Rearrange DWARFDebugLine::Row fields for smaller struct size

[sque]

Summary

On a 64-bit system, the DWARFDebugLine::Row struct is 32 bytes. Each field has the following byte offsets:

0-7: Address
8-11: Line
12-13: Column
14-15: File
16-19: Isa
20-23: Discriminator
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:

0-7: Address
8-11: Line
12-13: Column
14-15: File
16-19: Discriminator
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.

[aprantl]

LGTM. Thanks!

While you're in there, could you (in a separate commit) doxygen-ify the comments in this header?

[sque]

Sure, is there a script or command line tool that is typically used to doxygen-ify?

[aprantl]

No, it's a manual process. But pretty much the only thing that is really needed is to turn // in front of declarations into ///. For more details you can check out
http://llvm.org/docs/CodingStandards.html#doxygen-use-in-documentation-comments

[sque]

@aprantl I don't have commit access. Can you commit this for me?

[echristo]

echristo@dzur ~/s/llvm> git svn dcommit
Committing to https://llvm.org/svn/llvm-project/llvm/trunk ...

M       include/llvm/DebugInfo/DWARF/DWARFDebugLine.h

Committed r290931

llvm/trunk/include/llvm/DebugInfo/DWARF/DWARFDebugLine.h

struct Row {
     uint32_t Line;
     // An unsigned integer indicating a column number within a source line.
     // Columns are numbered beginning at 1. The value 0 is reserved to indicate
     // that a statement begins at the 'left edge' of the line.
     uint16_t Column;
     // An unsigned integer indicating the identity of the source file
     // corresponding to a machine instruction.
     uint16_t File;
-    // An unsigned integer whose value encodes the applicable instruction set
-    // architecture for the current instruction.
-    uint8_t Isa;
     // An unsigned integer representing the DWARF path discriminator value
     // for this location.
     uint32_t Discriminator;
+    // An unsigned integer whose value encodes the applicable instruction set
+    // architecture for the current instruction.
+    uint8_t Isa;
     // A boolean indicating that the current instruction is the beginning of a
     // statement.
     uint8_t IsStmt:1,
             // A boolean indicating that the current instruction is the
             // beginning of a basic block.
             BasicBlock:1,
             // A boolean indicating that the current address is that of the
             // first byte after the end of a sequence of target machine