On Sun, Mar 13, 2016 at 07:12:50PM -0400, Ganesh Ajjanagadde wrote: > This is based on the relatively well known xorshift128+ of Sebastiano > Vigna (https://en.wikipedia.org/wiki/Xorshift) that performs very well on the > BigCrush suite, is very efficient, and is thus used by a number of > clients: http://xorshift.di.unimi.it/ (see Introduction). > > Moreover, the implementation is in the public domain: > http://xorshift.di.unimi.it/xorshift128plus.c. > > Concretely, it is nearly as fast as av_lfg_get (which only returns 32 bits), > and has a much smaller cache (128 bits). Thus, the timings should also > be more stable. > > This is needed because av_lfg_get<<32 | av_lfg_get is far slower, and > likely less random as measured by BigCrush - most LFG's perform > quite poorly on the BigCrush suite: > http://www6.tw.freebsd.org/distfiles/testu01.pdf. > In particular, FFmpeg's seems to be Ran055 in the paper, see pg31. > > Sample benchmark (Haswell, GCC + -march=native): > 23200 decicycles in 624 calls of av_lfg_get, 1 runs, 0 skips > 23040 decicycles in 624 calls of av_lfg_get, 2 runs, 0 skips > 22810 decicycles in 624 calls of av_lfg_get, 4 runs, 0 skips > [...] > 19884 decicycles in 624 calls of av_lfg_get, 65532 runs, 4 skips > 19880 decicycles in 624 calls of av_lfg_get, 131067 runs, 5 skips > 19884 decicycles in 624 calls of av_lfg_get, 262136 runs, 8 skips > 19877 decicycles in 624 calls of av_lfg_get, 524276 runs, 12 skips > > 25380 decicycles in 624 calls of av_rand64_get, 1 runs, 0 skips > 28560 decicycles in 624 calls of av_rand64_get, 2 runs, 0 skips > 30112 decicycles in 624 calls of av_rand64_get, 4 runs, 0 skips > [...] > 22627 decicycles in 624 calls of av_rand64_get, 65536 runs, 0 skips > 22625 decicycles in 624 calls of av_rand64_get, 131072 runs, 0 skips > 22625 decicycles in 624 calls of av_rand64_get, 262143 runs, 1 skips > 22624 decicycles in 624 calls of av_rand64_get, 524286 runs, 2 skips > > Signed-off-by: Ganesh Ajjanagadde <gajja...@gmail.com> > --- > libavutil/lfg.c | 33 ++++++++++++++++++++++++++++++++- > libavutil/lfg.h | 26 ++++++++++++++++++++++++++ > 2 files changed, 58 insertions(+), 1 deletion(-)
why do you add this code to lfg.* (Lagged Fibonacci Generator)? its not a lfg also the LFG could be trivially extended/changed to 64bit if one wants only needs uint64_t being used also theres av_mlfg_get() which passes all tests, though slower of course and does xorshift128+ really pass all tests ? what if the bits are reversed so that the least significant and most significant are exchanged ? the text seems unclear about that anyway, iam fine with the addition of xorshift128plus but please put it in a different file above questions are more due to curiousity and not a objection [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB I am the wisest man alive, for I know one thing, and that is that I know nothing. -- Socrates
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