On a 64-bit system, the DWARFDebugLine::Row struct is 32 bytes. Each field has the following byte offsets:
24+: bit fields
The packing is fine until the "Isa" field, which is an 8-bit int that occupies 4 bytes. We can instead move Discriminator into the 16-19 slot, and pack Isa into the 20-23 range along with the bit fields:
20-23: Isa + bit fields
This layout is only 24 bytes. This 25% reduction in size may seem small but a large binary can have line tables with thousands of rows stored in a vector.
Patch by Simon Que!
Differential Revision: https://reviews.llvm.org/D27961