned on three bytes unaligned data: 0 sec, 140517 nsec aligned nontemporal: 0 sec, 467981 nsec naive: 0 sec, 206891 nsec c_is_faster_than_asm_a: 0 sec, 85294 nsec c_is_faster_than_asm_u: 0 sec, 85174 nsec c_is_faster_than_asm_u+1: 0 sec, 118674 nsec c_is_faster_than_asm_u+3: 0 sec, 118902 nsec c_is_faster_than_asm_u+4: 0 sec, 118370 nsec c_is_faster_than_asm_u+8: 0 sec, 118638 nsec

./test $(( 512*1024+1024*128 )) aligned: 0 sec, 167906 nsec unaligned on aligned data: 0 sec, 140650 nsec unaligned on one byte unaligned data: 0 sec, 239271 nsec unaligned on three bytes unaligned data: 0 sec, 251342 nsec aligned nontemporal: 0 sec, 458850 nsec naive: 0 sec, 364731 nsec c_is_faster_than_asm_a: 0 sec, 125240 nsec c_is_faster_than_asm_u: 0 sec, 118917 nsec c_is_faster_than_asm_u+1: 0 sec, 197348 nsec c_is_faster_than_asm_u+3: 0 sec, 196755 nsec c_is_faster_than_asm_u+4: 0 sec, 199757 nsec c_is_faster_than_asm_u+8: 0 sec, 197842 nsec