The cell size is not necessarily the same as "long" - it can be whatever the compiler wants, so long as alignment of non-bitfield fields is appropriate. It doesn't even have to be the same for every bitfield.
If the bitfield is declared as long, then based on that specific cell size the decision is made whether to pack the bits into the current cell or a new cell.
If a leading char member is followed by a uint64_t bitfield that is 57 bits wide, a new cell will be allocated for those 57 bits at offset 8. The char is considered to be a field of 8 bits allocated in an existing 64 bit cell, leaving 56. 57 cannot fit, and so the offset is bumped to the next cell alignment.
This is testable.
I'm only writing about GCC, not about ISO C, which specifies very little, allowing implementations latitude in choosing the underlying storage unit size and alignment for bitfields regardless of their declared type.
