Now, all the consecutive bitfields are wrapped as a large integer unless there is unamed zero sized bitfield in between. The patch is trying to make the bitfield to be accessed as separate memory location if it has width of legal integer type and its bit offset is naturally aligned for the type. This can significantly improve the access efficiency of such bitfield.
https://reviews.llvm.org/D30416 wants to achieve the same goal in llvm, but it is much more difficult because it has to deal with the significantly tweaked IR after all sorts of optimizations. The patch here is trying to do that in clang.
With the patch, we can remove most of D30416 except the illegal memory access shrinking.