Fix crashes appearing on CUDA CC 7 cards.

[griwodz]

Fixes the 2 issues with RTX 2080 that were reported and discussed in Issue #64

[griwodz]

The checks are failing in an apt-get update. What can I do about that?

[griwodz]

@simogasp may have observed a slowdown. While the changes were tiny, I noticed a little overhead in s_desc_loop.cu. popsift::any() implies a __syncwarp(), and __syncthreads() isn't actually required. So I moved syncthreads() out of the loop. Also I revised the indentation, which makes it a lot easier to see how small the change is with respect to develop.

[simogasp]

@griwodz shall we also update the cmake to specify the architectures to build according to the CUDA version? As of now, CCs > 6.5 are not built.

[griwodz]

It seems to work safely on everything up to CC 7.5 now, so it's probably a good idea.

[simogasp]

LGTM

As for cmake what about changing those lines to:

set(PopSift_CUDA_CC_LIST_BASIC 30 35 50 52 )
# versions greater than 7 support at least up to 6.x
if(CUDA_VERSION_MAJOR GREATER 7)
  list(APPEND PopSift_CUDA_CC_LIST_BASIC 60 61 62)
endif()
# versions greater than 8 support at least up to 7.2
if(CUDA_VERSION_MAJOR GREATER 8)
  list(APPEND PopSift_CUDA_CC_LIST_BASIC 70 72)
endif()
# versions greater than 9 support at least up to 7.5
if(CUDA_VERSION_MAJOR GREATER 9)
  list(APPEND PopSift_CUDA_CC_LIST_BASIC 75)
endif()

(info from here https://en.wikipedia.org/wiki/CUDA#GPUs_supported)

[simogasp]

const int lane

[griwodz]

Can we make a new PR for CMake and and the const?
I'm abroad and cannot test anything and I believe having the RTX fix merged is a good idea.
The APPEND needs a bit of testing because some of the CCs won't work. 32, 53, 62 and 72 are CC specific to the Tegra, which is an ARM-based system-on-a-chip and cannot do everything that I've used in PopSift. CC 52, on the other hand, should work.

[simogasp]

Oh yes, I've just listed the CC based on the cuda support, I had no idea which one should or should not be included. It was more to propose a different way to add the relevant CC.

Just out of curiosity, the tests that I ran were on an RTX 2080 without the proper CC set (7.5), do you think there is a significant difference when running the code from another CC that is not the one proper to the card? (i.e. should I expect that it runs even faster? :-) )

I'm not in a hurry to merge it, we can keep it here and merge it later.